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Vintage Radio

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.

By Rodney Champness, VK3UG

Recently, 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.

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The hole for the mains cord clamp was too big, so the cord was not securely clamped. As a result, the cord has pulled back through it, leaving the internal mains wires under tension (especially the Neutral lead).
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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 mid-air. This allows the leads to flex and short against other parts.

In fact, during the 1940s and early 1950s, chassis kits complete with top and bottom covers were produced so 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), 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 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

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