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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
A mainland Chinese radio
receiver from the 1960s
It’s not often that one gets to work on a radio set
that was manufactured in mainland China
during the 1960s. The set described here had
some interesting features, including valves that
were pin-for-pin compatible with western types.
Occasionally, one gets the opportunity to examine vintage radios
from behind the Iron Curtain. Many
of us are familiar with the Russian
transistorised multi-band portable
radios that appeared from time to time
on the market. One example was the
Selena, which evoked curiosity from
68 Silicon Chip
the electronic fraternity in the 1970s.
This interesting set used a turret tuner
to do the band changing, something
rarely used by western manufacturers.
But what about sets that were made
behind the other end of the Iron Curtain (or was it the Bamboo Curtain)?
What did the communist Chinese
make in the way of radio receivers?
They didn’t export valve radio equipment to the Free World and they were,
in fact, quite insular at the time.
An opportunity to see what they
did in the 1960s presented itself towards the end of 1997, when a friend
obtained a set from a market in Shanghai. I was keen to see this set and to
gain some idea of what the Chinese
were doing in electronics around
1963, the year the set was manufactured. It is quite an interesting radio,
with one or two unusual ideas.
The receiver itself is a 4-valve BC
superhet, designed for use on either
110VAC or 220VAC, with a transformer power supply. The various views of
the set show the wiring
style used and what the
various components
looked like.
Some aspects of the
set’s electronic and
mechanical design are
similar to our methods.
However, we tended
to use point-to-point
wiring during that era,
while this receiver used
tag strips quite extensively. This meant that
some wiring was unnecessarily long – it certainly wasn’t point-to-point.
In addition, the capacitors were generally
larger than the types
used here for the same
ratings. But some things
never change – they
suffer exactly the same
problems of excessive
leakage. The resistors
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The 455kHz IF transformers are unusual in that the adjustment slugs are at the
back of the cans. Obviously, the coils are mounted side by side, a technique used
in some early Australian IF transformers.
were similar to the ones used by the
Japanese of the same era but appeared
to be of better quality.
IF transformers
The accompanying photographs
show that the 455kHz IF transformers
are rather different to those used by
Australian manufacturers. The adjustment slugs are at the back of the
cans and it is obvious that the coils are
mounted side by side, as were some
early Australian IF transformers. The
aerial and oscillator coils are similar
to the slug-tuned coils of the same era
in Australia.
Aligning the oscillator and aerial
coils at the high frequency end of the
tuning range is a bit of a problem. The
trimmer capacitors are similar to the
all-wire types used by Philips and
some other manufacturers. They use
a 16-gauge (or thereabouts) enamelled
wire as one lead and fine tinned wire
wound around the enamelled wire as
the other lead. They can only be easily
adjusted once. I removed the one on
the aerial coil and replaced it with
a conventional trimmer capacitor,
which is easier to adjust.
Chassis layout
From the photographs, it can be
seen that the chassis layout is quite
conventional. However, there is one
thing I really do like about this receiv-
er when it comes to servicing – tip it
upside down and it rests fairly evenly
on the two IF transformers and the
power transformer. This makes it very
easy to work on the under-chassis
components. The set will also sit quite
nicely on the end that’s adjacent to
the power transformer.
It’s a pity more Australian radios
weren’t made like this – servicing
them would have been so much easier.
Getting the set out of the cabinet
is a breeze too. First, you remove the
plywood back panel (no cheap cardboard here) by removing four screws.
After that, you simply pull the two
knobs off, unscrew two bolts on the
back of the chassis and pull it out.
One point of interest is that the front
edge of the chassis is wedged into a
slot made in the plastic, which stops
it from moving around. Operatic sets
used a similar method of attaching
the chassis to the cabinet.
The set had previously been serviced on a few occasions and the
work was rather rough, so some of
my criticisms regarding the layout
are not entirely directed at the manufacturer. That said, the manufacturer must have had some training
on wiring from Radio Corporation,
as single-strand insulated wire was
used and the wires were all wrapped
around their respective terminals several times! This means that the parts
RESURRECTION
RADIO
242 Chapel Street (PO Box 2029)
PRAHRAN, VIC 3181
Tel (03) 9510 4486 Fax (03) 9529 5639
can only be easily removed by cutting
them out, as it isn’t easy to unwind
the soldered leads without cooking
everything in the near vicinity.
Circuit details
I find that having a circuit of a
set makes servicing so much easier.
Unfortunately, my trusty copies of
the Australian Official Radio Service
Manuals were of no help this time, so
I had to trace the circuit out myself.
I started by checking the valve
types, as this can give a good idea of
the style of circuit used. The line-up
included a 6A2, a 6K4, a 6N1-J and a
6Z4, none of which I’d ever heard of
before. They are all miniature types
and all have seven pins except the
6N1-J which is a 9-pin valve. These
valves are taller than a 6BA6 but
shorter than a 6AQ5.
In short, the valves were all “homegrown” types, the exception being the
6Z4 which appears to be a miniature
version of the 84/6Z4.
These unknown valves certainly
added to the difficulties of tracing out
the circuit. The set appeared to be a
superhet of some sort, with two coils
JULY 1999 69
CHINESE MAINLAND SET CIRCA 1963
Notes:
6A2 is pin compatible with the 6BE6
6K4 is pin compatible with the 6BA6
6N1-J is pin compatible with 6BQ7A (inc. shield) and is compatible
with the 12AU7/12AT7 except for heater pins
The 6Z4 is a near equivalent to the 6X4 (but is not pin compatible)
Fig.1: this is the circuit diagram of the receiver, as
traced out by the author. It is a 4-valve set with a
twin-triode output stage.
and a valve (6A2) close to the tuning
gang – obviously a converter of some
sort. As a starting point, I carefully
traced each lead, checked continuity
through the coils and traced out a
circuit up to the pins of this valve. It
looked like a circuit that a pentagrid
converter would use, so I checked out
a circuit based on a 6BE6 and the two
looked remarkably similar.
In fact, the 6A2 even used the same
pins as the 6BE6 for each function.
I then checked the next section
(around the 6K4) and this appeared
to be a conventional IF stage. I drew
the circuit out with the valve pins
numbered and then checked it against
a circuit using a 6BA6. The pin-outs
were almost identical!
The detector turned out to be a
germanium diode. From there, the
demodulated signal is fed to the volume control, which was followed by
some sort of 2-stage audio amplifier
(6N1-J). I was having some trouble
here as the speaker transformer had
gone open circuit in the primary. This
means that the screen of the audio
output valve cops quite a wallop and
can glow rather too brightly.
Initially, I suspected that the valve
was probably a triode pentode but
that didn’t appear to be the case when
I took a closer look. There was no
70 Silicon Chip
screen – just two cathodes, two grids,
two plates and one pin earthed. It all
seemed a bit strange until I checked
to see what the earthed pin did inside
the valve. It was a shield between the
two sections and this indicated that
the valve was a twin triode, not a
triode pentode as expected.
It is certainly unusual to find a
triode output stage. Interestingly,
the pin outs are the same as for the
6BQ7A twin triode RF amplifier, as
commonly once used in TV tuners.
The audio section even has negative
feedback!
The power supply is based on the
6Z4 and was quite conventional.
Unfortunately, one half of the transformer’s secondary HT winding was
open circuit. This version of the 6Z4
has similar ratings to the familiar
6X4, although its pin out is not quite
the same.
The final circuit is shown as Fig.1.
As you can see, it’s quite straightforward and it certainly makes the set
easy to service.
Repairs
It was now time to actually service
the receiver. First, the various capacitors were checked and those with
excessive leakage were replaced. The
resistors all appeared to be within tol-
erance but the loudspeaker transformer was faulty and had to be replaced.
The rest of the set appeared to be
in good condition so I applied power
and used a multimeter to check the
voltages. These were OK and the set
worked but its performance was initially quite poor.
This improved quite markedly after
a full alignment but there were still
problems. At times, the set appeared
to be unstable, particularly when I
added an extra RF bypass near the
front end of the receiver (more on
this later).
One of my pet hates is having to
guess where the dial pointer should
be when I align the oscillator, so that
the dial calibrations are accurate.
This is always a problem when the
dial-scale remains in the case of the
set when the chassis is withdrawn.
This receiver falls into that category
but because the pointer is so far behind the dial-scale, parallax error is
quite significant anyway. As a result,
the dial pointer position isn’t all that
critical, which is just as well.
Because of the similarity of the
6A2 and 6K4 valves to the 6BE6 and
6BA6 respectively, the latter were
substituted and the results were quite
satisfactory. I then tried a 6BQ7A in
lieu of the 6N1-J and the performance
decreased somewhat but was otherwise OK. I then substituted another
6BQ7A and it really went well except
that it was unstable.
The audio amplifier appeared to
be taking off due to RF signal from
the IF strip feeding back into it. This
problem was cured by placing a 47kΩ
resistor in series with the grid (pin 7)
of the audio amplifier output stage.
This was yet another example where
insufficient IF filtering in the audio
amplifier causes trouble.
I didn’t try substituting a 6X4 rectifier in place of the 6Z4, as a wiring
modification would have been necessary. However, I’m sure it would have
worked well had this been necessary.
Other problems
By now the set wasn’t performing
too badly but there were still a few
things to be sorted out. First, the
power transformer had one half of
its HT secondary winding open circuit and I suspect that it had been in
this condition for quite some time.
To overcome this problem, the two
plate leads (pins 1 and 7) of the 6Z4
were joined together and the faulty
winding lead was cut off.
This step increased the HT voltage
by about 20V. In addition, a 240V AC
supply is rather high for a set designed
for run off 220V AC, so a 180Ω 5W
resistor was wired in series with the
mains. This gave a nominal 220V AC
on the primary of the transformer.
Running the set for a few hours in
this condition showed no abnormal
temperature rise in the transformer
despite the open circuit winding.
As pointed out, the valves used are
similar to ones we know and they
draw the same heater currents, the exception being the 6N1-J which draws
0.6A compared to the 6BQ7A’s 0.4A.
For replacement purposes, the 6A2
= 6BE6, 6K4 = 6BA6, 6N1-J = 6BQ7A
and the 6Z4 = 6X4 (with some wiring
modifications).
Did the Chinese copy our valve
types and give them different type
numbers or was it just coincidence?
Summary
The Chinese receiver used tag strips quite extensively, while Australian sets of
the same era mainly used point-to-point wiring. In addition, the capacitors are
generally larger than the types used here for the same ratings.
In many ways, the set is not greatly
different from the average Australian
4/5 valve superhet radio of the era. As
already pointed out, the main difference concerns the use of a twin-triode
audio output stage.
I suspect that the pin-for-pin com-
patibility of valves and the general
similarity in many areas to sets in
the West is just too much of a coincidence. However, other areas of the
set’s design are quite original and
different.
It’s hard to judge what market it
was intended to fill but at a guess it
was probably intended for the upper
class market in China of that period.
A similar set here would have been
considered an austerity model. SC
JULY 1999 71
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