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Old 23rd Jun 2021, 9:25 pm   #1
EF80TVVALVE
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Default 500pF variable condenser

Hi all,

I'm looking for a 500pF variable capacitor that is used for the volume control on a HMV battery receiver. The one fitted has a short and needs changing, it has quite a long stem (approx 40mm) to reach through the cabinet so the replacement may need to be similar really to ensure a good fit.

Cheers!
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Old 23rd Jun 2021, 9:32 pm   #2
Aub
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Default Re: 500pF variable condenser

Variable capacitor as a volume control? It's usually a potentiometer.

Cheers

Aub
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Old 23rd Jun 2021, 10:55 pm   #3
Skywave
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Arrow Re: 500pF variable condenser

I suspect he means 500 kilohms.

Al.
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Old 24th Jun 2021, 7:58 am   #4
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Default Re: 500pF variable condenser

Possibly an early 30's TRF! Mick.
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Old 24th Jun 2021, 9:13 am   #5
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Default Re: 500pF variable condenser

Yes I did wonder about that. I suppose a reaction capacitor could alter the volume as the sensitivity increases?

Aub
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Old 24th Jun 2021, 9:23 am   #6
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Default Re: 500pF variable condenser

Which model is the set? It'd be interesting to see the circuit. VC as volume suggests reaction control maybe?
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Old 24th Jun 2021, 9:34 am   #7
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Default Re: 500pF variable condenser

I'm puzzled by what is on the right-hand side of the picture, it doesn't look like a variable capacitor, but then it isn't concentric with the shaft so doesn't look like a potentiometer either!

Andy
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Old 24th Jun 2021, 9:54 am   #8
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Default Re: 500pF variable condenser

Hi all,

The set is a HMV 148, I'm just going off the service data but the control knob is also marked 'vol'. I think Chris (Herald1360) is right in that it's a reaction control used to increase the volume. Strange in this set that the HF valve filament is controlled via a variable resistor to increase/decrease sensitivity, presumably to stop oscillation when the volumecontril is turned up to higher levels.
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Old 24th Jun 2021, 10:14 am   #9
Herald1360
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Default Re: 500pF variable condenser

VC3 in the HMV data is the reaction control. If that's labelled volume on the set then mystery resolved!

Is it just a basic solid dielectric type variable condenser?

Extending a spindle shaft shouldn't be too difficult, even some heavy duty adhesive tape might do though drilling and tapping the shaft and extension M3 (or for historical verisimilitude, 6BA) and using a piece of studding (or a sawn off screw) and some threadlock would be nicer engineering wise.
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Old 24th Jun 2021, 1:33 pm   #10
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Default Re: 500pF variable condenser

Hi Bren, they were usually a Dilecon type of cap and can often be dismantled and cleaned

Ed
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Old 24th Jun 2021, 2:46 pm   #11
EF80TVVALVE
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Default Re: 500pF variable condenser

Thanks all, I'll have a better look tonight and see if it can be repaired, it's really dirty and hasn't been stored greatly over the years so who knows, it could be all of that muck making it short out.
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