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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 16th May 2015, 9:53 pm   #1
Busman1950
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Default "That Cap"

Am I being thick or can someone PLEASE tell me what "THAT CAP" is.
It's driving me nuts.........
I've been repairing sets for years without realising that there is a monster lurking in the dark corners waiting to destroy all my hard work.
Thank you all for not laughing at my ignorance.
Bob
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Old 16th May 2015, 9:59 pm   #2
Station X
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Default Re: "That Cap"

The capacitor connecting the anode of the audio amp valve to the grid of the output valve. It has a high voltage across it which makes it prone to failure.

If it fails it may take the output valve, output transformer and mains transformer with it.
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Old 16th May 2015, 10:32 pm   #3
Boater Sam
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Default Re: "That Cap"

It is really a monster as you say. The anode of the 1sr audio amp has a high positive DC voltage on it.
The grid of the audio output valve needs a small negative control voltage to amplify.
Any leakage in THAT coupling capacitor puts a positive voltage on the control grid causing over dissipation in the valve, the excessive current ruins the valve if you are lucky. Or it burns out the output transformer and possibly the mains transformer if its your worst day.
If the control grid has a positive voltage relative to the chassis, it is usually the capacitor, but some output valves have a reputation for internal leakage causing the same problem. The UL41 is a case in point.
So even if the set works well, it is always a good idea to check the grid voltage.
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Old 16th May 2015, 11:14 pm   #4
kalee20
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Default Re: "That Cap"

Yup.

The majority of valve radios have a triode (or pentode) audio amplifier which drives the output valve via a coupling capacitor. And the capacitor is usually a paper dielectric jobbie, which over time goes leaky.

A little leakage in most capacitors does no real harm, but this is the exception! Leakage makes the output valve draw too much current. Distortion is often a result, and long-term the output valve, the rectifier valve, the output transformer, and the mains transformer could suffer.

So, most radios have 'that capacitor' and usually it needs replacing. That's why it is called 'that cap."
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Old 16th May 2015, 11:50 pm   #5
high_vacuum_house
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Default Re: "That Cap"

The real nasty with this cap (usually a wax paper or Hunts mouldseal) is that the radio can work perfectly well when it is leaking, only really giving distortion and/or a poor sound. It by itself rarely stops a radio from working altogether unlike say an O/C dropper resistor or dead electrolytic causing hum.

The radio can then carry on working without issue until the output stage has been well and truly cooked and transformers or output valves fail.
Christopher Capener
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Old 22nd May 2015, 10:28 am   #6
Busman1950
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Default Re: "That Cap"

Thank you all for clearing up this up, it's very reassuring that help is only a few strokes of a keyboard away.
I'm very grateful to you all.
Bob (Now a little less stressed) Hall
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