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Vintage Radio
HMV 42-71 dual-wave superhet
receiver
By Marcus Chick
This radio by His Master’s Voice was made in Australia from 1954 to
1959, using HMV’s type 42 chassis. It’s a mostly standard mains-powered
set with MW and SW reception, but a few surprises are hiding within.
T
his set came to me due to an
acquaintance downsizing and
moving into a retirement village. It was
described to me as a 6V radio. However, on collecting it, it was apparent
that it was mains-powered.
Editor’s note: this set was previously
reviewed by Rodney Champness in the
August 2003 issue (siliconchip.au/
Article/5648). You can read that article for a more detailed breakdown of
the model 42-71’s circuit.
The model 42-71 is a typical Bakelite table radio of the day, featuring
the then-recently-introduced miniature valves. For a multi-band set, it
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is unusual as this set is in the higher
price range. While it is designed as
what the Americans called “farm
radios”, it does not have the usual
RF amplification valve preceding the
frequency-changer valve. Nonetheless,
it is an attractive piece.
Not uncommon for the era, the set
shared its chassis with other cabinet
shapes to become ‘different models’.
After all, why reinvent the wheel? In
this case, the chassis is type number
42. You can get the service manual
for that chassis from Kevin Chant’s
website at www.kevinchant.com/
hmv3.html
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Uniquely to HMV, it uses 457.5kHz
for its intermediate frequency (IF),
whereas most in that era used 455kHz.
There was sound logic behind that.
The objective was to ensure that none
of the sub-multiple frequencies produced by the oscillator fell on a radio
station, especially the one you were
trying to listen to.
There were plenty of national and
international radio broadcasters back
then. To accommodate that, there were
three shortwave (SW) bands because
changes in the time of day, sunspot
activity and weather all conspired to
render some bands inoperable.
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SW1 covered 14.2 to 18.4MHz, SW2
covered 24.79 to 31.92MHz and SW3
covered 5.9 to 7.5MHz. SW3 is ‘band
spread’, so fewer station frequencies are over a larger area of the dial.
Then we have the broadcast band of
540–1600kHz, which had its station
spacing reduced from 10kHz to 9kHz
in 1978. That is why only 3SR and
2AY are anywhere near their original
positions.
Circuit details
Fig.1 shows the set’s circuit, which
follows the general plan for a superheterodyne radio of the day. V1 (6AN7),
the ‘frequency changer’, is actually two
valves in the same envelope. This was a
88
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step forward as the triode in the 6AN7
complements its hexode by becoming
a separate exciter for the oscillator.
Valves of this type were used in
shortwave sets as they provided better modulation (and thus better performance) at higher frequencies.
Restoration
A cable with a mains plug is no guarantee of its actual operating voltage, so
I needed to assess it first.
I never plug a set in to see if it works;
I cannot viably repair some sets when
they self-destruct after being plugged
in like this. Many of these sets were
abandoned after they broke down,
and as we who fix know all too well,
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certain bits deteriorate when the set is
not used for a long time.
I concluded that the incorrect
description came from the fact that
the visible valves have sixes written
on them. In other words, the heaters
ran from 6V, not the whole set.
The set only had one knob attached,
which was not entirely unusual, as
they were the long-shanked plastic
types with a clamp. Most of those were
bad news. Rigid plastic and movement
were never a successful combination,
and that plastic does decompose and
go brittle.
Under the dirt, there appeared to
be an almost-mint Bakelite cabinet.
So, after separating it from the chassis
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◀ Fig.1: the circuit diagram for the
HMV 42-71. Its most notable features
are the multiple wafer switches for
selecting between the MW, SW1, SW2
and SW3 bands and the somewhat
unusual 457.5kHz intermediate
frequency. Unlike some similar sets,
this one lacks an RF amplification
stage. The chassis diagram for the set
is shown at lower left.
and giving it a shower to get the dust
off, I decided it was in better condition than I first thought. The chassis
(which had not been showered) was
also in reasonable condition.
I took a good look at the chassis and
noticed several waxed paper capacitors as well as obvious heat damage to
the transformer wires. There were also
three aged Ducon electrolytic caps,
plus the mains cable didn’t seem to be
in great condition. There was no way I
was going to power it up in this state.
General restoration advice
I have no tolerance for wax paper
and some oil-filled caps; they inevitably become electrically leaky. I do not
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bother testing wax paper caps; finding
one good one in probably five hundred
is not efficient, so I replaced them all.
My “Honor” (Lafayette) RC Tester
manual quotes a non-polarised capacitor with a leakage resistance below
50MW (at valve working voltages)
as unsuitable for screen decoupling,
and less than 200MW unsuitable for
coupling. Therefore, I will not tolerate a leaky non-polarised capacitor,
regardless of whether it can be made
to function.
Consider that in a set like this, the
grids will draw next to no current. So
any positive voltage applied to the
control grid will destroy its bias and
can, or will, damage the valve. Even if
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it doesn’t, it will ruin its performance.
I usually touch a battery across
the output transformer’s primary to
ensure it is working and then perform
resistance checks on primaries and
secondaries. That eliminates a lot of
time-wasting and potentially damaging rework later.
Most Australian mica caps from
the late thirties are reliable, and they
should not touched, unless one end
is out of the circuit. If one of its wires
is out of circuit, I perform an insulation test to ensure there is no leakage.
I absolutely do not perform this test
on any capacitors that are across tuning coils etc (the coils and gang are a
matched set). Since the mica capacitors are installed during manufacture,
tampering with them is liable to cause
the set to not work properly.
The coils and gang are a matched set.
They have mica caps installed during
manufacturing to meet their specifications and tampering with them can
have catastrophic results.
Amazingly, there was one early
modern type capacitor in this set. It
stayed while the rest went. I was surprised to find no resistors worth changing as I went through the caps. As the
transformer wires had succumbed to
the heat of the rectifier and output
valves, I cut the wires short and spliced
in new lengths.
Adding some heat shielding
I cut out a piece of spare sheet metal
from some shed doors to make a heat
shield to fit between the 6N8, 6M5 &
6V4 valves and the transformer, protecting the new transformer wires. You
can see it in place in the photo of the
rear of the chassis.
The mains cable had also been
affected by the heat from the rectifier,
so I took the opportunity to cut off
the supply wires to the gramophone
socket and, as there was room, reroute
the new Earthed mains cable along the
side of the chassis with clamps (you
can also see this new arrangement in
that chassis photo).
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Silicon Chip
Australia's electronics magazine
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These three photos show the HMV 42-71 set after cleaning it, replacing all the old paper capacitors and some of the
resistors. The mains cable was also replaced, as the previous one was too degraded, and lastly you can see the heat shield
behind the 6N8, 6M5 & 6V4 valves in the photo at lower left. The P-clamp that holds the mains cable is too large, and uses
a ‘not-to-standard’ cable tie to prevent it from pulling out. Best practice would be to use an appropriately-sized P-clamp to
hold the cable securely.
This was also the point that I
decided that I needed to find out what
model the set actually was, to ascertain
the IF. As mentioned earlier, it is actually 457.5kHz, rather than the common
455kHz of most sets of that era.
I noted that the circuit required R11,
R12 & R13 to be three 10kW resistors
in parallel. Interestingly, this set only
had two. I left it as was. After completing a refit, I attached an analog meter
across the B supply to monitor it and
powered up the set via an isolation
transformer with a kill switch.
I also have neon lamps in bezels to
the primary and secondary of the isolation transformer so I can quickly see
if voltage is present, along with fuses
on both primary and secondary to protect the transformer itself.
Powering it up
The start-up was perplexing. There
were no dial lights; the heaters were
glowing, but there was no HT. Blown
dial lights are typical, so I fitted new
globes. There was power on the rectifier, but nothing coming out of it.
I hunted down another 6V4, as my
“Knight” tester will not check a 6V4.
With the new valve, I got a reading
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on the B voltage, although it was low.
That was obviously due to the missing
10kW resistor, so I replaced the two in
the set with three new ones in parallel.
Calibration
It quickly became clear that someone had been playing with all the
adjusting screws.
Initially, the calibration did not go
well; a symphony orchestra of various noises was getting in via the 36m
antenna. A new G10 LED light was
sending out a mass of RFI. Like the
other noisy LED lights I’ve purchased,
I returned it for a refund. That is why
I use a halogen desk light. The computer’s UPS was also chipping into the
EMI cacophony.
Clearly, regulations around RFI no
longer exist or are being ignored. AM
radio is being pushed out to hide the
fact that RFI is out of control.
After killing power to all the noisy
lights, plugpacks and such, I managed to get the calibration done. That
improved things immensely. I did it
with an entry-level signal generator,
calibrated with a Fluke frequency
counter and an oscilloscope as the
meter. The ‘scope is also helpful for
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checking for gross distortion and, if
present, helping to find its source.
The signal generator must not be
calibrated with the modulation tone
on. My oscilloscope, counter and signal generator are coupled via a dedicated attenuator box in a shielded
metal housing.
Naturally, there was that annoying
hissy crackle of a bad connection audible on start-up, which I quickly traced
to pin 3 of the 6M5 socket. After sorting that and burn-in testing the chassis,
I treated the cabinet with a beeswax
furniture polish and put the set back
together. It does pay to remove and
replace valves and to scrape any oxidation from the pins. They should also
be tested before putting them back in.
Note that the knobs shown in the
photos are from HMV but are not correct for the model.
Conclusion
This is an attractive radio that
cleaned up well. You have to be wary
of people having messed with the set
previously and introduced faults. Most
importantly, if the condition of the set
is unknown to you, make sure it’s safe
to power up before doing so!
SC
August 2024 91
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