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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
How NOT to build vintage gear . . .
plus cleaning away the gunk
There are lots of things to consider when
building or restoring vintage radios, includ
ing function, performance, reliability, ease
of service and safety. However, there are a
few things that you must not do.
A front view of the old PA amplifier with the cover removed. Note the three
volume control labels, one of which is actually for a 4-position switch! The
chassis can not be placed upside down with the valves in place, which makes
servicing difficult.
R
ECENTLY, I TOOK A LOOK at
an old 15W public address (PA)
amplifier that I had acquired some
years ago, my aim being to restore it
to working order. The unit had been
stored in my workshop for a number
of years and I hadn’t really looked at it
closely until now. From the outside, it
appeared to be a commercial unit but
as work progressed, it soon became
evident that this PA amplifier was
actually home-made.
In fact, during the 1940s and early
1950s, chassis kits complete with top
and bottom covers were produced so
94 Silicon Chip
that home constructors could make
their own PA amplifiers. Commercial
PA amplifiers were not particularly
common at that time and “Radio &
Hobbies” magazine described many
PA amplifiers over the years. This particular amplifier (not an R&H design)
had been built into one such chassis
kit, hence my initial impression that
it was a commercial unit.
Anyway, I placed it on the bench,
dusted it down and ran a damp rag
over the outside of the case to clean
it up. I then removed the top cover
which protects the valves, the power
transformer, two filter chokes and
the audio output transformer. This
was also dusted out and I then ran a
damp cloth over the chassis and the
transformers.
Next, I took the six valves out of
their sockets and carefully cleaned
them with warm, soapy water. With
octal valves, I hold them upside down
and rub the soapy water onto the glass
envelope to clean the dust and other
muck off the envelope. However, I
take care to avoid rubbing the labels
as they are all too easily washed off
along with the muck.
Once the envelopes were clean, I
wiped the valve bases with a slightly
soapy rag, taking care to ensure no
moisture could get down inside the
bases. Finally, I wiped the glass envelopes with a clean damp rag and left
the valves lying on their sides to dry.
By this stage, the top of the chassis
was looking reasonable although I
wasn’t particularly thrilled with the
previous owner’s paint job. First, the
paint had not been applied evenly to
the chassis, probably because spray
paint cans weren’t on the market at
that time. Even worse, the chassis
hadn’t been painted at all between the
HT chokes and the power transformer,
indicating that painting took place
with these parts bolted in place.
But what was really strange was
that the power transformer had been
painted red. It really looks out of place
but even so, I still thought that this was
a commercially-built piece of equipment with just some rather ordinary
paintwork.
The under chassis shocks
It was now time to work on the
under-chassis area. Leaving the valves
out (so that they couldn’t be damaged),
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This photo clearly shows some of the dodgy under-chassis wiring. Many of the
smaller components have long leads and are simply soldered together in midair. This allows the leads to flex and short against other parts.
I turned the chassis upside down and
removed the metal sheet shielding
the underside. This amplifier weighs
a hefty 14kg, so the valves would
have almost certainly been damaged
if they had been installed (some of
them are higher than the transformers
and chokes).
With the shield removed, I took a
look inside and was stunned at the
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poor quality of the work. The wiring
was so bad that there was no doubt this
was a home-made amplifier. Even the
worst manufacturers would not have
taken so many stupid short cuts and I
was unimpressed to say the least
It was so bad that there was nothing
for it but to abandon the restoration.
At the very least, it would require a
complete strip down and rebuild and it
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The mains cord attachment is definitely unsafe. Admittedly, the previous owner had used a cord-grip cable
clamp but its mounting hole was too
large and it did not grip the cord with
sufficient tension. As a result, the
mains cord has pulled through this
clamp, leaving the mains wires inside
the chassis under a great deal of tension (see photo). In fact, the wires are
under so much tension it’s a wonder
the connections haven’t failed.
Labels & valves
Another view of the dodgy under-chassis wiring. It’s so badly done that there
was no point in restoring the old PA amplifier to working order.
just wasn’t worth the effort. However,
since it was so badly laid out and built,
I decided that it would make a good
article on how NOT to build or wire
such equipment, or any vintage radio
gear for that matter.
So what is so terribly wrong that
I was forced to abandon the restoration? First, as mentioned, the valves
are higher than the transformers and
chokes, so the chassis cannot be upended without damaging these valves.
I have another PA amplifier that I can
easily tip upside-down with no likelihood of damage to the valves or any
other components (see accompanying
photo). However, that’s a relatively
minor point and I could live with this
inadequacy.
It’s the under-chassis wiring that’s
really atrocious. To begin with, there
are no tagstrips on which to mount
any of the small components and this
is obvious from the photographs. Instead, the valve socket pins and some
of the larger chassis-mounted parts
formed many of the tie points for the
smaller components, with the rest of
the components then strung together
in mid-air!
In some rare instances, stringing
components together in mid-air without tag strips or mounting boards may
be acceptable. However, when it is
systematic as in this particular chassis,
it is to be condemned. The leads were
twisted together and then soldered and
96 Silicon Chip
this makes the parts quite difficult to
remove if they become faulty. What’s
more, in some circumstances, the leads
and components could flex far enough
for short circuits to occur.
Many years ago, I had to service
a home-made high-frequency (HF)
transmitter that was wired in a similar
manner to this amplifier. Unfortunately, because of the types of components
used in a transmitter, there were not
many chassis-mount parts that could
be used as tie points. In the end, I told
the owner that I couldn’t service it as
it was and suggested that I rebuild it
– which I did.
I remember that in order to remove
the old, tangled web of floating wiring,
I first disconnected the wires going to
the valve socket pins and to the few
large fixed parts. I was then able to lift
the entire mess out as one complete
tangled clump of wires and small
components. It all reminded me very
much of an inner spring mattress!
Strange microphone sockets
Getting back to the amplifier in
question, the microphone sockets were
fitted to one end of the chassis. However, these were actually octal valve
sockets which are not really suitable
for microphone use. The other end of
the chassis carries a fuseholder, while
the back edge carries another of these
fuseholders, along with the mains
cord entry.
The front panel controls have metallic labels that could be bought from
the radio retailers of the era. These
nickel-plated labels look OK but one of
the three labelled “volume” controls is
actually a 4-position Oak switch! Your
guess is as good as mine as to what this
control actually does.
The valve line-up uses a 6SJ7 pre
amplifier followed by another 6SJ7
which functions as an amplifier. This
then feeds a 6U7G phase-splitter
which in turn drives push-pull KT61s
in the output stage. A 5Y3GT as used
as the rectifier. The grid lead of the
6U7G is not shielded and I suspect the
amplifier may have had some hum in
the audio as a result.
In short, if an item is being homebuilt, it’s important to take the time
to properly lay out the chassis and include proper tie points (eg, tagstrips).
It’s also important to keep inputs and
outputs separated as much as possible.
Valve shields will be necessary in
some cases to ensure stability and to
minimise hum, while shielded cables
are necessary for low level audio leads
and (often) radio frequency (RF) leads.
You also need to take into account
the number of parts to be fitted and
their size, so that everything fits in
the available space. And you need
to ensure that capacitors and tuned
circuits are kept well away from heatproducing sources such as valves and
high-wattage components.
It’s not unusual for the heat from
valves to damage cabinet tops, even
in commercial equipment. This can
occur even when asbestos or some
other material has been used to shield
the cabinet. Asbestos sheets were
sometimes fitted next to the output
and rectifier valves in some early
AWA radios. If you come across this
material, remember that asbestos is
a carcinogen and needs to be treated
with extreme care.
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These two photos show one of my other old PA amplifiers. Unlike the original unit, this PA amplifier can be tipped
upside down for servicing without damaging the valves, the chassis itself providing the necessary protection.
Unfortunately, chassis layout is often a compromise. Achieving the best
layout in terms of electronic stability
and performance sometimes means
that terminals, volume controls, on-off
switches and various plugs and sockets could finish up in inconvenient
locations. These problems can often
be overcome by using extension shafts
on controls or by placing the controls
in more practical locations and using
shielding and/or shielded cable.
This PA amplifier is a great example
of how not to do any of these things,
except that there’s nothing wrong with
the locations of the transformers and
chokes!
Cleaning chassis & parts
Now let’s take a brief look at some
of the techniques used to clean old
radio chassis. Unfortunately, some
of the methods used over the years
have proved to be rather destructive,
both to the chassis and to individual
components. In addition, some chemi-
cal cleaners can be hazardous to your
health if not used correctly.
Several years ago, a friend who is
an industrial chemist put together a
list of safe, non-destructive cleaning
techniques for vintage radios. With
his permission, I have rewritten the
information in a format suitable for
this column.
Both chemical and mechanical
methods can be used to remove the
accumulated muck and corrosion
from our radios. However, in this short
article, I’m only going to concentrate
on the mechanical methods.
It’s incredible what sort of material,
both organic and inorganic, can be
found stuck to old radios and to their
components. Getting the gunk and
corrosion off is often only partially
successful but if cleaning is pushed too
far, it can be quite destructive. Some
component materials react with the
gunk on them while in other cases,
the contaminant can soak in and be
absorbed.
Clearly, if you can identify what
the contaminant is, then you have a
better chance of correctly removing
it. However, it may sometimes be
better not to remove it completely
in order to avoid damage. The same
applies to the chassis – should it be
thoroughly cleaned and repainted or
just superficially cleaned and left in
its weathered state?
That’s a decision that must be made
by the restorer on a case-by-case basis.
Mechanical cleaning methods
OK, so let’s take a look at the main
mechanical cleaning methods:
Brushes: using a brush is one of the
best methods when it comes to removing dust from delicate objects. For
radio parts and chassis, you will need
a set of brushes with soft bristles, eg,
paint brushes. You can also use small
specialised brushes from art suppliers
and hobby shops.
Don’t use stainless steel wire brush
es to clean radio chassis or other parts.
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May 2011 97
The power transformer had been
painted red but there was no paint on
the chassis between the HT chokes
and the power transformer.
They can really do serious damage
(and yes, I’ve seen the results of this
type of heavy-handed treatment).
Vacuum cleaners: vacuum cleaners can be used in conjunction with
brushes, either as dust extractors or
blowers. However, be careful when
using a vacuum cleaner around a radio
chassis. They can easily destroy the
cone of a loudspeaker and possibly
damage other parts as well.
Don’t use a vacuum cleaner in
conjunction with volatile solvents,
as sparking at the commutator in the
motor could ignite the fumes.
Blowers: a speed-controlled airblower is the preferred device for
blowing dust and light muck out of
equipment. Wherever possible, this
should be done outside the workshop,
otherwise the dust will settle on other
equipment.
Using air-compressors to blow out
vintage radio gear is not generally
recommended. A high air velocity can
actually blow some parts to pieces or
worse, blow particles at high-speed
into your eyes if you are one of those
people who neglects to wear eye protection. In addition, the air from most
compressors will contain water and oil
if not heavily filtered.
That said, air-compressors do have
their uses, especially if fitted with a
small hand-held nozzle with an adjustable trigger that can regulate the
air flow.
Damp cloth & water: it’s amazing what
you can shift using just a damp cloth
and water (including valve numbers
and station markings on glass dials if
you are not careful). The new generation of “micro-fibre” cloths can pick
up an amazing amount of dust without
scratching but as always, there are
limits.
Abrasive techniques
Knives, chisels, screwdrivers and
various abrasives (eg, steel wool) all
have their place when it comes to
cleaning up old radios. Of course, serious butchery can be done with these
but sometimes their use is necessary to
get results. It’s just a matter of applying
common sense and a deft touch.
Steel wool, for example, can be used
as a fine abrasive on timber and other
hard surfaces, usually without scratching. It can even clean muck off valve
glass envelopes but again, take care to
avoid type numbers.
Sandpaper, emery paper and wet
and dry paper of various grades are
suitable for cleaning the chassis and
various other hardware items. Start
with the coarser grades and then
progress to the fine grades to remove
any scratch marks. Kitchen scourer
pads are good for cleaning chassis too,
particularly when soaked in kerosene.
Car polish can also be used as a
mild abrasive, as can toothpaste. Alternatively, you can mix baking soda
(sodium bicarbonate) in water to make
a mildly abrasive paste.
Baking soda is also found in some
dry powder fire extinguishers and
was used in the old “soda acid” type
water extinguishers. It is often used
to neutralise acid on lead-acid battery
leads and terminals and as a mould
inhibitor in refrigerators.
Devices which contain lots of brass
and formaldehyde plastic often exhibit
the formation of verdigris within the
device. A sachet of baking soda may
prove useful in preventing this if it
can be fitted inside the device. Note
that the sachet must be permeable, eg,
made of cloth.
Screwdrivers, knives and chisels
can be run along surfaces to lift or
loosen some of the muck. This must
be done with care to avoid scratches
but if the muck is really caked on, this
may sometimes be unavoidable.
In some cases, it may be necessary
to do some restorative work after the
surface has been thoroughly cleaned.
Rust converters and similar chemicals
can be useful here, as can fillers such
as car bog compounds.
In summary, provided you’re careful, the chassis and other major parts
can generally be restored to good
condition. Sometimes, you will need
to combine mechanical cleaning
methods with chemical cleaners but
don’t overdo it otherwise you could
SC
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98 Silicon Chip
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