My buzzing radio
Discussion
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.
I scoured the net for a decent radio to replace it but you can only buy nasty little DAB things.
Dear ST,
basics, probably grandmother-egg-sucking tutorial but best to check
Can/have you moved the radio, does the hum change?
Is the earth (if any) still connected, check plug and continuity to radio?
Is the radio valve or trannie?
If it is valves then be extra careful if you go rooting around inside, there's lots of VVV!
If you do put a cap. across the antenna then 22pF is a value to try. I'd expect it to reduce the sensitivity of the radio though.
You're fixing a symptom not the cause. I'd expect as it's old a capacitor at another point in the circuit, maybe power supply, has died,
regards,
Jet
basics, probably grandmother-egg-sucking tutorial but best to check
Can/have you moved the radio, does the hum change?
Is the earth (if any) still connected, check plug and continuity to radio?
Is the radio valve or trannie?
If it is valves then be extra careful if you go rooting around inside, there's lots of VVV!
If you do put a cap. across the antenna then 22pF is a value to try. I'd expect it to reduce the sensitivity of the radio though.
You're fixing a symptom not the cause. I'd expect as it's old a capacitor at another point in the circuit, maybe power supply, has died,
regards,
Jet
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...
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.
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.
It is a pleasure to see the schematic from an proper, decent quality radio
The fixed tap on the potentiometer is to provide an automatic "loudness" control. As you reduce the volume, the wiper approaches and then passes the fixed tap point and this boosts the bass response (or rather, attenuates the lower frequencies less).
This shows how the response changes as you adjust the volume (lower traces=lower volume).
If you fit a potentiometer without this tap the radio will still work but you will lose this loudness function, so the output may sound a little flat and lacking in bass at low volume settings. It may be possible to emulate this function with a stereo pot (if you could find one that fits) coupled with some circuit changes.
The fixed tap on the potentiometer is to provide an automatic "loudness" control. As you reduce the volume, the wiper approaches and then passes the fixed tap point and this boosts the bass response (or rather, attenuates the lower frequencies less).
This shows how the response changes as you adjust the volume (lower traces=lower volume).
If you fit a potentiometer without this tap the radio will still work but you will lose this loudness function, so the output may sound a little flat and lacking in bass at low volume settings. It may be possible to emulate this function with a stereo pot (if you could find one that fits) coupled with some circuit changes.
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