This is only a preview of the June 2000 issue of Silicon Chip. You can view 29 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Automatic Rain Gauge With Digital Readout":
Items relevant to "Parallel Port VHF FM Receiver":
Items relevant to "Li'l Powerhouse Switchmode Power Supply; Pt.1":
Items relevant to "CD Compressor For Cars Or The Home":
Purchase a printed copy of this issue for $10.00. |
VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
A Japanese 110V AC/DC set
Japan was exporting valve radios by the late
1950s and early 1960s, although still
struggling to recover following its defeat in
World War 2. The standard of the exported
radios at that time was mediocre and we
often looked down our noses at the sets. How
times have changed!
“They’ll never make radios as good
as we do”, was the familiar catch-cry
in those days. Famous last words! The
Japanese now make some of the best
radio equipment in the world and
Japanese electronic equipment now
fills our living rooms, work places and
cars. The big difference back in the
1950s and 1960s was that Japan was
then a cheap labour country, so the
radios were cheap to import.
A friend had a badly damaged JapaBelow: front view of the Japanese
110V AC/DC receiver circa 1960 with
hand-drawn dial calibrations and
stick-on labelling.
nese radio set that was no use to him.
Did I want it? “Yes please”, I said.
Being an inquisitive fellow, I wanted
to see what I could find out about it.
Unfortunately, quite a bit of the
cabinet had been broken and the
back panel, knobs and decorative
front panel were missing – hence the
stick-on Dymo® labelling and handdrawn dial calibrations shown in the
photograph. In fact, one whole end of
the cabinet with one loudspeaker was
completely broken away (the set uses
two 100mm speakers, one at each end
of the cabinet.
It had obviously fallen and not
bounced at all well off the floor!
I was interested to see how the
receiver would perform as I hadn’t
seen this model before. I have no idea
what brand the set is though, due to
the fact that so many bits are missing
and there’s no chassis labelling.
I glued the cabinet back together
with plumbers blue plastic cement. I
don’t recommend it but this was only
going to be a rough job to see how a
Japanese radio of this era performed.
The set was then given the once over
and any minor components considered likely to create trouble were
replaced.
110/117V AC/DC operation
I laboriously traced out the circuit and found that it operates from
110/117V AC/DC (and that means be
careful). Well, I got it up and running
on both the broadcast and shortwave
bands. The shortwave band tunes
from 3.8-12MHz and is very much
an afterthought, as there is only one
adjustment for that band (and minimal
adjustments for the broadcast band).
Despite the lack of adjustments in
the front end, it appears to be well
JUNE 2000 53
Rear view of Japanese 110V AC/DC receiver. The 240V-to-110V transformer was
fitted at the end of the cabinet near the power lead in. Do not touch AC/DC sets
unless you are very experienced and know exactly what you are doing – they
can be very dangerous.
aligned, with good sensitivity. The
manufacturer really did do a good job
of getting the prewound coils right,
so that few adjustments are needed.
With a few more adjustments, the radio would no doubt work even better.
The circuit diagram is quite standard, even allowing for the fact that it
is an AC/DC set. The valves all use
0.15A heaters and are strung in series
across the 110/117V mains.
The valve line-up is as follows:
12BE6, 12BD6, 12AV6, 50C5 and
35W4. Note that the first two numbers
indicate the voltage of the heater. The
50C5, a 7-pin miniature type, gets extremely hot. Its heater dissipates 7.5W
compared to 3W for the 6AQ5/6V6GT
and that’s even before the plate and
screen dissipation is taken into account. The 50C5 is designed to work
quite well with between 110-130V DC
on the plate and screen.
from the grid to the bottom of the fist
IF coil secondary. The 470kΩ potentiometer in the AGC line functioned
as a crude volume control.
The first audio stage is quite conventional, as is the audio output stage.
The main points of interest here are
the use of a 50C5 and the unbypassed
160Ω cathode resistor which gives
some degeneration.
As an aside, some of the audio circuitry is enclosed in a small hybrid
block of components. This method of
reducing the time to wire sets wasn’t
used for long. Instead, radios using
PC boards and individual components
proved much more popular. The main
drawback was that if one component
became faulty, it was not always easy
to isolate, As a result, the components
in the hybrid circuit were usually all
replaced and the unit thrown in the
rubbish bin.
Converter
Power supply
The converter stage uses the 12BE6.
This is a standard pentagrid converter and uses a tapped coil in the
cathode for the oscillator. The aerial
coil arrangement is also standard but
very austere, with few adjustments as
mentioned earlier. The intermediate
frequency (IF) amplifier is a basic
455kHz system using a 12BD6 which
is a 7-pin miniature valve similar to
a 6BH5.
The 12AV6 is used as the detector
stage and this has its two diodes
strapped together. The AGC line runs
The power supply is the conventional half-wave rectifier system used
in AC/DC sets. Incidentally, if the set
is used on DC, it is necessary to make
sure the above-chassis input lead is
positive otherwise the set won’t work
(yes, the valves light up but the valves
get no high tension (HT) voltage).
Because a half-wave rectifier is
used, the filter capacitors for the HT
rail are larger than normal at 30µF
each (all three of them) to reduce the
ripple. Any interference on the mains
is poorly filtered using just two .01µF
54 Silicon Chip
capacitors.
The chassis of the set can be earthed
as it is not directly connected to either
side of the mains. Instead, the “earthy”
side of the mains (ie, the Neutral) runs
around the chassis as a wire bus and
is connected to the chassis via one of
the .01µF capacitors.
However, this set does have one
potential “bitie” and that’s the earth
terminal of the phono input which is
connected to one side of the mains.
Just imagine this braided earth lead
going up to the earthed metal work of
the turntable! If the active side of the
mains was on the “earthy” side of the
input (ie, the active and Neutral were
transposed), this could be lethal. How
manufacturers ever got away with
such things is beyond me!
This model set was imported into
Australia as a 110V receiver. So how
was it used here? Well, in Australia,
it was converted to 240V AC by fitting
a 240V-to-110V power transformer
into spare space in the cabinet. It was
roughly fitted I might add but at least
it made the set safe as far as shocks
from the phono earth terminal were
concerned.
Summary
This radio is reasonably typical of
the standard of receivers manufactured in Japan at that stage. I had a
Lafayette HE-30 receiver of the same
general vintage and while it was better than this set, it isn’t all that much
better. Japan is now light years ahead
of that mediocre standard, as we all
know.
In summary, this was a rough and
ready set but it is stable and works
quite satisfactorily. However, I wonder
how well it would go in an electrically
noisy environment with virtually no
noise filtering on the mains input.
It’s an interesting little set – part of
the history of the era – but it certainly
isn’t one of my favourites.
Safe servicing AC/DC sets
Although AC/DC sets were quite
common in America and in Europe,
we in Australia and New Zealand have
been rather wary of dealing with “hot”
chassis sets. However, they were produced in small numbers in Australia
and New Zealand to suit some of the
small townships that had DC power
only but which might be converted to
AC later on.
These sets were also bought by
people who shifted around and could
not be sure if the next town they were
going to had AC or DC power. Some
of the last valve portables also were
“hot” chassis sets, having one side of
the mains connected to chassis when
they were used on mains in lieu of
batteries.
The first comment to be made in
regard to servicing such sets is be
careful – exercise extreme caution and
double check everything. Do not touch
these sets unless you are very experienced and know exactly what you
are doing – they can be death traps.
Depending on the circumstances,
there really isn’t a great deal of difference between grabbing the chassis
of a “hot” chassis set and earth at the
same time as grabbing 400V DC HT
in a receiver and chassis earth. The
effects can be identical – death.
So be careful with all receivers.
They can be lethal if you are careless.
With an AC/DC set, first check
which pin of the mains plug goes to
chassis or, if it is a better designed set,
to the negative bus that is insulated
from the chassis. Make absolutely
sure that it is the Neutral that goes to
the bus or to the chassis. Also, check
the capacitor that goes between the
chassis and the negative bus in the
receiver. This should have a rating
of 250V AC working and must be in
good order (in fact, it’s probably best
to replace it, just to make sure).
However, never assume that the
chassis will be at Neutral potential
when plugged into just any power
point. If the power point is wired
incorrectly (eg, Active and Neutral
transposed), the chassis could be live
(and that includes the pot shafts)!
Additionally, even if you are quite
sure that the set is wired in a safe
manner, use a rubber mat to stand on
and don’t touch any parts, including
the chassis, while the set is on (the
chassis will only be safe to touch if it
is actually connected to mains Earth).
Many people use a 240V-to-240V
isolation transformer to be even more
certain. A core balance detector such
as recommended in Vintage Radio for
May 1998 is cheap insurance.
One thing that cannot be done is to
run the set with a valve missing. That’s
because the valve heaters are wired
in series and if one valve is out of the
set, all the heaters go out. This means
that it isn’t possible to test the output
stage with the other valves removed
as can easily be done with receivers
using parallel wired heaters.
Another problem is wiring in dial
lamps (this set has none). They cannot
simply be wired in series with the
heaters, as these have low resistance
at switch on (ie, when cold) and draw
a heavy current. If you did connect
them in this manner, the dial lamps
would light up brightly – for a few
seconds – and then expire.
So how did the set manufacturers
overcome this problem? In the case of
the radio featured here, a dial lamp
could be wired between pins 4 and 6
of the 35W4 valve. The voltage drop
across this portion of the 35W4 heater
will be nominally correct for a 6-8V
150mA dial lamp.
Note however that the socket would
need to be well insulated to ensure
there were no shorts or shocks, as
both sides of the socket would be at
virtually full mains voltage above the
chassis or negative bus.
High-voltage valves
In Australia, we did not have the
range of high-voltage valves that were
available in America and Europe. As a
result, although the valve heaters were
wired in series, the voltage drop across
them was much less than 240V. Resistors wired in series with the heaters
accounted for the rest of the voltage
drop and often, in a higher quality set,
a barreter or current regulator would
be used as well.
A barreter consisted of an iron-wire
resistor mounted in a glass bulb containing hydrogen. This device gave a
constant current for a wide range of
applied voltages.
For example the 161 has a constant
ELECTRONIC VALVE &
TUBE COMPANY
The Electronic Valve
& Tube Company
(EVATCO) stocks a
large range of valves for
vintage radio, amateur
radio, industrial and
small transmitting use.
Major current brands
such as SOV-TEK and
SVETLANA are always stocked and we
can supply some rare NOS (New - Old
stock) brands such as Mullard, Telefunken, RCA and Philips.
Hard to get high-voltage electrolytic
capacitors and valve sockets are also
available together with a wide range
of books covering valve specifications,
design and/or modification of valve
audio amplifiers.
PO Box 487 Drysdale, Victoria 3222.
Tel: (03) 5257 2297; Fax: (03) 5257 1773
Mob: 0417 143 167;
email: evatco<at>mira.net
New premises at: 76 Bluff Road,
St Leonards, Vic 3223
current of 0.16A through it for applied
voltages of 100-200V. This was handy,
as it meant that the huge inrush current through cold heaters was avoided
and dial lamps could be wired in series with the mains (provided a 6.3V
dial lamp had less than 6.3V across
it to ensure long life). Additionally,
there was no need for a voltage tapping
for different voltages between 200V
and 250V - the current regulator took
care of all of this.
A 240V set using 0.15A heaters
required about 36W for the heaters
and series resistors. The HT circuit
probably required a further 12-15W,
making a total of about 50W - typical
of even AC-operated receivers of that
particular era. However, for some
strange reason, a few manufacturers
used 0.3A heaters, which meant that
the heater and series resistors used
72W before any HT current was taken
into account.
These would have been cosy sets to
operate in the middle of winter, as the
inside of the case really did get quite
hot. By contrast, the receiver described
in this article only used about 22W
SC
from a 110-117V mains supply.
JUNE 2000 55
|