Restoration of a WW2
Certainly looking it's age!
If all goes well I'll
start by making good any deficiencies and replace any bad components,
then get it working
Step two, I'll dismantle the
main mechanical parts and refurbish them, then reassemble it.
Finally, I'll test the receiver
and see how its performance compares with its specification.
Above is my second R1132A.
I bought this one in October 2014 and after removing it's outer
case discovered someone had swiped most of the valves. There
should be either 11 or 12 according to the circuit diagram, but
alas there were now only two. These were hidden away under a
metal shield on the right and were two examples of the ubiquitous
VR65 or Mazda SP61.
I have a very large
collection of valves and I hoped I'd be able to find enough in
my various boxes to fill the empty sockets. First I had to check
the numbers and see what the commercial equivalents were. Although
I'd got a previous model of the R1132A working in the past, it
had come with all its valves and I hadn't needed to check what
types these were. I was more than a little surprised to discover
the audio amplifier was an octode. After some rummaging around
I found a suitable valve. This is a 6A8G. I'd initially found
a small metal cased 6A8 which was rather rusty and as the top
cap lead needed bending a lot I carried on till I found the "G"
version. After an hour or so I'd found sufficient valves to get
the receiver fitted up. One valve was a problem. This was a P61,
a Mazda triode (coded by the Air Ministry as a VR66) from the
same stable as the SP61 but having its grid connected to the
base rather than the top cap. I did find a triode which I must
have had for ages, however, was it a P61? It had no metalling,
just plain glass and no number visible and, thinking very hard,
I reckon I'd experimented with it back in the 1950s when I'd
been interested in receiving VHF transmissions using super-regen
receivers. I suspect it was from the first set I'd ever built
and to make the valve work at the highest possible frequency
I'd removed the base. Later I used a twin-horned triode and then
a 955 and the P61 had been relegated to the junk box, miraculously
surviving 60 years.
I could have borrowed the P61
from my other R1132A or even bought one on Ebay, but it seemed
a good idea to restore this valve. The leads were just about
intact although pretty heavily tarnished and too short to use
without extending them.. First I had to sacrifice an SP61. I
found a tatty looking example with a wobbly base taped in place
and unsoldered its pins. This wasn't difficult. First I added
a blob of solder to the end of each pin to act as a heat reservoir
then gently pulled the glass envelope to release pairs of heated
pins from their grip on the wires. Little by little the pins
freed until the base came off. After cleaning up the pins and
checking that there was no solder obstructing the holes I turned
my attention to the old P61. It's leads needed extending so I
carefully cleaned off tarnishing and soldered short lengths of
24SWG wire to their ends after pushing short pieces of plastic
insulation down them to prevent shorting. I checked my valve
book to see the connections. MO16 was listed and, after orientating
the glass envelope carefully, located the wires connecting the
electrodes in their respective pins. The glass fitted nice and
snugly and, after soldering the wires to the pin ends, snipped
off the excess. A spot of superglue to secure the new base and
I was ready to test the valve (and to confirm it was indeed a
See the steps in restoring
the P61 below
Not exactly a common base,
this is a Mazda Octal and is different to the International Octal,
having the heater pins (1 & 8) wider apart than the IO base.
This example was taken from
an old SP61 and solder inside the pins has been removed ready
for the extented wires on the bulb to be inserted.
The base fitted; ensuring
the glass bulb is orientated correctly. Later I added more superglue
around the base and added aluminium foil.
I set up the AVO valve tester
dials and checked the restored valve. It worked. It is
a P61 and emission is pretty good.
While the valve tester
was fired up I checked the EF39s I'd lined up for the receiver.
All were fine. Note that the R1132 uses EF39 valves which are
variable mu types, somewhat similar to the 6K7 but not identical.
They are not interchangeable with non-variable mu types, although
one could use the latter in positions not connected to the AGC
line (such as the BFO).
Above you can see the
IF amplifier and audio stages. The four IF transformers have
chunky tuning slugs best twiddled (carefully) with a metal screwdriver
having a blade width of around 3mm. There wasn't much evidence
of hand capacity effects.
The primary aim is to see if
the receiver is restorable without too much effort, so I soldered
three wires to the Jones Plug. HT, LT and ground and connected
these to a power supply. I set the HT to the recommended 210
volts and checked the current consumption. It was not far from
the 50mA suggested in the manual. I plugged in a pair of headphones
to the monitor socket. Nothing, completely dead so back to the
I located the output transformer
and found a length of TV coax cable was soldered to its output.
Connecting the phones again produced nothing so I checked the
HT connection to the transformer primary. This read 199 volts
but the 6J5 anode read only 0.6 volts... odd.
I turned off the power and checked
the transformer primary. Open circuit: explaining no doubt why
the set had been ditched and its valves removed. Why had this
happened? Often a decoupling capacitor at the output valve anode
is responsible, but in this set the anode connects directly to
the transformer and nothing else so it'll have to remain a mystery.
Turning to my transformer junk box I selected a small low voltage
mains transformer (with three secondary windings) and wired it
in place of the old output transformer. It'll do to prove the
receiver works at least.
I connected a small loudspeaker
to one of the low voltage windings and turned on the power. I
heard a faint hum from the speaker... so far good. I connected
a coax cable from my signal generator to the new PL259 aerial
socket (the previous owner had removed the wartime coax socket
so I found a panel-mounting "UHF" socket which fitted
perfectly in the original hole). There was a healthy crackle
and I hadn't even turned on the generator output... very promising...
I tuned the R1132A dial to mid-way,
turned on the generator RF output set to 100mV having selected
110MHz then stepped up the frequency little by little. Suddenly
the loudspeaker burst into life.. The set was actually receiving
a 120MHz AM signal modulated by a 1KHz tone.
I twiddled the dial and discovered
the set tuned 106 to 130MHz. The coils had seemed intact but
were pulled apart explaining the shift of 6MHz from the original
spec of 100 to 124MHz. Reducing the generator output revealed
the set could just about hear 20uVolts, fading out completely
at 10uVolts. What about the image? The IF is said to be 12MHz
so tuning the set to 120MHz -24MHz =96MHz should bring in the
image. With the dial set to receive 120MHz I set the generator
to 96MHz and could only hear this when the level was 10mVolts.
So, the set is generally working.
Sensitivity needs to be improved and my guess is this can be
put right by looking at the AGC and gain control circuits. Currently
the pre-set gain control has been disconnected and who knows
how the grids and cathodes are wired. There is only a tiny change
in the S-meter reading if the input is cranked up to hundreds
of milli-volts. The AGC circuit is one of most complicated I've
ever seen so probably there's a problem somewhere, inadvertently
put in by the last experimenter...
Before I started I decided to
regularise the output transformer modification and removed the
original with the open circuit primary and substituted the replacement
which was roughly the same size and shape. I did a quick check
with a wattmeter and this seemed to indicate that the best of
the three output winding measured around 100 ohms. My junk box
speaker was marked 25 ohms, however I need to carry out further
tests once other problems are sorted out.
I started to investigate poor
gain by checking the IF stages and quickly found a screen resistor
marked 68Kohm that was difficult to measure, but I finally decided
it was more than a megohm. I fitted a new 68Kohm resistor and
the screen voltage increased to about a hundred volts or so and
the overall gain stepped up a notch. After another half an hour
I'd found that adding a couple of decoupling capacitors added
a further notch of gain. At this point I must admit to have mis-remembered
the IF value and set the signal generator from its setting of
120HMz to 6MHz. I noticed that the best response from the loudspeaker
was about 5.9MHz so started setting the slugs in the transformers
to set this response to 6MHz. Gain started to improve quite dramatically
and eventually I found the IF strip would burst into oscillation
as slugs were peaked. This receiver uses lots of metal plates
and special decoupling arrangements to minimise feedback, but
in my example is let down by valves with poor metalizing. In
fact the last IF stage uses a red EF39 with a loose base and
clearly a break between its metal coating and pin 1 of its base,
so I unplugged the valve and temporarily wrapped stranded wire
in the gap which had developed between the glass and bakelite
base, connecting the bare wire to pin 1. This immediately reduced
the unstable gain and killed the oscillation.
I then decided to move back
to tests using VHF input frequencies. The set was noticeably
much more sensititive and was responsive to inputs of only 5uVolts.
Previously an input of 20uVolts produced nothing and around 50uVolts
could be heard just comfortably. I was now able to check the
action of the beehive trimmers and discovered they were able
to be peaked on incoming test signals. A short length of wire
enabled me to hear very weak broadcasts from Bournemouth Airport,
but overall the set is far too unresponsive and audio is too
Above is the VHF front
end as it was before I started restoration. Somebody has tweaked
the tuning condensed vanes to produce less swing than the designers'
intended and pulled apart the coils to increase the maximum received
Below shows the new aerial socket
and P61 plus the oscillator voltage stabiliser.
The next step was to readjust
the IF transformers using the correct input of 12MHz. I noticed
that the set responded to 10uVolts injected at the top cap of
the SP61 mixer whereas previously it needed 10mvolts to get the
same response, this being an indication of the level of the 2nd
harmonic from my Wavetek signal generator. This time I tuned
the first IF transformer which I'd initially omitted to do as
it was awkward to reach. Both windings were off tune and resetting
them resulted in the set responding to an input of only 2uVolts
at 125MHz. As the audio level from my speaker is very low unless
I crank up the input to 500uVolts I need to determine what's
wrong. Also the signal strength meter is pretty unresponsive
until very large inputs are applied. This points to the AGC line
and possibly a poor choice of audio output transformer. As a
start I measured the resistors around the EB34 valve and found
very large discrepancies between the marked and actual values
6A8 anode load 100kohm read
169kohm and following 6J5 grid leak 330kohm read 550kohm with
6A8 anode at 185volts.
6A8 screen resistors 68kohm
and 100kohm read 79kohm and 1.2Mohm with the screen voltage at
At the EB34 various 330kohm
read 460kohm, 800kohm, 379kohm and 1.05Mohm and a 220kohm read
I haven't yet checked all the
various decoupling capacitors, many of which may be leaky. Some
are waxed types and others are mounted in metal cans fixed to
Here's a collection of components
removed from the set. Often you'll find one specific value resistor
is bad; in this case 330kohm and a couple of 68kohm, a 100kohm
and a 220kohm.
Old WW2 decoupling condensers are always
leaky although it isn't necessary to swap them all unless they're
very bad or in a critical circuit position, such as in an AGC
line where impedances are vey high and leaky condensers will
severely degrade the AGC performance.
Long ago the control adjacent
to the RF gain knob has been disconnected and various wires left
floating. I've also discovered that twiddling the BFO tuning
knob dramatically affects overall sensitivity which is a bit
odd, and I haven't found the tiny diode valve in the AGC circuit.
This is supposed to be an EA52. Was it removed or perhaps a later
After another session with the
old receiver, I changed the audio output transformer from the
12 volt mains type to one from my junk box which looked better
as it had a primary resistance of something like 1000 ohms. It
gave me more output and I'm now using a standard speaker. To
correct the deafness I changed lots of resistors and capacitors.
Each change improved the audio output by just a little bit but
all told the audio is now pretty good and loads of air traffic
signals are coming in with a length of wire as an aerial.
Below you can see the replacement
audio output transformer next to the audio filter choke. It's
carrying temporary wires to a loudspeaker and wattmeter (the
latter is what I use for peaking the output). New parts are fitted
Because this set
has loads of gain the designers' have taken a lot of trouble
to provide common earthing points at each valve to prevent unwanted
feedback, hence the strange 5-pin solder tag assemblies located
above the valves.
The next step will
be to check the IF response as now that signals are reasonable
I can detect some humpiness when tuning across them. At the low
end of the band I can hear extremely strong (and very distorted)
FM broadcast stations and there seems to be a lot of spurious
signals across the whole band including what sounds like an analogue
video signal somewhere around 120MHz. Sometime I need to add
some aluminium foil around the repaired P61 because it has no
As far as sensitivity is concerned
I can now comfortably tune a 1.5microvolt signal. The S-meter
is a 5mA milliammeter monitoring the anode current of V5, a variable-mu
EF39. The AGC applies a negative bias to its grid which reduces
the anode current. The design caters for really strong signals
viz. No signal =4.7mA; 20uV =4.5mA; 100uV =3.4mA; 1mV =2.9mA;
10mV = 2mA and 100mV = 1.4mA. I'll check the actual readings
next time I turn on the receiver.
As the output circuitry has
all been disconnected I'd like to re-establish this but, because
the replacement audio output transformer has only a single output
winding I'll either need to add a virtual earth centre-tap or
leave the output floating. I'll also check the BFO which I think
is working OK. I re-established the output circuit but the existing
low frequency choke attenuated the audio so much I shorted it
out. Now the attenuator works OK and the audio can reach 200mW.
The disconnected preset gain control was really redundant so
I connected it up as a tone control.
After looking fruitlessly for
V12 I suspect this example of receiver doesn't use such a valve
and never has. I suspect the diodes in the EB34 are used an AM
detector and AGC rectifier and V12 maybe was a modification wired
as a noise limiter?
Basically, I'm now happy with
the performance of the R1132A. It's not rebuilt as original but
it now makes a useful air-band receiver. I guess the next step
is to tidy up the wiring and then detach the front panel, repaint
it and add new markings for the controls.