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VINTAGE RADIO
By JOHN HILL
Restoring a Tasma TRF receiver
I had an interesting & most demanding repair to
do recently, involving a 1931 model 65 Tasma
console. The old Tasma is a very basic 5-valve
TRF (tuned radio frequency) receiver & it was in
a woeful state of disrepair.
The Tasma belongs to a radio collector mate who bought it sight unseen,
except for a photograph which was
sent to him from Queensland. What the
photograph didn’t show was that the
receiver had no valves or loudspeaker and had a totally burnt-out power
transformer.
On delivery of the Tasma, its new
owner was so disheartened with his
purchase that he placed it in an auction. However, after a conversation
with me about replacement transformers and other parts, the dilapidated
Tasma wreck was quickly retrieved
from the auction rooms. In due course,
the chassis and two electrodynamic
loudspeakers (a Jensen and a Rola)
found their way onto my workbench.
A Jensen speaker was originally fitted
to the Tasma and that make was to
be given preference as a replacement
over the Rola.
Oh, how I wish that I had kept my
big mouth shut! On seeing the Tasma
for the first time, I soon realized why it
had been sent off to the auction rooms.
It looked as though it had spent most
of its life in a tropical rainforest. I am
inclined to think that Queensland
weather is not kind to vintage electronics.
The burnt-out power transformer
was interesting in that it was constructed more like a modern transformer
rather than one from the 1930s. As
shown in one of the accompanying
photographs, each winding is placed
side by side instead of one on top of
the other, as was usually the case in
that era.
Rewinding the transformer was
considered at one stage but it was
more than I could handle, as both the
primary and high tension windings
were open. And having it rewound
professionally would be quite an
expensive repair job – probably at
least $100.
As luck would have it, the owner
had a discarded old Hypressco chassis which would hopefully supply
a suitable power transformer with a
2.5V low tension winding. That too
was on my workbench, waiting to be
cannibalised for spare parts.
Other problems
After a complete strip-down & repaint, the derelict Tasma chassis looked as
good as new. This particular receiver now works better than ever, following the
discovery of a manufacturing fault.
82 Silicon Chip
A quick check over the Tasma chassis revealed that there were other serious problems apart from the defunct
power transformer. Two of the RF (radio frequency) coils had open primary
windings and they would either need
repairing or replacing. When you are
faced with a rotten job – it’s usually
rotten all the way!
The 3-gang tuning capacitor had its
problems too, with dry rusty bearings
and the three sets of moveable plates
about 45 degrees out of alignment with
each other. In addition, all the paper
capacitors were leaky and the large
block capacitors used in the high tension filter were particularly bad. Some
of the resistors had gone high too and
the wirewound high-tension dropping
resistor had several dead taps on it,
This close-up view shows the new power transformer cover. Made from light
gauge sheet steel with spot welded seams, it is identical to the original apart
from being 10mm higher.
indicating either poor connections or
a break in the resistance wire. Finally,
the tone switch had also been badly
strained and wasn’t making contact
at any of its four positions.
As I said before, rotten all the way!
Sorting the transformer
When faced with such a job, it is
hard to know just where to start. I
decided to check out the replacement
power transformer to see if it would
work in OK.
The Hypressco chassis had its
share of problems too. The rectifier
socket had a great hole burnt in it and
all the valve pin connectors were just
dangling on their respective wires
underneath.
Although the power transformer
was a large 2.5V type, it was not the
original. There was another set of bolt
holes in the chassis that suggested
there had been a transplant at some
time in the past. Checking out the
transformer soon revealed that it had
suffered a coronary in one half of the
high-tension winding. Repair prospects at that stage of the proceedings
did not look very promising.
As the transformer was particularly
large and robust looking, I thought I
might try feeding the good half of the
winding into a silicon diode bridge
rectifier to supply the set’s high tension. In fact, the bridge rectifier setup
worked quite well except that it required a sizeable wirewound resistor
to reduce the voltage to a level that
would work in with a 2kΩ field coil.
What’s more, the owner wasn’t really
happy about his Tasma being “hot
rodded” to such an extent, as he likes
things to be reasonably original. The
thought of silicon diodes and large
20W resistors did not appeal.
The next alternative was to use the
half high-tension wind
ing with the
80 rectifier connected as a half-wave
unit. Surprisingly, this worked better
than expected. It produced the correct
voltage and is completely hum free
while still using only 10µF electrolytics either side of the field coil.
As the set wasn’t working at this
stage, all the power transformer tests
were done using a test rig that produced a 50mA load. Although the high
tension arrangements are not a desirable set up, the receiver has run for
prolonged periods of up to four hours
without the transformer becoming any
hotter than moderately warm.
There was another problem yet to
be solved regarding the replacement
transformer. As the substitute unit is
about 10mm higher than the original,
the transformer cover would no longer
fit. No problems! The local sheetmetal
man made up a similar but deeper
cover and after a coat of paint no one
would ever know the difference.
Although some readers may strongly
disapprove of all these devious goings
on, everything seems to be working
well in the power transformer department and once the cover is on it
even looks OK. I believe it is better to
improvise and have a receiver working
than to have it original and either not
working or costing a fortune to repair.
The Tasma’s original power transformer (left) had many
charred windings & was a total write off. Removing &
stripping the tuning capacitor (above) was the best way
to clean it & lubricate the spindle bearings.
February 1995 83
Disaster struck at about fifty turns
when the wire broke. A strand of copper wire a mere eighth of a millimetre
in diameter is not very strong and coil
winding requires a reasonable amount
of tension. When winding, one always
hopes that the coil does not break or
slip out of one’s aching fingers. If either happens, it’s a case of “oh well;
start again”!
In the end, the outcome was quite
successful although re
winding RF
coils is always a tedious job.
Other repairs
Although the replacement power transformer only had one half of its hightension winding intact, it was still able to supply the Tasma’s needs. Note that
this photograph was taken with the experimental bridge rectifier still in place.
The two open-circuit RF coils were
next. First, a rough sketch was made
of the wiring connections so that
everything would go back where it
should. This is a good precaution to
take before unsoldering anything – RF
coils or otherwise.
The RF coils are identical and they
had the same fault. Fortunately, the
open primary winding is wound over
the top of the secondary which made
the repair a good deal easier than if it
had been the other way around.
The problem was the much dreaded
“green spot”. The fine silk-covered
wire had several spots of corrosion
in it which could be clearly seen as
it had come through the silk. The 60
turns of wire were counted before the
damaged coil was removed.
Not having silk covered wire, I
had to compromise. Enamel covered
0.125mm wire is about 0.01 millimetres larger than the wire originally
used. It would have to do!
The inductance of the primary
winding of an RF coil is by no means
as critical as the secondary winding
which is connected to the tuning capacitor. Variations in the secondary
would cause tracking problems when
tuning. A turn or two over or under
on the primary would make very little
difference.
There was a lot of wiring that needed to be replaced and the connecting
leads from the coils to the valve top
caps and tuning capacitor were all rewired. Resistors which had gone high
were replaced and the paper capacitors
all replaced with modern high-voltage
polyester types. The ineffective tone
switch mechanism was also repaired
and a new wirewound volume control
fitted.
One problem encountered is that the
Tasma has a few odd looking original
components in it that are a little different from normal. For example, there
were a number of square shaped fibre
formers bolted to the underside of the
chassis (see photograph). These little
units are either wirewound resistors
or radio frequency chokes.
The two shown in the photograph
are resistors. One is the output valve’s
centre tapped filament resistor and
this is con
n ected to the second
unit – a 500Ω cathode bias resistor.
These square shaped components
are not the usual readily identifiable
wirewound, centre-tapped and bias
resistors.
After checking out the two available loudspeakers, it was not difficult
to choose one. As the Jensen had an
open field winding, the Rola was
the one for the job. The speaker was
wired directly to the receiver (no
speaker plug and socket), which
makes handling the set rather awkward from then on. If it had been
mine, I would have been tempted to
fit a socket and plug.
Early tests
These odd looking square components are wirewound resistors. One (right) is
the centre-tapped filament resistor (for the directly heated output valve), while
the other (left) is the 500Ω cathode bias resistor.
84 Silicon Chip
Upon trying out the Tasma, the
best that could be said for it was that
it was a really poor performer. This
was despite that fact that the correct
valve types had been fitted: two 24As,
a 35, a 45 and an 80, as indicated by
This microscopic spot of corrosion was sufficient to stop the receiver from
working. In fact, the Tasma had two faulty RF coils due to “green spot”
corrosion. Note that the primary winding is wound on top of the secondary
winding, which made repairs much easier.
This photo shows the rewound RF coil, prior to installation in its metal cover.
It was hand-wound with enamel-covered wire of a slightly different gauge & this
restored it to full working order.
the valve location chart inside the
cabinet.
But there was a very good reason
for the weak response. A close examination of the wiring underneath
showed that the screen grid on one
of the 24As had never been connected to the high tension supply. The
screen had a bypass capacitor but no
screen voltage. Running a wire from
an adjoining screen grid connection to
the unconnected screen gave a huge
improvement to the set’s performance,
which improved even further when
the trimmers were properly aligned.
It would appear as though this
particular receiver had been a dismal
performer all its life and would have
given only mediocre reception on the
strongest of signals.
Well that’s about all there is to report
on the Tasma repair. There was a lot of
time and effort spent getting this one
going again, believe me! While some
of the repair techniques may be questionable from a purist’s point of view,
the nicer alternatives would have cost
hundreds of dollars.
However, outwardly the receiver
looks quite acceptable and it is working better now than at any other time
SC
in its life.
February 1995 85
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