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VINTAGE RADIO
By JOHN HILL
A couple of odd repairs
I recently had two radio receivers to repair for
a collector &, in each case, there were unusual
problems. Both radios were small post-war
4-valve bakelite cabinet types – one a Kriesler &
the other a little Philips Philipsette.
Now some people make it difficult
for repairers in that they tinker with
things before they take it to someone
to fix. I know this to be a fact for I have
done so myself from time to time and
I'm sure that I'm not the only one to
do so. It is, therefore, only fair that
someone has now done it to me.
In the case of the Kriesler radio, the
owner had removed a component and
lost it. What's more, this component
was supposed to be a fairly mysterious one, being described as, "about so
long, as thick as a finger, hollow and
burnt black".
It was its blackened colour that
prompted the owner to remove it because it must have been the problem.
However, even when the charred part
was removed, it was still unidentifiable and what to replace it with was
a mystery. Whether it was a resistor,
a capacitor or some other component
neither he (nor I at that stage) had
any idea.
There was one consolation, however. The position from which the strange
component had been removed had
been marked.
When I finally started working on
the set, it was quite obvious what the
missing part was. It was positioned
between the two positive contacts
of a twin high-voltage electrolytic
capacitor and could only be a high
tension filter resistor. Yet it was not
the usual setup.
It would appear that the missing
resistor was a high wattage wirewound
type because all the current from
the rectifier flowed through it before
anything was connected to the high
tension supply.
By contrast, in most small 4-valve
receivers, the high tension for the output valve comes from the input side of
the filter and a one or two-watt carbon
resistor is used in conjunction with a
second electrolytic capacitor on the
output side to supply the other valves.
The value of the missing resistor
could only be guessed at. Something
around 5kW and 10W was used as a
starting point. It did little to bring the
set back to life.
Voltage checks
This photo shows the 4-valve Kriesler that had the missing component. It also
had other problems – mainly faulty paper capacitors.
80 Silicon Chip
As nothing seemed to be self-destructing, I did a few quick checks
with the voltmeter. There was around
240V on the input side of the filter
resistor but less than 100V on the
output side. A 1kW resistor was substituted with very little difference in
output voltage.
In this set, A 30kW resistor connected to the output side of the filter
applies high tension to the screen grid
of the IF amplifier valve. This screen
resistor had about 100V on one side
and zero volts the other. Based on this
evidence, it looked like the resistor
was open circuit.
Wrong! – when the resistor was
removed, it checked out well within tolerance and was replaced from
whence it came.
So where to from here? This particular screen connection on the
IF valve also applies voltage to the
6AN7 frequency converter valve via
a connecting lead. When this lead
was disconnected, the screen grid
on the IF valve suddenly had voltage
applied to it.
By this stage of the proceedings,
the fault was fairly obvious – a short
circuit at the point where the screen
voltage of the IF valve is applied to the
6AN7 frequency converter socket. As
the socket connection at that point had
a 0.05µF bypass capacitor to chassis,
it seemed likely that this component
could be faulty – and it was.
After disconnecting the suspect
capacitor (an original paper capacitor
I might add), it was found to have
a complete short circuit. Replacing
this faulty capacitor restored the set
to working order once again. But although the set was now working, the
high tension voltage was still only
150V at the output side of the filter.
This small Philips 4-valve receiver is a mighty performer for its size. It had
a number of problems, including a faulty valve, faulty capacitors & power
transformer faults.
Capacitor checks
So far only two components had
been replaced: the filter resistor and
the faulty screen bypass capacitor. All
the remaining paper capacitors were
originals and it seemed that they too
could be a little suspect.
Checking the capacitors with a
voltmeter revealed that three of them
had high tension voltages across them
and these were replaced with modern
polyester equivalents. This step saw
the high tension voltage rise to 210V.
The remaining paper capacitors
were all replaced with 100V greencaps.
Looking back, I don't suppose there
was anything really spectacular about
this particular repair. It was fairly
routine and systematic as it followed
the trail from the missing resistor to
the shorted paper capacitor, then onto
the other leaky capacitors.
It does show, however, that one
must look beyond the broken down
component and locate the real cause
of the problem. The real fault in the
old Kriesler was four ailing capacitors,
not the obvious overloaded resistor.
A short-circuited 0.5µF capacitor was one of the problems encountered with the
Kriesler repair. The routine replacement of paper capacitors can automatically
solve many obscure receiver faults.
The final touch to the Kriesler repair
was an alignment check. This was
most essential as the adjustment slugs
in the aerial and oscillator coils were
many turns out, thus displacing the
tuning to a considerable degree.
The Philips receiver
Next was the little Philips Philipsette and what a great receiver they
were for their size. This one looked
a bit of a wreck though; it was very
dirty and had no control knobs on
it. The missing knobs could be a
problem as they are special little red
ones that are unique to this particular
receiver. I was fairly sure that I had
no spares.
My concern about the knobs was unfounded. On withdrawing the chassis
August 1995 81
electrolytic. The latter looked particularly bad, as the seals at the positive
ends were ruptured and split. Despite
their appearance, they seemed to be
working all right but, of course, they
were all replaced.
Replacing the paper capacitors
cured the distortion problem. The
exact fault may have been a leaky
coupling capacitor to the grid of
the output valve. A leaky capacitor
in this position is bound to cause
distortion. The high tension voltage
rose 20V after the capacitor job was
finished.
By the way, the term "high tension"
is relative when referring to one of
these little Philips receivers. The
rectifier, a 6V6GT, operates with only
about 110V on the plate compared to
a typical plate voltage of 250V.
A completely dead ECH35 valve and a few sick capacitors were all that
prevented the Philipsette from working.
The power transformers used in many Philips & Mullard receivers share this
common fault – an exposed high tension winding. The winding protrudes
outside the paper insulation that separates the layers (probably caused by the
paper shrinking with age). This is not the transformer used in the Philipsette
in the story but a similar one in worse condition, to show the problem more
clearly.
from the cabinet, two red knobs fell
to the floor.
They had been loose inside, rolling
around on top of the chassis. Why they
hadn't been lost is a miracle.
Valve problem
The little Philips had a valve problem – the ECH35 frequency converter
was very dead in the heater department and needed replacing. The remaining three valves tested OK.
82 Silicon Chip
Removing the dirt and grime from
the chassis was next, then the valves
were refitted for a quick try out. Within
15 seconds from switch on, the set
burst into life.
But working and working well are
two different things. The sound was
harsh and distorted and it became
worse as the volume was increased.
Like the previously mentioned
Kriesler, the Philipsette had all of its
original capacitors, both paper and
Alignment OK
These neat little radios are sods
of things to align because all of the
adjustments are made with those
rotten-to-work-with Philips trimmer
capacitors. You know the ones – those
with the external coil of fine wire. As
the alignment seemed to be very good,
I chickened out and left it alone, declaring the repair finished.
Now both of these receiver jobs
were done to a set price. If they had
been mine I would have fitted a new
dial cord, cleaned the back of the dial
glass and maybe installed a new output
transformer. But when working to a
fixed price, such niceties have to be
ignored. These extras take time and
money and if a customer will not pay
to have such things done, then he must
live with the consequences.
The Philipsette was working away
on the bench while I was cleaning the
dust out of the cabinet. Then, quite
suddenly, the clear reception went soft
and garbled. To make matters worse,
the power transformer was rapidly
overheating.
Faults such as this are annoying to
say the least. One minute you have a
receiver working normally; the next,
there is something sadly amiss.
HFT short
When a transformer suddenly overheats, it usually has a short circuit in or
across one of its secondary windings.
In this case, the valves and dial lamp
were still lit, so it appeared as though
there was a high tension short.
A careful examination was made
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of all valve socket connections. In
particular, I checked for loose wires,
blobs of solder and broken insulation
but everything checked out OK. Even
withdrawing all the valves did not
prevent the transformer from overheating.
It then occurred to me that if the
short was still there when the rectifier
was withdrawn, then the fault must be
on the transformer side of the rectifier
socket – perhaps in the transformer
itself.
A close inspection of the power
transformer revealed a blob of solder
wedged firmly between one side of
the high tension winding and the core
laminations. A molten drop of solder
could have only fallen in there when
the chassis was upside down. As I had
done my work with the chassis on its
end, I didn't put it there!
Removing the solder returned the
set to normal operation. The solder
was acting like a thermal switch and
only caused trouble when heat expansion of the windings caused the
solder to short the HT winding to the
laminations.
In addition, it was noticed that
some of the high tension winding
was exposed and a couple of turns
were hanging out in the open. This
is a common fault with this make of
transformer because the windings
come quite close to the edge of the
paper that separates each layer. The
loose wires were coaxed back in place
SC
and held with silicone sealant.
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This small blob of solder was shorting
out the high tension winding of the
Philip's power transformer. The
short-circuit only occurred when
the transformer became hot enough
for the expansion of the high tension
winding to sandwich the solder
against the core laminations. There's
always something different that can
cause trouble.
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