This survivor from 1936
has arrived here to be put back in service. The set has been
stored in damp conditions so the case is in poor shape and there's
a good smattering of rust inside, but it's complete and I see
no reason that it cannot be sorted out.
The first thing you'll
spot is the 120 volt battery which is probably over 50 years
old.. I'll attempt to date it later (note.. it's post WW2). The
set uses British valves and remarkably these are complete with
intact metallising. This is important as there will be lots of
gain and this may result in instabilty if screening is poor.
Besides the HT battery there would have been a 2-volt accumulator
but examination of the circuit reveals there was no grid bias
battery. Instead the HT negative feed establishes bias for the
valves. This is pretty important because, without negative grid
bias, the valves would draw lots of HT current making the set
expensive to run.
Above you can see tell-tale
marks from a woodworm infestation. Apparently they've taken a
particular liking to the speaker cloth, but in fact this means
that interior woodwork has probably suffered. From the colour
of the dust inside the case I think the things have long gone,
but I'll need to paint on a woodworm treatment fluid as a precaution.
Most battery operated sets had
no dial illumination so there's a nice clear dial with markings
on paper. Many receivers from this era and to the end of valve
sets used luminous dial markings or clever light-guides to illuminate
station information. These needed dial lamps which would consume
as much power in a battery set as the valves themselves hence
doubling the trips to the local shop for recharging the accumulator.
Initially I thought the dial had staining but once removed from
the chassis I could see it was a strange semi-reflective finish.
One can date an early receiver
by studying the station names marked on the dial. Click
to see this page on dating a set.
Before I could remove
the chassis from the cabinet I had to detach the four knobs.
This is frequently a difficult proposition and in this case it
was especially tricky. There are no grub screws, instead the
knobs are held in place with internal steel springs which had
rusted in place. Two spindles are steel and two are brass making
removal of the former extra difficult. There are a few ways of
removing stubborn knobs. I tried a method wrapping string around
the back and pulling which failed to budge them. I have a pair
of pliers with hooked jaws and managed to get these behind one
of the knobs. After heaving on this I heard a crack and the thing
slid off. The other three knobs were pushed in place leaving
too little space for the pliers, but I removed the four chassis
securing bolts and this allowed the chassis to be pushed forwards
leaving just enough space for the pliers. To help deal with the
rust I'd dripped pentrating oil onto the shafts and this helped
to get the other three loose without damage.
Once the chassis was ready for
removal I unsoldered the speaker leads after marking these because
oddly there are six separate wires.
In the picture above you
can see the dial cord on the right. Instead of the usual string
it looks like pyjama cord or old fashioned bootlace. No chance
of this breaking.. which is a good thing because the tuning condenser
has seized solid and if it had been standard dial cord it would
The dial is interesting
because it uses two pointers. The upper one is a standard pointer
coupled to a slow motion drive made from a pair of brass gears
and the second a pointer screwed to the tuning condenser shaft.
The latter rotates through only 90 degrees so one side carries
medium wave and the other long wave markings (these are wavelengths
in metres as is usual in the UK). You will just be able to discern
in red in the inner long wave scale the marking "Aircraft".
Curious listeners to long waves might appreciate that these wavelengths
are still in use today where short range MCW broadcasts identify
airfields. For example, our local airport in Christchurch, which
is named "Bournemouth" has a transmitter on 339KHz
or 885 metres sending the callsign "BIA". No doubt
standing for the grandiose title "Bournemouth International
This receiver uses old
British-based valves, and there's a good selection. From left
to right...the RF amplifier and frequency changer/oscillator
is a relatively rare TP22, a triode pentode with a 9-pin base.
The IF amplifier, a B7-based VP210 is a variable mu pentode,
the audio amplifier, detector and AGC rectifier is an HL21/DD
using onlt a B5 base and the audio output valve, a B7-based QP230
which is a slightly exotic double pentode for push-pull operation.
All have 2 volt filaments consuming a total of around 750mA.
Anode currents for these valves are pretty low and in total would
only drain 5 to 10mA from the HT battery depending on the strength
of the tuned broadcast and audio output setting.
The tuning condenser has
three ganged sections, usually indicating an RF amplifier stage
in front of the mixer, and there are two sets of RF coils, not
because the set has a separate RF amplifier, but it has bandpass
circuits aimed at providing decent reception of distant stations
in the presence of strong local broadcasts. Also the low IF of
only around 125KHz demands fairly sharp tuning to reduce image
reception. Tuning condensers like the example above have a section
which is adjustable so that perfectly tracked RF-Oscillator ganging
can be maintained across the tuning range otherwise too much
of an image may be leaked through resulting in a beat whistle
spoiling reception. In this example you can see the nearest section
has its tuning capacity reduced by the bending of sections of
the aluminium vane.
The first job is to sort
out the tuning mechanism. Turning the spindle fails to turn the
dial pointer because a number of the parts are seized due to
rust. Initially I tried to lubricate the moving parts without
dismantling everything. The mechanism is overly complicated and
looking between the tuning condenser and the rear of the dial
revealed something amiss and it was only by removing the dial
that I could examine the problem. You can see the remains of
a flexible coupler fastened to the tuning drum.
The other part of the
coupler is fitted to the dial pointer gears via a collar held
in place by a grub screw. The design of the old coupler allows
for variations in the fitting of the dial and gears.
Looking around for a part
suitable for making a new coupler I found an old solder reel
that had the correct diameter... Below the reel drilled and held
in place on the tuning drum and then fitted with the second part.
The new coupler isn't
very flexible so I drilled out the fixing holes to enable it
to be accurately lined up with the front panel holding the dial.
Once the driving hole was central I tightened the fixing screws.
The tuning spindle
pulley was slipping because the rubber washer was hardened and
worn to a shiny finish so I found a couple of rubber VCR pulleys
which will provide sufficient friction for the drive belt.
The re-sleeved tuning
To centralise the tuning
mechanism the tuning condenser fixing screws need to be slackened
then tightened when the dial had been fitted.
The gearwheels have an
The first components I checked
were the electrolytic capacitors. The replacement Radiospares
is marked 8uF (HT smoothing, C19) and checked out as pretty well
perfect but the original 50uF (grid bias smoothing, C20) was
open circuit. Both of these may have been important if the receiver
was used with a battery
eliminator to reduce hum. Click
the link to see a collection of these.
The next problem, after
the tuning mechanism fault, was the push-pull driver transformer.
This had an open circuit in its output winding. Oddly the primary
winding which is fed by a capacitor that invariably would be
expected to have leaked HT current and damaged the winding, was
OK, measuring the correct value of 700 ohms, but one of the two
centre-tapped secondary windings was open.circuit. This would
have resulted in low audio volume. As the secondary winding is
closest to the transformer core and its the inner winding that's
failed the transformer is u/s. I need to find a replacement having
a centre-tapped secondary winding measuring 2,500-0-2,500 ohms
and a primary of 700 ohms. The ratio might be anything from 2:1
to 5:1. I could measure this perhaps because half the winding
I measured the input at the
primary winding as 5 volts 1KHz with 15 volts at the centre tap
to the good end of the secondary.. so the transformer is a 3:1
or 1:3 whichever way you want to express the gain. What about
a replacement? Back in 1936 it wouldn't have been a problem,
but over 80 years later the choice is quite restricted. There
are basically two avenues.. one is to source a close match to
an expensive specialist transformer and the other to use a small
mains transformer. Looking at the parameters. First the construction
of the transformer. Most interstage transformers used extremely
thin wire (in fact the size of the wire is likely to be the reason
for failure as it can fuse open circuit if a problem occurs such
as a bad condenser or even damp conditions followed by corrosion)
and can accommodate a huge number of turns resulting in a high
inductance. The end result is a DC resistance and an AC impedance
for the primary and secondary windings.
Starting with the primary winding.
Once this has been determined the secondary winding will automatically
be specified once the step up ratio is set. For a 3:1 step up
the secondary will have three times the number of turns as the
primary. Its resistance and impedance will then be decided by
the size of the wire.
Looking at this specific receiver
circuit you'll see an 0.1uF blocking condenser in series with
the primary winding so no DC current will flow. At this point
we should consider this condenser as a source of a problem because
it needs to be completely leakage free and that is a tall order
for an 80 year old condenser so it must be replaced, but maybe
not because the primary winding is OK. Now the secondary winding
connections... these connect directly to the grids of the QP230
with the centre tap connected via 100Kohm to HT negative. Looking
at the circuit diagram you'll notice there is no grid bias battery,
but instead a resistor chain which will pass HT current. Within
a very short time, once the filaments are heated, each valve
will establish anode (and screen current if applicable). The
current will flow through the bias chain and very soon will settle
to a steady state value determined by the HT current together
with the bias applicable to each valve. What in fact happens
is a surge in HT current which lasts for only a short duration
with the amount of current flowing mainly determined by the zero
bias characteristics of each valve.
The most important valve as
far as the interstage transformer is concerned is the QP230.
With zero bias the valve's pair of anode currents can total 50mA.
The bias chain equals 920 ohm so assuming say an instantaneous
HT current of 55mA the bias applied to the QP230 will be about
minus 50 volts. Now the QP230 will be cut off. The current flow
through the bias chain drops and the bias reduces to say minus
8 volts resulting in around 3mA of anode current. The end result
or steady state HT current will eventually in short order stabilise
at say 6mA. During this phase I wonder if the interstage transformer
is stressed? I guess if there's a strong crackle from a dirty
switch there may be a large voltage swing and conceivably this
might encourage the QP230 to draw grid current, but the grid
feed is via a 100Kohm resistor which would limit its effects?
All told, I think the transformer just failed from corrosion
in its copper windings brought on by damp conditions, as evidenced
by the rusty chassis.
The old transformer is said
to have a primary DC resistance of 700 ohms and across the whole
secondary winding 5 Kohms. Assuming a ratio of 1:3 it looks like
the secondary is wound from thinner wire than the primary winding
having twice the resistance of the primary wire. The metal core
is not very large in cross section being about 20% the area of
a 50 Hz transformer core so I initially thought the inductances
of the windings may not be especially high, but of course depends
on how much wire is used (see below for
I put a 32nF capacitor across
the primary winding and measured the resulting resonant frequency,
repeating for the good half of the secondary. I found the primary
resonance was 130Hz and the secondary 50Hz, giving me 47H and
316H respectively. Allowing say 1nF for winding self-capacity
these figures drop to 35H and 214H. These seem pretty high so
I then checked the windings in series with the same capacitor,
and ignoring the windings self capacity, I got 44H for the primary
and 390H for the half-secondary which are in the same ball park.
No doubt, if I allowed for the self capacitances of the windings
the final numbers would probably be closer. In fact it's relatively
easy to measure self capacitance of coils using a square wave
and oscilloscope. Having got a rough idea of the winding inductances
one could calculate their impedances although the results will
vary significantly depending on the value of their self capacitance.
Trying a few numbers an impedance of at least 300 Kohm would
result for the primary winding impedance.
There are some transformers
that might be suitable for the interstage transformer but I'm
not sure about their winding resistances or their winding inductances
so I tried an experiment using three small 1.5VA transformers
designed for 440 volt to twin 12 volt windings. These have a
10Kohm primary winding.
Feeding in 5 volts RMS at 1KHz
gave me about 225mV output across the two 12 v windings in series
which I connected to a 12 volt winding of a second transformer.
This resulted in about 7 volts output. Not a gain of three but
at least a possible decent match to the valves.
Next I tried using a small 230
volt to 2x 20v transformer to drive the two 440 volt and this
should produce a voltage gain... this worked OK giving me a gain
of between 2 and 3 and the resistance of the primary winding
being only about 1Kohm compared with 10Kohm for the 440v transformer
means its a much closer match to the original, so I will need
to test the transformers in the receiver and see how this compares
with bench tests.
Below, the output compared
with the input showing a step up of two and below this a picture
of the two outputs with a 90 degree difference in phase. The
input resistance is about 1,000 ohms and the output 10Kohm +
Views of the new interstage
transformer mounted adjacent to the QP230 socket.
Above.. I treated the
chassis with a rust remover which left the patina....
Now, the output transformer.
this is just as old as the interstage transformer and has a centre-tapped
primary winding totalling 750 ohms and a secondary DC resistance
of 0.2 ohm (see below for a picture).
Sure enough, both primary windings
were open circuit. Thinking about why this should be so, I remembered
the electrolytic condenser wired across the bias chain. This
is measuring open circuit, but if it were to have failed initially
showing a serious leak the QP230 would be zero-biased and draw
loads of current. This would result in a transformer dissipation
of about 2 watts if 50mA was drawn through the 750 ohm winding.
I'll try a 115v-0-115v mains
transformer in its place.
The loudspeaker is a Rola marked
as having 8 ohm impedance. The QP230 output valve has an anode-to-anode
output impedance of 16 to 18 Kohm so the output transformer needs
a step down ratio of something short of 2,000:1 and of course
it needs to be centre-tapped for the HT feed. A typical 6V6 output
load is around 5 Kohm (ratio of 625:1) suggesting the QP230 output
transformer has a lot more turns and therefore a higher resistance
than typically found making it more susceptible to accidental
If I was faint-hearted I'd have
ditched this receiver as too hard to fix and it was even more
likely when I'd removed the duff output transformer from the
loudspeaker and discovered the loudspeaker was also open circuit.
I was interested in tracing the fault so dismantled the thing.
It was clear that some jigs are needed to assemble the cone and
the magnet assembly because of the tiny air gap between the cone
and metalwork. There was some corrosion in the wire from the
cone to the coil, but I cleaned some to bare copper and found
the fault to be in the coil itself. This has two layers of thin
enamelled copper wire and there must be a break in the inner
layer so at this point I decided a cheap replacement permanent
magnet speaker will have to do.
Possibly the output transformer
had developed a short between primary and secondary and placed
a high voltage on the speaker coil?
I tried a mains transformer
with twin 115 volt inputs and 12 volts output... not a very good
match in theory but I'll see if it works. I mounted the new output
transformer on a metal strip and fixed it between the tuning
condenser and the QP230 using existing chassis holes and wired
it up. Next, I removed the audio coupling condenser and tested
it for leakage. With 200 volts across the condenser in series
with a 100Kohm resistor, I found about 190 volts across the resistor.
If the condenser was good this voltage would have been close
to zero. I also changed the bias decoupling condenser marked
50uF at 12 volts working whose positive terminal connects to
chassis. I fitted 100uF at 63 volt working.
I then connected a variable
HT supply to the HT leads and checked the total HT leakage. This
was about 1mA at 120 volts. I also tested the bias supply and
found it changed quite a bit as the HT was varied, settling to
minus 6.8 volts with the HT at 120 volts.
I checked the valve filaments
and all were intact so it's still a goer... Next was to replace
the valves, connect up a loudspeaker and connect the low tension
for the valve filaments. These consume about 700mA so I set my
PSU to 2 volts and set the current limit at 500mA then carefully
increased this whilst watching the voltage. At something under
700mA the voltage stabilised at exactly 2 volts. Then I connected
a variable HT supply whilst monitoring the current, stopping
at 120 volts with the current at around 10mA.
With the receiver of this vintage it's
possible to check the output stage simply by touching the top
cap of the triode amplifier. This produced a strong hum in the
speaker (proving the new transformers are servicable). Note that
just because a valve carries a top cap doesn't necessarily mean
that this is the control grid connection. In fact the VP210 (below)
has its anode connected to the top cap. I'd temporarily fitted
a pair of knobs to the tuner and wavechange switch but adjusting
either didn't have much effect on the faint hiss/hum from the
speaker. Touching a long wire aerial to the front end resulted
in only faint crackling noises so I connected a signal generator
to look for the IF. After some experimentation I found I could
hear a signal at 110KHz and to hear this the signal generator
was running 1000mV.
This is the first IF transformer
which follows the TP22 frequency changer and is designed to run
at precisely 128.5KHz.
The two rusty screws had been
fully tightened. These are compression trimmers and their excessive
capacity is the reason for the 110KHz response. By unscrewing
these, together with those on the second transformer I was able
to shift the receiver response to the correct IF of 128.5KHz,
and was able to reduce the generator output to something like
20mV from its original 1000mV. In doing this I found the automatic
volume control was doing a good job of maintaining a constant
It became apparent that the
anode coil in each transformer peaked nicely but neither of the
two grid coils tuned. Looking at the circuit diagram there are
a few critical components that might explain this. One is the
decoupling condenser C5 which provides a return path to ground
from both IF amplifier grid coils.
Having proved the receiver was
essentially working I tried to hear a test signal on either the
long or medium waveband. It became obvious that the wavechange
switch needs cleaning but in a working position I was able to
hear frequencies from about 550KHz up to about 1450KHz but nothing
at all on the long wave setting so I set the generator to 828KHz
corresponding to a strong medium wave broadcaster and found I
could faintly hear it but only with a long wire connected to
the top of the input coil adjacent to the right of the dial.
Tuning across the station told me the IF response was very lumpy
indicating the same tuning problem as when adjusting the IF tuning.
The very poor sensitivity can
be due to a variety of things... chief culprits being the various
decoupling condensers C4, C10, C11 and two RF couping condensers
C12 and C13. There's also the tuning condenser trimmers... these
may have been screwed tight like those on top of the IF transformers?
That might explain why the highest frequency I could hear was
1450KHz. The dial shows Bournemouth which was close to 200 meters
in 1936. 200 meters corresponds to 1500KHz and usually sets tuned
slightly higher than this. Another reason the waveband is too
narrow is the oscillator padder condenser which is marked as
1,081pF. If this has gone down in capacity, even by a small amount,
the tuning dial range would be reduced. Why no long wave reception?
Probably the wavechange switch which is an extremely simple affair
carrying only three single pole switches.. Preselector, RF tuning
I fitted new capacitors in place
of C4, C5, C10, C11, C12, C16 and C17.... C5 (0.1uF) actually
measured 15uF and the others about double their marked values,
but connecting them to 200 volts via 100Kohm resistor showed
all were leaky with between 70 and 160 volts across the resistor
(meaning all had leaks of around 2mA. This would have resulted
in low valve screen voltages and also reduced signal strengths.
C16 and C17 across the output transformer would reduce the potential
audio output. After re-testing the receiver and re-aligning the
IF to 128.5KHz, not much had changed except I could now hear
a very weak Radio 4 on 198KHz so thankfully the oscillator is
working OK on medium and long waves.
It's also possible that one
or more valves needs replacing, however, tweaking the LT up to
2.2 volts does not alter the audio at the loudspeaker which tends
to indicate their emissions are satisfactory..
Above are the faulty transformers..
upper left is the interstage transformer and upper right the
output transformer. Ordinarily these would be awkward to replace,
but because they are both push-pull transformers it's impossible
to obtain a replacement from their era.
Opposite is a selection of the
leaky condensers replaced by modern equivalents.
Worryingly the IF secondaries are
still not tuning and the front end seems also to be completely
flat with no change in signal strengths when adjusting the tuning
condenser trimmers. Clearly there are at least two faults remaining,
although feeding 20mV into the TH22 I see 200mV at the detector
diode of the HL21DD socket and tuning to 198KHz I see a gain
of 200 which is presumably coming from the frequency changer
and the IF amplifier. It looks like I'm going to have to make
some resistance checks of the coils and then remove the IF cans
and inspect the circuitry.
It was easy to detect
the fault. With the can screwed to the chassis my meter measured
42 ohms across the anode winding and 43Mohm across the grid winding.
Each complete can and transformer
was easy to remove as only two 4BA nuts hold them in place and
the four connecting wires, like most in this set, are only touch-soldered
in place. However it was difficult to remove the coil assembly
from the can because the metal bar at the bottom of the coil
former is bent by machine after passing through slots in the
aluminium can. The two slots needed to be cut and their edges
bent back then two nuts removed from the top where the trimmer
assembly is fitted.
In the centre of the ceramic
top there's a screw sealed by resin. The screw holds the trimmer
assembly to a wooden plug which was jammed into the coil former.
Over the years the wooden plug shrunk allowing the trimmer assembly
to turn independently of the coil former.
I suspect vibration from the
loudspeaker produced sufficient movement of the coil former for
the black connecting wires to rub against the coils.
Here's a view of the break.
In fact it was a design fault.. although I can't say exactly
when the radio stopped working but the problem was slowly moving
to the point where an IF coil went open circuit. Interestingly,
one of the can securing nuts was missing.. was this an echo of
a previous repairer who'd given up?
The IF coils comprise two tuned
windings inside the cans and separated by a few centimetres .
These transformers were never intended to be repaired after manufacture
because the fixing screws are bent through the sides of aluminium
cans and the cans have to be cut to remove the coil assemblies..
you'll note the plural...
Both transformers had
an identical fault (same wire.. same position), hence my diagnosis
as a design flaw. There are four cotton covered wires stretched
between the trimmers at the top and the pins at the bottom. These
were in contact with the coils and at that point you could see
green verdigris where friction between the cotton and the coil
had removed the coil insulation. At the crossing points of the
thin wires and the connecting wires a small cardboard strip was
glued in place, but in a few instances the connecting wire was
outside the protection of the strip and pressing on the coil.
The coils are made from single stranded wire rather than better
quality Litz wire so were easy to fix... it was the outer turns
on each coil that had fractured so no real problem making the
repairs.. I was careful to bend the black connecting wires away
from the coils before putting back in their cans.
Here you can see the simple
Note the dark discolouration
on both coils. This is transference of pigment from the black
wire as well as some verdigris. It's spread over a couple of
millimetres because the coil former is held loosely to the fixed
trimmer assembly to which four black wires are connected and
any vibration would have wobbled the coil former but not the
can itself. The fixed trimmer assembly wires then rubbed on the
outer surface of the coils. Any damp in the air would be absorbed
by the cotton covering then, once bare copper was exposed from
friction, verdigris was produced resulting in eventual fracture
of the thin wire.
In the second transformer,
the paper strip did not quite cover the coil and again wear followed
by damp and corrosion resulted in the break.
To avoid a repeat of the problem
I superglued the coil former to the trimmer assembly. The junction
between the two is via a wooden plug in the top of the coil former.
The plug had shrunk slightly so the coil former was loose.
I applied superglue around the
circumference of the wooden plug on each transformer then bent
the black wires away from the coils.
When I'd finished all four coils
had much the same DC resistance of about 42 ohms.
After re-fitting the two
IF transformers the radio turned from deaf to very noisy. I re-aligned
the IF to 128.5KHz and this time all four trimmers were able
to peak the signal and attaching a long wire aerial brought in
dozens of broadcasts. These sets are tricky to align because
of their tendency to burst into oscillation and I found the trimmers
on the variable condenser need looking at as their springs which
open and reduce the capacity are too weak and need bending outwards.
Next, I'll set aside the
chassis and take a look at the cabinet. It needs a woodworm treatment
first. Here's a set of pictures taken as work proceeded.
When you're restoring
the cabinet of an old radio it's not usually a good idea to strip
off the old finish. An old radio like other "antiques"
needs to retain it's patina, When faced with hundreds of woodworm
holes you have a problem but although I think the blighters have
long gone I still brushed on lots of woodworm treatment fluid.
This set isn't too bad and if you want
to see a worse case look here and scroll down to the Model
100 phone, then see it's refurbishment.
The only difficulty I met with
this cabinet was removing the old speaker cloth. This was glued
to the rear surface of the cabinet front and on top of the cloth
was the speaker baffle. This was not only secured by six completely
rusted screws, but wedged in place by two side cheeks each held
in place with four more rusty screws.
I eventually removed 14 screws,
the baffle and the cloth. A couple of the screws sheared off
and one had to be chiselled out.
I have a box of various speaker
cloths so I'll pick out the best looking after I've finished
work the cabinet exterior.
The panels on this cabinet
are in fair condition with only the top
almost devoid of paint, a not uncommon condition because owners
often plonked a large plant on the top and then over-watered
it so the plant pot overflowed destroying the finish. I rubbed
the panels down with 800 grade emery cloth and oil then brushed
on a wax paint. I tried filling some of the woodworm holes but
it was a waste of time because they remained visible.
Below.. methods of fixing back
loose veneer and re-gluing a loose top and then after preparation
Originally the cloth was
stuck to the rear of the front panel but it's easier to stick
it to the speaker baffle then adjust the tension using staples.
The glass clips in place.
Shortly the chassis will be
fitted but first I'll complete testing and replacement of old
power supply wiring.
Now, back to the receiver chassis.
I'd noticed a few minor problems during initial work... the tuning
condenser preset weren't working properly and there was an intermittent
fault which reduced the audio output whilst drawing an extra
25mA or so of HT current. The top of the tuning condenser had
some patches of rust and the trimmer washers were heavily corroded
so I removed the parts.
Removing the adjusting nuts and
the phospor bronze strips revealed a problem. Corrosion was preventing
sufficiently low resistance contact for the trimmers to operate.
Once the rust had been cleaned away
and new washers fitted all three trimmers worked perfectly allowing
alignment of the medium waveband. These trimmers are adjusted
once the dial has been set to 214 meters. The coil inductances
are designed such that once medium waves are aligned using the
trimmers and the long wave oscillator trimmer adjusted correctly
both medium wave and long wave alignment is excellent.
Before final alignment
of medium and long waves was completed I set the IF amplifier
to exactly 128.5KHz. The official alignment details advise adjusting
the four trimmers on the IF coils for maximum audio output, but
I found a double hump when tuning across Radio 4 Long Wave so
I used the spectrum analyser and found the response as initially
set up was pretty poor. I believe this is due to the action of
the automatic volume control circuit which is very sensitive
and results in distortion of the IF response. However, using
the spectrum analyser and after twiddling the trimmers several
times the IF ampifier suddenly took on a decent shape with a
single tuning peak and excellent skirts... below. The auto setting
was used and this resulted in odd cursor settings. The response
is better than +/- 13.5KHz @ -50dB or +/- 5KHz @ -10dB. The 3dB
bandwidth looks about 7KHz which is a trifle low for high fidelity
but ideal for night-time listening on a noisy and crowded medium
During alignment the intermittent
fault got really annoying. I traced this to the output valve
which drew an extra 25mA HT if tapped gently. I substituted a
QP22B valve in place of the QP230 and the intermittent magically
disappeared. The total HT current read about 10mA at 120 volts
and LT 675mA at exactly 2.0 volts. With a long wire aerial the
medium and long wavebands are now filled with stations. Time
to return the receiver to its refurbished cabinet and consider
a suitable power supply.
As HT batteries and accumulators
are not readily available the best bet for powering this old
receiver will be a mains power supply. Back in the 1930s it was
not uncommon to buy a battery eliminator to use in place of an
HT battery and sometimes it was possible to also replace the
accumulator but this would be a relatively rare option because
of annoying hum from inadequate smoothing. Nowadays it's possible
to generate a 2 volt DC supply with no hum. What isn't common
these days is the availability of a mains transformer suitable
for directly producing a 120 volt HT voltage so one must use
a voltage multiplier. Conveniently, transformers salvaged from
equipments requiring several voltages may be at hand in most
electronic workshops, and with their secondaries wired in series
will produce, using a voltage doubler, a voltage suitable for
the HT for this sort of receiver. The peak DC output from the
transformer shown below will be root 2 times (27 + 14 + 12) volts
= 74 volts. Using a simple voltage doubler this will around 148
volts. This can be fed through a choke salvaged from a switching
power and result in an HT voltage under a load drawing say 10mA
of about 130 volts.
The filament supply can be developed
using a second transformer having a rating of say 10VA. I selected
a 6 volt transformer whose output is rectified by a full wave
bridge and fed to an adjustable voltage regulator set to exactly
2 volts by the 500 ohm pot. The 750mA drawn by the receiver poses
no problem currentwise and the PSU delivers a rock steady 2 volts
with no ripple. The 317T regulator has a small heatsink to limit
The power ON LED is in series
with a 12Kohm resistor. This also serves two other purposes..
to draw some current to limit the HT voltage and to drain any
voltage when the PSU is turned off. The LT output is prevented
from rising too high by the inclusion of a 3.3 volt zener diode
which will blow the LT fuse if for any reason the voltage rises
too high. I fitted a 2 amp fuse in the LT circuit and a 100mA
fuse in the HT circuit.
The outputs are carried
by sockets for the HT and screws for the accumulator spade terminals.
To the right of the terminals you can see the (ten turn) adjusting
screw for the LT voltage. Note that the McMichael requires an
isolated HT negative feed to develop correct bias voltages for
I tested the completed
receiver using the new PSU and found it was so sensitive it picked
up stations without an aerial.
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