My buzzing radio

[Simpo Two]

My veteran Grunding radio recently developed a fault whereby it makes a loud buzzing sound - mains hum I think - whenever it's on. However if I touch the aerial the buzzing stops. Something called hand capacitance I believe. Is there anything I can clip/stick/attach to the aerial to kill the buzz? I tried an earth wire but it made little difference.

I scoured the net for a decent radio to replace it but you can only buy nasty little DAB things.

[Simpo Two]

Sound logic but what faradage and what do you connect the other end to?

[Simpo Two]

Sound logic once more!

NB That should be Gruding not Grunding.

Goddammit, GRUNDIG is really hard to spell!

[Simpo Two]

Thanks - it started after I gave the sliding volume control a good 'sliding' as it was getting noisy. I don't know if that broke the pot... I now get a loud buzz and very faint sound regardless of the position of the slider or radio. It's an Elite Boy 700, which sounds horribly gay...

[Simpo Two]

OK, we know the problem now. The volume control is a sliding 100K dual gang pot and the little metal spring contact between the knob and the tracks has broken off. It's a bespoke piece of hardware 58mm long, 6mm wide with (I think) a travel of 49mm.

My boffin writes: 'The volume control pot has an extra function as you can see from the circuit diagram. The Grundig pot has four connections where normally you would expect to find three. It is unique in that it has a tapping point (the fourth connection) along the resistance of the pot. There is only one slider. This arrangement is I believe, due to some tone balancing circuit and no doubt very clever for its time. Look at R102 and how C68 taps into it approx half way up (lower centre of diagram).'

Circuit diagram:



I need a pot of the same dimensions/spec to replace it, but I fear that's impossible. Closest I can find it http://uk.mouser.com/Passive-Components/Potentiome... but nothing is quite the same.

There's a very good Elite Boy 700 on eBay now but it's 100+ miles away and collection only. Another option is to mount a rotary pot somehow, maybe on top of the now empty slot.

If anyone has any ideas on a replacement sliding pot I'd be very grateful.

[Simpo Two]

Excellent work Mike, thank you. It looks like there's no suitable sliding pot so the plan is to fit a rotary dual gang instead, and have a little knob where the slider was.