R107 MkI
Just arrived for an overhaul
is this early example of the R107 receiver.
Note that what you read below
describes various problems as they were identified and experiments
made to determine their rectification. Sometimes it took quite
a long time to arrive at the true causes and some ideas proved
wrong before effecting repairs. Some of the parts concerned are
now, like myself, just about 80 years old and obviously will
not be perfect. Another point to remember is that this example
was refurbished by MoD, probably 70 years ago and it's clear
that some errors were made in the process. |
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I first located a lead with
an R107 plug, tested this and then checked across the pins on
the receiver which proved to be open. The first step was to clean
the fuse located behind the mains transformer which had a verdigris
coating (this is used for 12 volt DC operation only) then
waggle the on/off switch which had duly followed the usual practice
of being lazy. Plugging in a mains supply brought up the valve
heaters including the rectifier but no sign of life from the
set. At the front is a test panel which includes an HT point
which slowly rose to 58 volts then dropped back to 56 volts as
the valves warmed up. Waggling various controls showed no signs
of crackling so time to delve into the innards. I could perhaps
take a short cut and connect an external HT supply to the receiver,
but before I try this I'll investigate further.
It was at this point I discovered
that the HT voltage must be top secret as it wasn't given anywhere
in the manual (ZA3220). When you consider that the key to fault
finding is the set of voltage readings at the test panel you'd
think that the HT value is rather important as it will influence
the quoted readings. I eventually found a figure of 250 volts
quoted in brackets in a document labelled 3STC Aug '42 VJN. It
was added to advise that if all the test readings were bad the
HT voltage might be the cause. Many of the power supply components
are not exactly accessible but to compensate there are several
"tag boards" labelled A, B, C and D which carry important
connections, including on Pin 1 the HT output from the HT smoothing
condenser. B1, C1 and D1 should therefore have 250 volts present. |
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You'll note the test point
labelling cleverly relates directly to the valves. Measurements
are made between HT (+) and the test point. |
Circuit |
Test Point |
Reading volts |
V1A |
1A |
15.0 |
V2A |
2A |
11.0 or 5.0 |
V2A' |
2A' |
9.5 or 0 |
V1B |
1B |
11.5 |
V1C |
1C |
16.5 |
V1D |
1D |
16.5 |
V2B |
2B |
9.5 |
V2B' |
2B' |
20 |
HT |
+ |
250 wrt chassis |
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The four tag boards left
to right A, B, C and D.
A lot of effort has been made
to help diagnose faults.
The tags aren't marked but their
circuit designations are numbered 1 to 12 top left to bottom
right. |
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As the red lamp wasn't
lit I checked and found the bulb was missing. The receiver can
be operated from a 12 volt battery so valve heaters are connected
in series-parallel. The dial lamps, like valve heaters are 6.3V
and are connected in series but the bulb under the red cover
is fed via a resistor. The bulb ratings are also top secret as
they're excluded from the parts lists. As long as the dial lamps
are matched (because two of the three are wired in series) I
guess their rating isn't too important.
Having failed to make sense
of the connections to the 6X5 rectifier (which was lit OK) I
decided to connect a DC supply in place of the set's HT at pin
1 of tag board D and chassis ground.
There wasn't any sign of trouble
as I wound up the voltage with 250 volts drawing some 35 to 40mA
but nothing from the speaker. I plugged in headphones and was
rewarded with a slight hum and, by adjusting various controls
and connecting a long wire aerial, turning to Range 3, I was
able to tune a strong medium wave broadcast. After fiddling with
controls I found the speaker switch was completely unresponsive
so I added a jumper wire across the switch and Smooth Radio was
suddenly loud and clear. The BFO responded to its switch but
was miles off frequency.
So what's the problem? There
are a few possibilities including an open circuit HT secondary
winding, a bad 6X5 or a bad component buried in the power supply
area. My Bernards Valve guide gives 6X5 anodes at pins 3 and
5, heater at pins 2 and 7, and cathode at pin 8 but a quick continuity
test failed to prove connections to these to be right. |
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It didn't take too long
to figure out what was wrong and it's something pretty weird.
In fact weird is becoming quite a frequent thing with radios
visiting my workshop. You can see below what looks to me to be
wiring that looks different to other stuff. Those two blue wires
are connected to the 6X5 rectifier at pins 4 and 5. The black
wires (plus that faded black wire) connect to chassis. Resistance
readings across pins 4 and 5 and either one to chassis match
the secondary winding (around 320 ohms and two readings of 160
ohms) so it appears the transformer is OK.
I can only explain what I found
by accusing a dabbler rewiring this area and mistaking the connections
to the valve socket. If pins 3 and 5 rather than pins 4 and 5
had been soldered the HT would have probably been OK. According
to the circuit diagram a single condenser viz. C20A should be
wired across the secondary not a pair connected to ground. Those
two blue wires, plus the orange and grey are very close to metalwork
and should perhaps have been routed at the near edge where there's
more room. |
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I should really delve
under the power supply chassis and sort out the mess... |
Circuit |
Test Point |
Correct volts |
Actual volts |
V1A |
1A |
15.0 |
7.4 |
V2A |
2A |
11.0 or 5.0 |
5.3 |
V2A' |
2A' |
9.5 or 0 |
0 |
V1B |
1B |
11.5 |
13 |
V1C |
1C |
16.5 |
14.6 |
V1D |
1D |
16.5 |
15 |
V2B |
2B |
9.5 |
10.7 |
V2B' |
2B' |
20 |
24 |
HT |
+ |
250 wrt chassis |
250 wrt chassis |
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With exactly 250 volts
from an external supply I measured the test points (left).
Not perfect but at least Smooth
Radio is booming from the loudspeaker and switching from AVC
to OFF indicates at least there's partial AVC action.
After rewiring the rectifier
and finding the set failed to work, the last owner must have
got so fed up he just dumped it....
Rather than stripping the PSU
to access the 6X5 pins.. why don't I just insert a shorting wire
from pin 4 to pin 5 after checking the latter isn't wired to
anything else? |
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I tried shorting pins
3 and 4 and the HT increased to 90 volts so I rechecked continuities....
about zero ohms from the bottom leg of the 250-0-250 winding
to pin 4 of the 6X5 and about 1Mohm from Pin 4 to pin 8. About
350 ohms across the full winding and a good ground point.
Powered, the winding supplies
about 600 volts AC to my meter.
Either there's a wiring error
or possibly small resistors to act as fuses have been added as
shown here.
I'll need to dismantle the PSU
to find out... not too easy a job as the picture below suggests?
Those screws look suspicious?? |
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In fact the 6X5 valveholder
is to the right of this metalwork underneath an RFC (just visible
under the diagonal strengthening bar) marked as L17A on the PSU
circuit diagram. It's difficult to see the wiring but I decided
on a simpler approach by soldering a couple of wires to the 6X5
pins and connecting these via tiny 51 ohm resistors to act as
fuses to the 250-0-250 winding. This worked and gave me about
170 volts HT. A little lower than expected, maybe due to a bad
reservoir or smoothing condenser and listening on headphones
indicated there was some hum.
I checked the voltages across
the resistors and measured 700mV (14mA) and 100mV (2mA) to pins
3 and 5 respectively. Apparently the 6X5 is not perfect and will
help explain the low HT. An external HT supply drew around 40mA
at 250 volts.
Now that the set is working
I'll see if rewiring the 6X5 base is a practical proposition
and look for a replacement valve or even cheat and fit a couple
of silicon diodes... in fact this would be a very simple mod
as all the connections are on top of the transformer. |
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I fitted a pair of BY299
diodes (800V x 2A) as these were robust and left my two 51 ohm
(fuse) series resistors in place. No provision for surge as I'd
previously checked the HT line by pulling it up to over 300 volts
without a problem. The HT was around 270 volts at switch-on dropping
to 235 volts with the valves warmed up. The voltages across the
resistors were equal and measured 750mV RMS (15mA each leg).
Plenty of volume for Smooth Radio and Absolute Radio but the
set was very deaf on the two shortwave ranges with just a faint
squeak of 80m SSB. Next I'll recheck the test panel voltages,
start alignment, beginning with the IF amplifier which runs at
465KHz and fix the BFO frequency. Many years ago I heard the
R107 BFO was so potent it could be heard miles away by ships
at sea. I wonder if this was true.
I'll cut the two blue wires,
isolating whatever weird circuit is present under the chassis
then tidy up the wiring to allow the 6X4 to be refitted as a
dummy then the transformer cover can be replaced. |
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Circuit |
Test Point |
Correct volts |
Actual volts 1 |
Actual volts 2 |
V1A |
1A |
15.0 |
7.4 |
13.8 |
V2A |
2A |
11.0 or 5.0 |
5.3 |
4.7 |
V2A' |
2A' |
9.5 or 0 |
0 |
0 |
V1B |
1B |
11.5 |
13 |
11.5 |
V1C |
1C |
16.5 |
14.6 |
16.4 |
V1D |
1D |
16.5 |
15 |
16.5 |
V2B |
2B |
9.5 |
10.7 |
9.2 |
V2B' |
2B' |
20 |
24 |
20.3 |
HT |
+ |
250 wrt chassis |
250 wrt chassis |
216 wrt chassis |
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With the PSU using a pair
of BY299 diodes instead of the 6X5 the test points are reading
slightly differently and with no anomalies. The IF amplifier
was pretty close to optimum at 465KHz but Range 3 needed tweaking
at its HF end. Ranges 1 and 2 are still deaf probably due to
misalignment.
In order to make RF adjustments,
using an audio wattmeter across the loudspeaker, I fitted a large
smoothing capacitor to eliminate hum. |
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One design feature to
help servicing the R107 is the provision of holes in the front
panel. These are covered by removable plates (which you can see
in the picture at the top of this page), allowing access to the
IF transformer trimmer condensers. Some similar wartime receivers
need to be adjusted using a short screwdriver in a cramped space.
A stroke of luck for a change
was the BFO whose slow motion drive wasn't fastened to the tuning
shaft making tuning rather hit and miss. Once I'd positioned
the drive and tightened the loose grub screw the BFO tuned across
465KHz.
Front end alignment is quite
easy as the trimmers and coil adjustments are nicely grouped
together in a logical fashion. Front to back.. Aerial, bandpass
input, bandpass output and oscillator. Top to bottom Range 3,
Range 2 and Range 1.
Incidentally, to align the IF
amplifier I used my new TinySA. Coupled
via a 20dB attenuator and a BNC flying lead with croc clips.
It worked perfectly. Initially set to 465KHz AM at 1KHz I reduced
the audio to 500Hz as my ears got fed up with the former. It
was running at its default of -7dBm and via the attenuator, used
to protect it against any stray voltage, I initially clipped
it to the top of V1C the first IF amplifier then for checking
Range 3 at 3MHz and 1.2MHz to the aerial connector. I then checked
the results with my HP8640B and found
it was set up perfectly. Later I'll use my
DSA815TG because merely tweaking the IF trimmers rarely produces
perfect IF response. To check the BFO frequency a fully tunable
signal source is far better than a digital type, and when confirming
you have the true and not an image or spurious response a properly
tuneable signal generator is far better. For example sweeping
across a particular shortwave frequency might reveal dozens of
spurii if the amplitude of the signal is too strong. Sometime
(when I've retired) I'll do an experiment to reveal this...
Below the front end adjustments...
you follow the general rule to first check the mechanical tuning
setup, then trim condensers at the HF end and twiddle coils at
the LF end until the tuning range exactly fills the required
band and you're happy with the flatness of the background noise
level. Bear in mind that in this day and age the background noise
level picked up by an aerial may fluctuate considerably. I'm
sorry that many listeners will never have the opportunity afforded,
in the 1950s for example, when with headphones you could (metaphorically)
hear a pin drop in New Zealand on 40m. |
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A few things to do are..
remove and clean the loudspeaker switch, detach the HT connection
from the rear of the muting plug. A better idea might be to remove
the muting plug and fit a new IEC mains plug in its place, using
the wires currently going to the two pin mains plug. The reason
being that no safety earth is provided with the existing plug.
The loudspeaker switch one detached very reluctantly woke up
after applying switch cleaner and jiggling it a few hundred times
(due to a rusty return spring). I suppose I should also check
valve top caps for any positive DC voltage... |
Next I tackled the slow
motion drive as this felt rough and slipped in places. I decided
to remove the whole thing and take it apart. Not too difficult
but tricky reassembling because you need to know precisely which
parts to fit first. The method is to assemble the bearings before
sliding into the outer sleeve, taking care to get the outer parts
the right way round. The one with the locating screw slides in
first with the notch last. The outer three balls are fitted first
then the inner three balls slip into the gap at the end with
the lever and note that they only fit in one sequence. The inner
three 6BA screws need to be quite tight and the outer ones adjusted
for drive smoothness.
The rough feel was because grease
had migrated out of the bearing surfaces leaving metal-metal
contact which had worn the bearing surfaces. This was much worse
at the inner bearing. |
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Left, the slow motion
drive removed from the receiver and below the component parts.
The two bearing shell pairs are each connected by a set of three
6BA screws and linked together within the outer cylinder to which
the outer shells are held in place. One by a tiny screw and the
other by a pin engaging with a slot in the cylinder edge as you
can see on the left. The inner bearing is driven by a shaft coupled
to the outer bearing and just enough tension is applied to the
sets of screws to overcome friction and rotate the tuning condenser.
In this design the cylinder carries the direct drive knob which
remains stationary as the inner slow motion knob is turned. The
dial pointer is clamped to the thicker output shaft which also
carries a pin to limit travel to 180 degrees. |
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I'd noticed that there
was a lot of play in the pointer both before and after I'd put
in new grease. The cause is the copper disk under the three screws
tends to move backwards and forwards as drive is applied. I'm
unsure how to fix this other than perhaps supergluing the disk
in place. Maybe star washers between the copper disk and the
bearing would stop movement? The root cause is turning force
from the inner bearing pushes the 6BA screws across the hole
width in the outer bearing because the holes in the bearing are
larger than 6BA. Even though the movement is quite tiny it's
amplified at the dial pointer causing backlash. The solution
was simple. The three screws at the rear of the drive were too
tight. Slackening these resulted in well nigh perfect tuning
and I was able to listen to SSB on 40m, however, turning on the
set this afternoon resulted in the pair of HT fuse resistors
burning up... the last time this sort of problem occurred it
was a large grey condenser. These are made in the same way as
older waxed tubular versions wound with paper and aluminium foil.
I guess a better solution is to fit a proper fuse in the centre
tap ground connection and maybe a pair NTC thermistors in place
of the fuse resistors? |
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When I investigated the reason
for the burnt fuse resistors I found the main circuit was OK
but one of the diodes had failed short-circuit. I then realised
the full wave rectifier was driven by an HT winding that supplies
250 volts DC when loaded with say 50mA but when unloaded I measured
300 volts RMS whose peak is 424 volts and across the whole winding
double this at about 848 volts. Bearing in mind these figures
are probably even a little low because of mains fluctuations
and noise, the use of an 800 volt diode is definitely a bad idea.
I swapped the pair of BY299 to BY255 which are rated at 1200
volts and also fitted a pair of 120 ohm eighth watt fuse resistors
in the diode cathode legs which now have 1.7 volts DC across
each. This represents only about 15mA or 30mA HT drain. The HT
voltage at the smoothing condenser measured at 210 volts with
test points below. These (Actual volts 3) are now very close
to optimum.
Circuit |
Test Point |
Correct volts |
Actual volts 1 |
Actual volts 2 |
Actual volts 3 |
Final Test |
V1A |
1A |
15.0 |
7.4 |
13.8 |
12.9 |
6.2 |
V2A |
2A |
11.0 or 5.0 |
5.3 |
4.7 |
11.0 and 5 |
5.7 |
V2A' |
2A' |
9.5 or 0 |
0 |
0 |
8.2 and 0 |
0 |
V1B |
1B |
11.5 |
13 |
11.5 |
11.3 |
13.1 |
V1C |
1C |
16.5 |
14.6 |
16.4 |
15.6 |
14.4 |
V1D |
1D |
16.5 |
15 |
16.5 |
15.8 |
14.3 |
V2B |
2B |
9.5 |
10.7 |
9.2 |
8.9 |
11.5 |
V2B' |
2B' |
20 |
24 |
20.3 |
19.75 |
26.3 |
HT |
+ |
250 wrt chassis |
250 wrt chassis |
216 wrt chassis |
210 wrt chassis |
263 wrt chassis |
As you can see in the picture
above, all the components are readily accessible apart from those
under the diagonal re-enforcing bar but as HT current is only
relatively small and the various test points now show normal
conditions I don't believe wholesale swapping of parts is necessary.
I measured most of the resistors and found all were high in value.
Typically 20% high. During initial testing I'd noticed swinging
the HT from 250 volts down to less than 100 volts had no real
effect on the audio output and that being so resistor values
being universally high doesn't matter. Keeping the colourful
originals therefore is a sound idea. Alas, there's an exception
as for some odd reason the panel lamp doesn't come on because
the 100 ohm series resistor in the manual is actually 150 ohms
and this measured 180 ohms. The latter requires a 6.3 volt lamp
rated at say 50mA. Rather than fit a standard 200mA lamp and
change the resistor (R1B) to 30 ohms 1W I fitted a red LED soldered
to an old lamp base. This works OK and runs with 4.6 volts RMS
across it and, being a diode, requires no external rectifier.
My guess is the panel lamp was too bright and a mod was issued
to raise the series resistor by 50% to dim it. |
Replacing a valve rectifier
with silicon diodes in an old receiver is not as straightforward
as it first seems. A valve rectifier warms up much the same as
the receiver's valves so a potential voltage surge isn't that
important. Diodes on the other hand will immediately place their
rectified voltage at their cathodes and this voltage can be very
much higher than the receiver is used to accommodating.
WW2 and 1950s/1960s equipment
might have used an electro-mechanical time delay device to prevent
damage from a voltage surge but is there a simpler method using
modern components? |
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The above sketches show
the basic R107 HT power supply, the silicon diode version I'm
using and a theoretical circuit using thermistors.
My BY255 diode solution is very
compact and uses a very high value smoothing capacitor to remove
hum and as a side effect delays the HT from establishing across
the valve circuit. This delay is due to the fact that the capacitor
is initially seen as a short-circuit fed through resistance made
up from a number of elements. The HT transformer winding has
a resistance of about 330 ohms, the RFC about 10 ohms, the LFC
about 100 ohms and a pair of small resistors for fusing open
in the event of HT failure each of 120 ohms. If the circuit resistance
is say 600 ohms the initial surge, given an open circuit HT of
350 volts will be around 0.6 amp reducing rapidly as the capacitor
charges and the valves warm up. Because the target voltage at
the R107 circuit is say 250 volts (and it may not even reach
this) we're looking at a time constant of only say 20% of RC
so our 600 ohms and 470uF is effectively a couple of seconds
and by this time the valves will be warming up and drawing current
thus limiting the voltage surge.
A more elegant solution might
be shown in the third sketch above. T1 and T2 are thermistors
having a cold resistance of say 100 ohms dropping to say 1 ohm
when hot. T3 is a PTC thermistor having a low value when cold
rising to a very high value when hot. The circuit looks like
a good solution but there's a technical difficulty with T3. Thermistors
are designed to limit current surge not to limit voltage surge
so are primarily low resistance devices. There are some that
have a quite high resistance but have only a small current handling
capability. They also have a limited voltage capability which
means faced with say 350 volts of HT we need to use lots of these
devices in series. Current handling being fairly low means we
may need to use two or more of these series-connected chains
in parallel.
Another technical difficulty
is to arrange the time delay for resistance changes to take place.
Thermistors are quite small and need to react swiftly in order
to minimise heat dissipation, whereas our valve circuit would
like maybe 10 seconds of delay. So there's a conflict which may
lead to the thermistors getting too hot. |
Another option is this
which uses T1, T2 and T3 as they were intended, to handle current
surge. It also helps to arrange a calculable time delay for the
establishment of HT. A downside is the power handling capacity
of T3 as well as its current and voltage handling capacity. In
fact such a device as required for T3 may not exist.
Can T3 be constructed from available
devices? In the previous circuit T3 is a PTC type but here it's
an NTC type of thermistor. Perhaps an NTCLE100E3102JB0 would
work? Will it just go POP at switch on or maybe BANG or would
a suitable choice for T1 and T2 help? I'll invite readers to
work this out for themselves. |
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Having been diverted I
got back to R107 alignment. The IF response curve was OK but
slightly shifted HF. I corrected this initially in narrow then
checked it on wide with the following results. I injected the
signal at the top cap of V1B and measured the response at the
detector diode in V2F.
I'm using my high voltage probe
(to safeguard my DSA815TG) which has an inderterminate gain so
ignore the power levels in the traces. The relative values are
fine but the actual numbers are irrelevant. |
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Narrow response. |
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Wide response. |
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All the IF trimmers peaked
nicely except one.. this was the lower trimmer in T1C which suggests
something wrong. Overall gain is fair with Range 3 pretty good
but the shortwave ranges are a bit deaf so I'm guessing there's
a bad component in the connections between V1C and V2F. Oops..
yet another senior moment... I asked the WS19 Forum if anyone
had any advice concerning T1C and was informed I'd published
the very reason for the problem and here it is below **.
The IF cans rely solely on earthing on a couple of metal bars
being in contact with the inside surface of the IF can and in
the earlier example these bars were not only badly tarnished
but painted. It makes me wonder if a large number of R107s have
exactly the same fault as a result perhaps of the same person
doing the assembly. My guess is, if that were so who suspects
the loss of gain is due to this? Maybe umpteen examples are in
use with a deaf IF stage with owners blaming bad band conditions?
Rather than strip out the can, a rather tedious exercise, maybe
a small hole drilled through the centre of the base and a pair
of self-tapping screws used to renew contact with the can? Click
either picture to see the original refurbishing article.
** In this case Sod's Law ruled that the fault
should be due to something completely different |
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A practical solution is
to fit a couple of screws with solder tags and provide a new
grounding point for the IF can.
Before I tackled this I decided
to check everything that was supposed to be grounded and everything
tested fine so that bad earth on the previous R107 displayed
the same fault but this time the problem lies elsewhere.. there's
no option but to remove the IF can. This is very tricky as some
screws are hidden and most are loctited but after a bit of ingenuity
I managed. My original thought was a bad micamould condenser
and two are connected to the lower untuning coil. |
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The circuit diagram on
the R107 lid and in the manual have an error. The same error,
which is a missing connection, associated with T1C.
Is it Sod's law that dictates
this very schematic error points to the fault I'm experiencing?
Two condensers joined in series are designed to tune the lower
coil in conjunction with the trimmer. I looked at them.. they're
bolted together to the base of the IF can with two 6BA screws,
loctited and each with a locknut. I removed the screws and cut
off the condensers from the trimmer. One marked 0.001uF measured
989pF and the second marked 400pF measuring 388pF. Pretty good
BUT they fell apart when I measured them because they should
have been.. but weren't soldered together! Was it the very same
bloke that made a mistake wiring that 6X5 rectifier that forgot
to solder the condensers?
I think I'll assume this... |
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Above, T1C with the two
micamoulds removed (right). They're held together because one
leg is just bent around the other but not soldered! The lower
one (400pF) in series with the upper (0.001uF) were in parallel
with the trimmer with the junction soldered to the coil and the
brown covered wire at bottom left. |
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Look closely at these
two drawings. The one on the left is the same as the circuit
diagram marked on the inside of the R107 front lid and you can
see an error..the right hand diagram is consistent with the assembly
inside the IF can. |
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I noticed a fault when
first checking the RF trimmers at the can carrying the anode
coils for the RF amplifier V1A. The gain for Range 2 infrequently
increased dramatically (reaching normal level) when touching
the can making me suspect a bad solder joint or maybe a bad earthing
point. It didn't really happen with Range 3. The proper gain
disappeared as suddenly as it had appeared and I couldn't repeat
the gain change by tapping the can. |
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After detaching the cans
from the RF stage coils (eg L4A and L4B) and the paxolin board
holding the three trimmers and coils L4A/L5A/L6A I found the
centre coil and its trimmer very sensitive to touch ***.
Inputting -90dBm of modulated AM at 3MHz into the aerial on Range
3, I could comfortably hear the tone. I then tuned to Range 2
and tuned to 3MHz. The input had to be raised to -40dBm to hear
the same level tone and by jiggling L5A the tone would increase
by 30dB. I found a pair of plastic covered wires stretched tightly
across the edge of the RF chassis, one of which might have been
shorting but I'm not 100% convinced as the coil still seemed
intermittently sensitive to touch. I might detach the coil and
see if there's a poorly soldered joint. Because of the cramped
layout it's not possible to see connections to the coils and
trimmers. In the meantime I tried my homebrew noise source to
try and evaluate what's going on. This is really easy to use
compared with a signal generator and I was able to roughly equalise
the RF response over Ranges 1 and 3.
At this point I decided to deal
with the red loctite used on the coil adjusters. I found the
one way to get rid of this was to hold a scalpel to the thread
and rotate the screw. I also used a 2BA die. First screw the
nut downwards against the top of the coil, withdrawing the screw
as far as possible then carefully position the die and by hand
rotate it down the brass thread.
*** This turned out to be a faulty wavechange switch.
The wire from the Range 2 coil wasn't being selected at the switch. |
This is the output from
the noise source scanned from zero to 30MHz. |
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A view of the RF amplifier
and Mixer grid coils which form a bandpass filter for each of
the three wavebands.
The trimmers and coil adjusters
are very well placed and logically laid out for alignment. The
covers for both coil sets have been detached. |
|
I found Range 2 to be
very odd. I can peak the noise at the LF end and at the HF end
but tuning across the range shows a huge dead spot about a third
down from the HF end. At first I thought this is due to inadvertent
switching from true to image reception eg. the LF end and the
HF end are tuned separately to the true and image signals. I
have come across this problem before which I think was due to
a bad coil or too much or too little stray capacitance. I confirmed
this wasn't the case by using my spectrum analyser which method
I'll describe below.
The R107 coils are unusual,
according to the handbook, using eddy currents to modify their
inductance. It's said that each coil has a copper disk rather
than a dust-iron core and the recommendation is not to insert
the disk too far into the coil for fear of losing gain. This
effect is demonstrable using the noise source. |
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In a similar way to checking
IF response you can check a receiver's RF alignment. To do this
connect the tracking generator to the aerial socket whist monitoring
the grid of the frequency changer. See
here which is a test of the RF25 unit. Instead of a complete
scan as in the case of a fixed frequency IF amplifier you see
two signals for a correctly aligned receiver. These are the response
at the frequency to which the dial is set plus the local oscillator
(LO). The R107 has the LO higher by 465KHz than the tuned frequency
so the narrow spike seen on the right is the LO. Two useful checks
can be undertaken. First, by tuning end to end, you can readily
see any amplitude change in the LO. Ideally the amplitude of
the spike shouldn't vary too much. The shape of the RF response
is very interesting as it should smoothly rise in amplitude before
plunging down with much the same slope. In the R107 there are
three sets of tuned circuits which help keep the curve shape
relatively narrow. The narrower the curve and the higher is the
top of curve the better is the attenuation of the LO and the
image. Set the frequency to the high frequency end of the dial
and use the trimmers to set the shape of the curve, then retune
to the lowest frequency and adjust the coils for best curve shape
again. Then repeat a few times (as with standard alignment).
Quite tiny trimmer changes can make significant curve changes.
With care you can also check
dial settings by monitoring then setting the LO frequency to
match the dial (knocking off 465KHz).
What did I see? Well I was able
to perfectly align Range 1 and 3. Notable was the effect of altering
the coils. The normal method of using a fixed frequency and an
audio wattmeter has coil adjustment pretty vague and woolly compared
with the SA method where even small coil changes were quite apparent.
So Ranges 1 and 3 checked out well; however Range 2 gave the
same result as shown above with a pronounced dip somewhere between
5 and 6MHz. I've shown 100% as the norm but Range 2 develops
a lot less response to the noise generator than Ranges 1 and
3. I can now confirm though that the notch is nothing to do with
a crossover from true to image signals, but instead a resonance
effect. Somewhere in the (faulty) front end is a circuit rejecting
something like 5MHz. Because the low gain can correct itself
intermittently it can't be a wiring error but must be an open
or short circuit or faulty component which is introducing a series
connected coil and condenser across the signal path or a parallel
tuned circuit in series with the signal path.. ie. a rejector
circuit.
Springing to mind is something
like an IF rejector tuned circuit designed to deafen a receiver
to a signal close to the IF. The aim of that is to place a narrow
band short circuit across the aerial input circuit.
I studied the circuit diagram
and noticed the wavechange switch not only selected the desired
range coils but also shorted out coils not being used. That being
so if I looked at the trimmer stators (which are easy to get
at and the coils not so) to identify what I confidently exected
to be either an undesired short or open circuit within the set
of RF anode or mixer grid coils. I checked the resistance of
each stator to ground. The mixer grid stators were a solid 6Kohm
to ground (in fact the rotors are wired together to ground for
the first and third coil sets and decoupling condenser C5C for
the second set) whatever position was the wavechange switch but
the anode coil stators were very odd. These were either 50Kohm
or 500Kohm to ground and these resistances seemed to be completely
random at any setting of the wavechange switch. In fact turning
the switch sometimes gave 50K and a moment later in the same
setting 500K. I donned my magnifying goggles, illuminated the
switch wafer with a torch and immediately spotted a switch tab
lying on top of the pair of contacts instead of between them.
The adjacent contacts were skewed at 45 degrees and all the switch
tabs were the same. None were feeding into their contact sets,
and mostly either resting on the upper contact or with the tiniest
space away. The whole rotating part of the wafer was very slightly
out of place, just far enough for its tabs to miss threading
between the contact sets. It was a simple enough task to gently
pry the contact sets into line with the tabs.
What was happening then is shown
in the sketches below. RF energy was being coupled to the mixer
Range 2 coil from a combination of wiring plus unshorted coils
for Ranges 1 and 3. The unselected coil for Range 2 was absorbing
RF energy at the natural frequency of coil and trimmer (without
the tuning condenser). Range 3 was working OK with its switch
contacts in good order and Range 1 not too bad but in the Range
2 position of the wavechange switch the combination of faults
matched those shown below. Once corrected the Range 2 coil was
properly selected and overall gain dramatically increased. At
this point I hadn't replaced the two screening cans. |
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The sets of coils are
housed in metal cans and the wavechange switch is normally protected
with a heavy metal cover.
I initially thought the R107
had been placed on something that had fouled the wavechange switch,
stressing the wafer, but because the cover was in place the switch
was either poorly manufactured or damaged by a previous owner.
When I removed the metal cover I did notice the screws were a
mixed bunch and not original. |
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Checking alignment showed
that hardly anything now lined up other than the local oscillator
and a fresh re-alignment is required. I also need to identify
why the audio output is relatively low (apart from the LF end
of Range 3 which covers some local medium wave broadcasts) although
I still have an audio wattmeter, which accounts for some loss,
connected across the loudspeaker..
Realignment went ahead with
the screening cans back in place but the notch had re-appeared
around 5MHz in Range 2. Testing is not easy because of the way
the receiver is put together but I removed the two screening
cans and the paxolin top from the anode coils again. There are
a few possible reasons for the notch and I decided to see if
I could rule out a bad coil. I connected my multitester to each
coil in turn and measured its inductance starting with RF amplifier
anode coils. These measured 50uH and 10uH plus "Low inductance"
(probably 1.5uH?). Each was checked with the wavechange switch
in its correct position and should have been shorted out in the
other two settings, however in Range 3 the 10uH coil failed to
be shorted so there's still a remaing problem with the wavechange
switch. I used a different method to test the mixer coils (as
I'd left its paxolin panel in place) by measuring the inductance
at the tuning condenser. The coils measured exactly as before
so the RF amp anode and mixer coils are OK. The table below shows
measurements across the RF amplifier anode and mixer coils. **
missing short.
Tuning Range |
Range 1(anode) |
Range 2(anode) |
Range 3(anode) |
Range 1(mixer) |
Range 2(mixer) |
Range 3(mixer) |
Range 1 |
290mohm |
140mohm |
140mohm |
290mohm |
250mohm |
250mohm |
Range 2 |
600mohm |
10uH |
10uH ** |
250mohm |
10uH |
250mohm |
Range 3 |
290mohm |
290mohm |
50uH |
250mohm |
250mohm |
50uH |
This second table takes the
possible tuning range of the variable condenser plus strays plus
trimmer in pF then calculates the natural frequency of an unshorted
coil plus trimmer and you can readily see the Range 3 coil left
unshorted would absorb 5MHz from Range 2 (exactly the problem).
The components list gives C14A as 300pF x 4 which lines up perfectly
with the table below given a trimmer of say 30pF plus strays
including the tuning condenser minimum value. Coil values were
those measured plus a guess at that for Range 1. If this analysis
is correct shorting the Range 3 coil with Range 2 selected would
remove the notch.
|
COIL |
CAP |
FREQ |
COIL |
CAP |
FREQ |
CAP |
FREQ |
|
|
Tuning |
Low end |
|
Tuning |
High end |
Trimmer |
Resonance |
Range 3 |
50uH |
350pF |
1.2MHz |
50uH |
50pF |
3.2MHz |
20pF |
5MHz |
Range 2 |
10uH |
350pF |
2.7MHz |
10uH |
50pF |
7.2MHz |
20pF |
11.3MHz |
Range 1 |
1.5uH |
350pF |
6.9MHz |
1.5uH |
50pF |
18.4MHz |
20pF |
29MHz |
|
I played around with the
three wave ranges for ages with the screening cans on and then
removed. One puzzle was why I'd discovered that the notch in
Range 2 had seemingly gone, when later it had re-appeared. Problem
solved.. at least now to my satisfaction. Using the homebrew
noise source I noticed the deafness in Range 2 had to a large
extent gone away. In fact the noise level in Range 2 was now
equal to the noise level in Range 3. Rembering that Range 3 was
pretty good means that now Range 2 is also pretty good.. but
only with both the bandpass filter screening cans removed...
put one anywhere near the coils and Range 2 noise drops and with
even one in place Range two is completely deaf. My guess is the
coils shift in inductance as the screening can is fitted so they
are no longer able to be tracked across the range. In fact even
with the cans removed correct tracking is tricky. Oddly Range
1 appears deafer with the screening can detached than in place
and is very difficult to track. I now need to carry out some
further experiments to resolve the issues.
Range 1 was indeed pretty deaf
with the screening can off but with the can in place pretty good,
with the noise source producing much the same results as with
Range 3. What if I fooled the Range 1 coils? I offerered up a
small piece of tinplate between Range 1 anode and mixer coils
and up went the trace on the DSA815 by more than 10dB and up
went the noise level to the original level. Very odd, but I'm
putting it down to "eddy current tuning" (the method
used in the R107 handbook).
It's all very puzzling as presumably
the 1939 design team knew what they were doing. My guess is that
ageing is involved. Maybe metal oxidation, bad earthing, poor
condensers or even the refurbishing carried out in the 1950s
provides the explanation? As one possible reason for problems
with the front end is bad condensers I decided to remove the
decouplers. All were identical 0.05uF x 350V (500V peak) and
metal clad. Results are given below. I replaced all seven with
new polyester 68nF x 250VAC. Of course as you'd expect with this
R107.. absolutely no change.
Measuring the condenser leakages
revealed they're a lot better than wax coated types. C5M was
the worst being 95Kohm with the others ranging from 194Kohm to
1Mohm. Interestingly as they were connected to a circuit carrying
400 volts in series with 360Kohm about half were slowly improving
whilst the remainder slowly worsening. |
CONDENSER |
C5A |
C5B |
C5C |
C5D |
C5E |
C5F |
C5M |
MARKED |
0.05uF |
0.05uF |
0.05uF |
0.05uF |
0.05uF |
0.05uF |
0.05uF |
CAPACITANCE |
300nF |
109nF |
56nF |
204nF |
174nF |
99nF |
589nF |
% LOSS |
12.0 |
4.9 |
1.3 |
8.6 |
8.0 |
4.7 |
15.0 |
ESR OHMS |
110 |
- |
- |
130 |
- |
- |
110 |
|
I made a small tin bracket
which I placed between the two Range 1 bandpass coils and this
improved the receiver's response across the band by 10dB. During
this process I noticed that pressing on the Range 2 coil increased
the audio output by 20dB. I tried poking at the wiring etc but
only pressing on the coil had any effect until the imrovement
just went away and couldn't be repeated. The investigation would
appear to need the coil removing. I also noticed that the same
coil refused to track, meaning that the trimmer needed twiddling
to improve a -70dBm signal from nothing to audible. Maybe the
coupling coil is dry jointed or the coil copper slug has detached?
I found waving ferrite or brass near the anode coil altered its
tuning so the 10uH coil itself must be intact as are connections
to its trimmer. I suppose I could wind a new coupling coil in
place between the anode and mixer coils... |
After puzzling over the
bandpass filter tuning problem I decided to discover what "eddy
current tuning" meant and soon realised the reason for my
difficulties. It all hinges on the use of ferrous or non-ferrous
tuning slugs. Ordinarily mutual coupling between iron cored coils
increases inductance so each time an alignment pass is made the
coil adjustment reinforces alignment but the reverse happens
with "copper" tuning (called "eddy current"
tuning). For starters, looking at the drawing opposite, all six
tuning coils interact to some extent with each other. Enclose
a pair of screening cans over the input (A) and output coils
(B) and the major interaction is reduced to four coils, and of
course the non-ferrous can has a dramatic effect on the inductance
of every coil. I also noted that the R107 uses a special screen
over the tuning condenser, presumably to minimise coupling between
the different tuned circuits. Of course I'd detached this in
order to work on the receiver. Things had not been improved by
the bad wavechange switch and an intermittent in L5A. Clearly
every part has to be in place when aligning the receiver. |
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Attempting to resonate
L4A and L4B to transfer the optimum RF signal across Range 1
is difficult because as L4A is brought to resonance it raises
the resonant frequency of L4B and vice-versa. Also, because unused
coils are shorted by the wavechange switch their resonant frequency
is dependent on their inductance plus trimmer. If this resonance
occurs within the tuning range of the selected waveband it will
alter the inductance of the tuned coil in such away as to detune
it at one specific point. This results in a notch which is quite
obvious using a noise source connected to the aerial connector.
Presumably this notch can be minimised by using a specific alignment
technique. |
Now.. can the R107 be
fixed? I decided to check the RF amplifier tuned circuits to
see if I could precisely determine their characteristics. What
I found does explain the set's deafness because all the coils
have too much inductance. They all resonate to a frequency lower
than the dial setting. To make things simpler I've quoted the
average of the coil/trimmer settings eg. for Range 2 at 7MHz
the coil/trimmer range covered 4.8 to 5.2MHz so a false resonance
would occur when the setting was 5.2MHz.
It's not too easy to check the
bandpass filter resonances so it remains to be seen if these
are as bad as those in the RF amplifier.
During checking I discovered
the grid leak was a 22K resistor measuring 26K instead of R2A
quoted at 250Kohm. |
|
COIL |
LOW DIAL |
TUNED CIRCUIT |
HIGH DIAL |
TUNED CIRCUIT |
RANGE 1 |
L1A |
7.0MHz |
6.0MHz |
17.0MHz |
14.6MHz |
RANGE 2 |
L2A |
3.0MHz |
2.50MHz |
7.0MHz |
5.0MHz |
RANGE 3 |
L3A |
1.2MHz |
1.15MHz |
3.0MHz |
2.25MHz |
|
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Is it possible perhaps to use
the same technique as used on a local oscillator circuit namely
to insert a padding capacitor in series with each of the three
RF coils? Access is not to difficult at the switch contacts.
S1A/A or even a common padder at C13C/C4A to the switch wiper... No.. I tried this and it wasn't successful.
The following table attempts to calculate the actual inductances
of the three aerial coils assuming the tuning condenser is 300pF,
a trimmer setting of 15pF and varying strays and coil capacitance
7pF. The next step is to roughly work out the number of turns
required and then to count them. It's then possible to work out
how many turns are required to produce the correct coil value.
The latter will be calculated in another table using dial markings.
Plans have now changed because I measured the aerial coil inductances
and they seem to be exactly as they should be or even less! (see
below).... back to the drawing board.. Is the tuning condenser
the right one at 300pF? Not easy to measure it in-situ so I measured
one that had the same size rotor plates and it measured 470pF.
It has 15 vanes compared with 10 which makes about 313pF for
the R107 example. Next I checked the grid coupling condenser
C13A and it measured correctly as 200pF. |
|
COIL |
LOW DIAL |
C in pF |
L calc |
L actual |
TEST |
HIGH DIAL |
C in pF |
L calc |
L actual |
TEST |
L test |
RANGE 1 |
L1A |
7.0MHz |
322 |
1.6uH |
1.1uH |
6.5MHz |
17.5MHz |
52 |
1.6uH |
1.1uH |
14.3MHz |
2.29uH |
RANGE 2 |
L2A |
3.0MHz |
322 |
9.3uH |
8.8uH |
2.9MHz |
7.0MHz |
52 |
9.3uH |
8.8uH |
7.25MHz |
19.5uH |
RANGE 3 |
L3A |
1.2MHz |
322 |
54uH |
53.8uH |
1.2MHz |
3.0MHz |
52 |
54uH |
53.8uH |
2.2MHz |
96uH |
|
Using the experimental results
above (results were in the column marked "TEST"), L1A
is 2.29uH (43% high), L2A is 19.5uH (110% high) and L3A
is 96uH (77% high).
Previously I'd measured the
bandpass coils with an LCR meter as L3B=50uH and L2B=10uH which
is promising although I'm starting to think that either the coil,
due to changes in the wax covering or the former on which the
coils are wound has gained extra self capacity so although their
inductance might be OK their additional capacitance reduces their
resonant frequency. On the other hand their adjusting cores are
said to be brass and introducing this material into the coil
should decrease their inductance but I'm fairly sure the opposite
is happening. Finally I found the true design values of the coils
marked in red below from James Cookson G4XWD. If anything these
are even more puzzling unless the brass cores can swing their
values considerably lower.
Note that unlike a normal receiver
that uses dust iron tuning slugs the R107 uses brass cores. Ordinarily
a coil will be would to provide the minimum inductance required
to match the HF end of the tuning range, but in the case of the
R107 the opposite applies. Coils are wound to match the LF end
of the tuning range, bearing in mind stray capacitance and trimmers
will enable their resonant frequency to be lowered. This is not
the case with the R107 as with the core and trimmers set for
the LF end the dial must correspond with the HF end. In fact
the whole alignment process hinges on the brass cores swinging
the coil inductances high enough.. and that's not happening here. |
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In an effort to align
the front end of the receiver I'd noticed R2A had been reduced
to 22K from 250K and wondered if this had been a modification.
Sure enough whilst atempting to align the bandpass filter the
receiver had burst into oscillation which was tamed by resoldering
the old R2A resistor back. I've had some experience of this kind
of problem so looked at grounding points. There are some very
odd examples. Firstly the aerial coils are commoned to a small
solder tag held under a post on the left of the screening can.
These posts are tarnished and not very tight so I fixed this.
Next I looked at the tuning condenser and tightened its securing
screws. The grounding of the bandpass filter is also poor and
I found that adding additional decoupling capacitors grounded
to the edges of the cans considerably improved RF stability.
These connect to the live sides of C5C and C5D. I was then able
to revert to the 240K grid leak and overall front end gain increased
even though tracking was still some way from ideal. |
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It was easier to add this
simple audio amplifier than replace several of the RF coils.
The audio from the detector is typically raised from 600mV to
14 volts although the aim is not to produce lots of audio power
but to amplify weak signals.
The TIC49 is nothing special
except it has a CE rating of 350 volts. I'm running it at around
5mA collector current which places the collector at about 50
volts. The base resistors act to give auto-bias.
While testing the amplifier
I found the reason for the low HT voltage... this being an open
circuit reservoir condenser. Adding 4.7uF brought the HT up to
310volts. |
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Above is the simple audio
booster located adjacent to the base of V2B' mounted on a 5-way
tagstrip. To the right is the new HT smoothing capacitor and
I fitted a small HT reservoir capacitor with the pair of silicon
diodes inside the transformer cover. The amplifier provides a
gain of about 27dB. Of course Sod's Law came into play when,
just after checking the amplifier, I carried out a final experiment
on the RF coils. During earlier testing I'd noticed Range 2,
being very deaf would come to life for a brief moment then die
again, but no amount of fiddling with the circuit would repeat
the increase in gain. I decided to use a hot air gun to remove
the wax coating and maybe warm up the coil in case the wax had
somehow added extra intrinsic capacitance which might explain
reluctance to resonate. The coil winding emerged and I initially
wondered if there was an intermittent short between the coupling
and tuned windings. With the receiver switched on I found slight
movement of the trimmer produced a slight crackle. The stator
was firmly soldered as was the rotor whose black covered wire
connected to the coil. I checked the connection from chassis
to the rotor and noticed it varied from 1.3Mohm to something
higher but very intermittently so, but the Range 3 trimmer rotor
showed a solid 1.3Mohm. I pulled the black wire and much to my
suprise the bare end slipped out from under the coil.. it hadn't
been soldered but before melting away the wax had been fairly
secure. I tinned the bare end and soldered it to the outside
of the solder tag on L5A.
Before testing further I removed
the wax from Range 1 bandpass coils L6A and B whilst checking
their resonance. Refitting the two bandpass filter screening
cans showed more gain even before fresh alignment and was now
quite reasonable for Range 1 and 2 but after proper realignment
all three amateur bands 80m, 40m and 20m had decent CW and SSB
signals. Using my homebrew noise source I can still see a resonance
effect from Range 3 coil in Range 2 and maybe Range 2 in Range
1 but overall performance is now quite acceptable. Some care
is now required to get the best results because of overloading
and a little instability from the extra audio gain, improvements
I made to RF grounding, new RF decoupling capacitors and the
substitution of the higher value grid leak for the RF stage.
The final change I made was
to fit a 1uF x 600V reservoir capacitor which resulted in an
HT of 268 volts rather than over 300 volts using 4.7uF.
I now need to tighten the trimmers
and put back detached metalwork. Handling the set shows the amount
of audio gain to be good for SSB/CW but needs to be reduced together
with RF gain for reception of strong broadcasts, but an essential
job is to make the set relatively safe. It uses a special 2-pin
mains plug without any provision for a safety ground and additionally
carries a connector (used in conjunction with a trasmitter for
muting audio) with a set of exposed pins of which one carries
HT of about 300 volts. I decided to fit an IEC mains socket after
removing the plug carrying HT. |
It's necessary to remove
the side strenthening panel to get at the redundant muting connector
which is held in place with 4BA screws and locking nuts. Also
in the way is the HT block condenser used as a reservoir and
for smoothing. With these parts removed I cut the muting circuit
wires, tucked these out of the way and removed the muting plug.
This left a hole in which I fitted an IEC plug on a small metal
plate. This connects via a pair of leads to a couple of chokes
below the chassis whose leads I disconnected from the old mains
plug.
When I'd first tested the receiver
it had several HT problems already covered above. Clearly the
reservoir condenser wasn't working and I'd fitted a 1uF x 600V
capacitor. This was chosen to lift the HT, now supplied from
silicon diodes, to a level commensurate with the design value.
Too much reservoir capacity and the HT would have risen to over
320 volts because the forward resistance of the new diodes is
very much less than that of the original 6X5 rectifier. The new
smoothing condenser is actually 470uF x 450V and is aimed not
only at smoothing the HT, but to delay its rise a little.
As you can see on the right
this old block condenser is marked "NOV 43" and its
two 8uF sections measure 282pF and 95pF so useless as a reservoir
or for smoothing, in fact useless for anything other than maybe
a paperweight (if the R107 was being restored it would be worthwhile
to remove its innards and fit a pair of new capacitors).
Below a view of the new mains
lead and its fitting using the original 4BA screws. That second
block condenser incidentally measured 4uF, precisely as its marking
states and has an ESR of a mere 0.49 ohms. |
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How does the R107 handle now
that it's been brought back to life? Sensitivity on a long wire
is now excellent and the extra audio amplification can bring
weak 40m and 80m SSB signals to good loudspeaker strength. The
AGC produces about minus 4.5 volts on strong signals which isn't
enough to dampen overall gain sufficiently so very strong broadcast
signals can overload the output valve. This means that audio
gain and RF gain controls need to be balanced in order to cope.
However without the additional amplification ageing of coils
in the front end would mean poor results on shortwave signals.
To receive LSB the BFO is set right of centre and USB the same
amount left of centre. Fidelity of audio is a trifle sharp but
I found that using the narrow IF setting and/or the crash limiter
results in a more mellow sound. The headphone and line output
sockets can feed stereo headphones if the plug is not fully inserted
and of course the audio level can be adjusted independently of
the loudspeaker setting if either headphone socket is used. I
found the balanced aerial input is insensitive. This might be
a design characteristic or merely a problem due to ageing of
the aerial coils. Image response is negligible due to the design
of the set. The tuning control is very light and unusually the
inner knob does not rotate in sympathy with the slow motion knob.
Adjustments for smoothness or backlash can be made by tightening
or loosening three 6BA screws on the rear of the drive or alternatively
at three screws hidden by the inner knob.
The original front dial lamp
is very uncommon, having a very low current consumption to keep
its brightness low and I fitted an LED in its place. |
As I dragged over the
outer case to mate with the R107 I noticed it looked brand new
but the rivets holding the inner extrusion to the outer case
had mostly failed. Also with this set is the external cover..
this is certainly not new and looks like it's had a very rough
life. Surprisingly the case and receiver both have identical
serial numbers but the lid looks really battered so had it spent
its life protecting the front of the receiver?
I drilled out a selection of
the broken copper rivets and secured the inner extrusion with
6BA screws. |
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