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Vintage Radio

Last month, we discussed the Hotpoint Bandmaster J35DE in general terms. This month, we describe how it was restored to its original performance. The only instrument required was an inexpensive digital multimeter and the same general ideas can be applied to most vintage sets.

By Maude Findlay, MIE Aust, VK2PW

Many of the articles on vintage radios in these columns give some of the details of restoration but rarely would a set which looks potentially good on initial inspection turn out to require so much work to restore it to its original standard of operation. And while some people might simply turn the set on and hope for the best, that is not likely to be successful in many cases.

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These faulty parts all had to be replaced in the old Hotpoint Bandmaster radio. Most are capacitors but there are also quite a few resistors, a couple of dial lamps and the 6J8G mixer valve.
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This under-chassis view shows the radio after the above parts were replaced. It's normally fitted with a perforated steel cover.

The starting point with this set was the power cord. It was originally fitted with a twin-lead conductor power flex which is not deemed safe these days, especially when a 60-year old power transformer is being used. It might be OK for the present but that cannot be guaranteed.

Accordingly, a 3-conductor flex was fitted, with the chassis correctly connected to mains earth for safety. The power cord was securely anchored with an IP68 cable gland and the green earth wire terminated to the chassis with crimped lug, screw, nut and lockwasher. This works well although using a cable gland may not be an approved method when it comes to anchoring mains cords.

The next step was a resistance measurement of the primary of the power transformer. Measured via the power plug pins it was about 50Ω and from the plug pins to the chassis it was a large number of megohms. So that was OK but the set has a double-pole rotary power switch operated by one of the front-panel knobs and it seemed very tired. Turning the switch backwards and forwards produced an occasional flicker on the meter but not the original 50Ω reading. So it had to be replaced but obtaining the same switch was impossible.

A used switch potentiometer with a double-pole switch rated at 240VAC 2A was found and fitted as a replacement but its shaft was too short. This was extended using a short section of shaft from another pot. They were joined using a threaded bush from yet another pot, the whole lot being glued together with JB Weld epoxy adhesive.

Terminating wires to the switch was yet another hurdle. The solder tags on the switch pot are of thin sheet metal and not designed to take the strain of stiff wires with mains insulation. For this reason, the mains wires were extended with flexible hook-up wire which was in turn covered with thick plastic tubing.

Fortunately, the original volume control, which is separate from the power switch, was quite usable.

Then we come to the valves. A natural tendency among these new to radio restoration is to pull out the valves and wipe away the dust and grime but this can be a real trap since it is so easy to clean off the label marking. Then how do you identify them? Four of the valves in this set are of similar size and have no connection to a top cap, so it would be easy to mix them up.

So before pulling any valves out of the chassis, do a quick diagram showing the location of each valve and its type. Then put a sticker on the base of each valve and label it as well.

Turning the chassis upside down is another hazard because it needs a rear support to stop it from resting on one of the IF transformers. A length of angle bracket bolted to the back of the chassis provided the necessary support.

Then we could have a detailed look at the components underneath. One manufacturer produced paper capacitors in a black plastic which melted at soldering temperature. Servicemen in the 1950s referred to them as the “black death”. It was expected that most of these would be leaky. Surprisingly though, most of the capacitors were OK, both with regard to leakage and capacitance, except for a couple where the ends broke off when the multimeter was connected!

Ultimately though, most of the paper capacitors were replaced with modern metallised polyester types (greencaps etc) as the leads of the originals were so fragile.

The resistors were carbon composition, most about 35mm long and 6mm diameter, and were probably rated at 1W dissipation. Measurements showed that most of the resistors were high in value, some by a factor of two to one but the 325Ω V4 cathode resistor and the 50Ω resistor for the back bias circuit measured both very close to their marked values. They appeared to be wirewound types.

Electrolytic capacitors

Electrolytic capacitors in old valve radios usually have a high leakage or if not, they have dried out and have low capacitance. Still, replacements are available from a number of suppliers.

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