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VINTAGE RADIO
By RODNEY CHAMPNESS, VK3UG
Restoring the butchered set
Restoring a vintage radio that someone else
has had a go at can be a difficult job.
Sometimes the fault will be quite subtle but
all too often, the previous restorer will have
made a complete mess of things.
It’s not unusual to come across a
set that has been really butchered.
When you see such a set, it makes
you think that the person who did
the work on it should be granted the
striped apron award and then hung,
drawn and quartered.
Often, these sets are obtained for
what appears to be a reasonable price
and the seller often says that there
isn’t much wrong with the radio.
Sometimes however, the seller has
tried to get the set going but has finished up with a mess that’s bigger than
when the work was started. This is a
case where a little knowledge can be
dangerous. Caution is needed in restoring sets that haven’t been “got at”
and an enormous amount of caution
is needed where as set has obviously
This view shows the wiring around the 6M5 valve socket of the Little Nipper
radio that I was given to service. Before removing any parts, it’s a good idea to
make a drawing of the connections so that it can be easily reassembled later on.
86 Silicon Chip
been “got at” and “butchered” into
the bargain.
In some cases, the restorer has
been very careful with the work but
has been unsuccessful because they
didn’t understand how the circuit
worked. In other cases, everything
has been done correctly and the lack
of success is due to a faulty new part.
Yes, that happens occasionally and
people with considerable experience
get caught as well.
A snap diagnosis
When I built my first radio in 1954
(a “Marconi” 1-valve kit), I couldn’t
get it to work. I then took the typical
but totally useless approach of a novice and pulled it to pieces and rebuilt
it – more than once, actually – and
it still didn’t go! It must be the 3V4
valve, I reckoned, so I sent it back to
the supplier and they sent me another
one and the set then worked.
For once, the diagnosis of “it must
be the valve” was correct but it often
isn’t. I had no test gear, virtually no
radio knowledge and no hope of finding out what was wrong. My so-called
diagnosis was nothing more than a
lucky guess.
Of course, once the set was operating, I became the local radio expert
– at least, in my opinion. I was soon
brought back to earth. A cousin and I
tried to get his 1-valve (1D8GT) radio
going a little later on and we had no
success with it at all. Like mine, I
wondered some years later if we had
inadvertently put HT voltage on the
filament of the valves! We’ll never
know.
In circumstances like this, it is better to get some advice from a restorer
more experienced than you are. When
we lack the experience of years in the
trade, it’s easy to overlook things that
Fig.1: the circuit of a late-model HMV “Little Nipper”. Substituting incorrect component values can really upset the
performance of a circuit like this.
a more knowledgeable person would
detect quickly.
For example, I got bogged down
trying to get my VHF amateur radio
station going on the 144-148MHz
band soon after I got my licence. I
literally didn’t have a clue and so a
friend and I bundled all our amateur
radio gear into the car and travelled
100km to the nearest VHF amateur
radio operator. He helped both of us
get the equipment going, explained to
us what he was doing and encouraged
us in various ways.
We never looked back from that
time onwards. And so it is with new
restorers. A little help at the right time
and you’ll really start to have a satisfying time restoring your radio gems.
The things people do
What people manage to do to the
sets they are restoring could fill
a book. My first story concerns a
friend’s brother-in-law. He acquired
a 6V vibrator mantle set which he
asked my friend about and was told
that it was a battery operated set.
Obviously, this advice didn’t sink
into the “smart-alec” brother-in-law’s
head, as he promptly removed the
50A battery clips and substituted a
3-pin mains plug. He then plugged
the set into the 240V mains supply.
There are no prizes for guessing
what happened next. All the valves
now have no filaments, while the fate
of the vibrator is unknown As for the
rest of the set, heaven knows what
damage has been done. A perfectly
good set was instantly turned into
junk and it’s now a very doubtful
proposition for restoration.
This same scenario often occurs
when 32V sets are bought or sold to
the local secondhand/antiques shop.
Unfortunately, 32V sets look like their
240V AC cousins and usually have
3-pin plugs on their power cords.
Plugged into 240V, things light up
brilliantly for a fraction of a second
until the fuses in the set blow – that
is, if they haven’t been replaced with
a 2-inch nail (the original 300A slowblow fuse). Remarkably, many 32V
sets survive such harsh treatment but
be aware that all may not be well in
such a set.
Then there are the sets that someone has actually got into and “serviced”. These are the real worry and
before even switching them on, it’s
advisable to obtain a circuit diagram
from the Historical Radio Society of
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MAY 1999 87
Fixing the butchered set – continued
Australia or the New Zealand Vintage
Radio Society. Alternatively, you may
know a fellow enthusiast who can
supply a copy.
If in doubt, trace the circuit out to
determine whether it is as it should
be. If not, a rewiring job lies ahead
before the set can be turned on. Sometimes, the exact circuit diagram will
be difficult to obtain. If this happens,
select a circuit that’s similar (eg, for a
slightly different model) and use this
as a starting point for the restoration.
Naturally, different valves require
slightly different operating conditions
and the Miniwatt Technical Data book
can help you here.
The Little Nipper
I once had a late model Little
Nipper HMV radio to service. These
sets are quite reliable and, as shown
in Fig.1, the circuit is quite straightforward . This particular set suffered
from instability in the IF amplifier.
In this circuit, the AGC bypass/
filter capacitor (C9) not only filters
the AGC line but also acts as part of a
neutralising circuit with C8 (this just
goes to show that pentodes, as well as
triodes, can benefit from neutralising
in RF circuits). I found that the restorer had installed the wrong value for
C9 (about 10 times the correct value).
This in turn upset the neutralisation
and caused the instability. As soon as
the correct value was installed for C9,
the set performed quite nicely.
In another case, the restorer re-
t
Shop soiled bu
!
HALF PRICE
placed the diode detector RF filter capacitor (C15) with another capacitor
that he thought was of the same value.
Apparently, it wasn’t easy to read the
value on the original and not having
a circuit to refer to, he used a .01µF
capacitor when the correct value was
100pF. As a result, the set was very
“bassy” and had little audio gain.
Once again, changing the capacitor
fixed the problem.
Capacitor values are not usually
critical except in tuned circuits. Gross
deviations from the correct values can
create problems but one step up or
down from the nominal value is rarely
a problem. Note also that some of the
nominal values that have been used
for years through force of habit are
not necessarily the optimum values.
On the other hand, resistors tend
to be more critical and so the correct
values should be used in that part of
that particular set’s circuit. By following the general component values,
as shown in Fig.1, the performance
should be quite reasonable.
The worst sets
The worst sets to get back into operation are those where the restorer has
decided to replace things “willy-nilly”, in an effort to get the set going. In
some cases, all the paper capacitors
are taken out and then a new batch
is put back in.
Unfortunately, many people forget
to draw diagrams of where things
come from and often end up fitting
the new parts in the wrong places.
The result is a unique circuit that
doesn’t work.
I make it a policy to replace one
component at a time so that I don’t
forget where it came from. And if I
have large component such as a valve
socket to replace, I draw a diagram
on a piece of paper that shows all
the connections, so that I know what
goes where.
Sets that have been abused in
various ways are not good choices
for first-time restorers to cut their
teeth on. Experienced restorers are
not keen on them either and for good
reason – they can be more trouble than
they’re worth.
If you do have a set that falls into
this category it’s best to seek advice
from an experienced vintage radio
restorer. That way, you won’t spend
a lot of time on a set that’s not worth
restoring or that’s beyond your capabilities.
Manufacturing faults
Finally, note that some sets had
faults built into them right from when
they were manufactured. If you can
detect the errors made (and they may
not be easy to find), you may well be
able to say “it goes better than new”.
I’ve encountered a few stinkers like
that over the years and they generally
become first class sets once the problems have been ironed out. Of course,
the faults are usually very subtle and
take some hunting down.
That said, restoring a vintage radio
set that someone else has given up
on is a very satisfying experience.
SC
Have fun.
14 Model Railway Projects
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For Level Crossings; Diesel Sound Simulator.
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