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Old 8th Apr 2018, 4:05 pm   #1
martin.m
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Default HMV radiogram 1609, severe mains hum (solved)

I love 1950s radiograms and don't like "scrapping" them but we don't have space for any more of them in the house (My wife's view). I decided to save the complete chassis with the front panel and tuning scale to build into a cabinet. The radiogram had been powered up by the previous owner who said it worked but had "a bit of a background buzz". On switching on, within a few seconds there was a loud 100Hz hum from my test speaker. The noise became louder and louder as the valves warmed up.

This model uses a U84 full wave valve rectifier. The smoothing circuit is simple and consists of 16uF + 8uF capacitors in the same can, with a 430 ohm resistor and a choke wired in series between the two capacitors. HT is then fed to the audio stages, IF amp and mixer/oscillator stages without any further smoothing. As a temporary measure I connected a modern 22uF 450v capacitor from the rectifier cathode to earth. This reduced the level of hum but it was still very loud. Another 22uF 450v was connected in parallel with the final smoothing capacitor. The level of hum reduced but was still unacceptable. I didn't have a spare rectifier or KT81 output valve to try so considerable time was spent head scratching. Then I did what I should have done in the first place, disconnect the original smoothing can while leaving the new capacitors in place. Result, lovely silent background and faint distorted sound could be heard. Obviously the original smoothing can had suffered an internal short between the two positive terminals which had applied raw HT from the U84 cathode directly to the audio stages.

I fitted a period smoothing can, changed some of the waxy capacitors and a couple of 220k resistors feeding the anode of the DH49 audio amplifier triode. When switched on, the set sprung to life giving loud clear sound and I was able to tune in Radio China International on one of the SW bands. Now to brush up on my woodworking skills.
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Old 8th Apr 2018, 8:38 pm   #2
G6Tanuki
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Default Re: HMV radiogram 1609, severe mains hum (solved)

Rather than an internal short, it's possible that the twin capacitor has lost its 'common' connection: I've seen this a couple of times - usually it's caused by the thin strip of aluminium that's riveted to the earth tag then interwound with the capacitor foils having rotted away inside the can.

When this happens you get no smoothing effect - the 2 capacitors are essentially then working in series as a 'bypass' round the smoothing resistor or choke.
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Old 8th Apr 2018, 11:32 pm   #3
martin.m
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Default Re: HMV radiogram 1609, severe mains hum (solved)

Yes, that sounds a more likely explanation. The hum was still there with the KT81 output valve removed from it's socket. The ripple current must have been flowing through the audio output transformer primary winding via the tone control circuit. I have never come across this type of fault before. Incidentally, the Gram / Radio switching seems complicated on this model. In the "Radio" position extra top cut is applied to the audio, negative feedback is disabled and the output valve cathode bypass capacitor is taken out of circuit. I doubt it made much difference.
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