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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 10th Jan 2017, 12:10 am   #1
TNC
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Default Reversible capacitor

Have been asked to take a look at a 1948 Pye 6volt car radio. I have found the schematic in a volume of the red Radio and TV servicing books but am puzzled by the description of two of the capacitors in the circuit....they are noted in the component listing as "10 Reversible (15V)"

I'd be very grateful if some kind soul could tell me the modern description of that capacitor. Given the low voltage rating I am wondering whether it's similar to the metallic cased non-polarised capacitors I have seen which nevertheless had a red and a black end?

Thanks, Trevor.
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Old 10th Jan 2017, 12:22 am   #2
Argus25
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Default Re: Reversible capacitor

Possibly they are 10uF 15V NP (non polar types). These are used when the circuit configuration could at times have two opposite average polarities across the capacitor. The times that this crops up can be in radio AGC circuits where with no signal, the polarity is in one direction and with signal it reverses, in transistor radios. Electrolytics don't like reverse polarity. One trick I have seen to make a NP electrolytic is to wire two in series, back to back, though not as good as a single NP part probably. Or maybe since this is a tube based car radio , it could be configured for + or minus ground which is why they used "reversible capacitors". All that is a guess, it would be good to see the schematic and see where the capacitors were and the polarities they could be exposed to. It could just be as you suggest, that the two capacitor terminals are isolated from the aluminium case, but some are like that anyway, but not all.
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Old 10th Jan 2017, 12:55 am   #3
Nicklyons2
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Default Re: Reversible capacitor

If the component are indeed 10uF 15v, personally, I would replace them with modern non-polarised types. Polyester 10uF at voltages 63 and less are only 'liquorice allsort' sizes which are probably no larger than the original electrolytics and will give far less trouble.
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Old 10th Jan 2017, 11:24 am   #4
MotorBikeLes
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Default Re: Reversible capacitor

For a 1oUFd reversible capacitor (most frequently met in audio coupling but also in some TQ vert amps etc.) the normal procedure in the absence of the correct replacement would be to use two 22UFD in series, but with opposing polarity.
Les.
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Old 16th Jan 2017, 1:09 pm   #5
TNC
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Default Re: Reversible capacitor

Thanks very much for your advice/comments.
I ought to have mentioned that the component sits at the "in volts" side of a
6volt vibrator HT generator.

I managed to find an addtional and more detailed parts listing which specifies it as "10mf Elec reversible 15 volt" ...and I wonder if the reversible is more to do with whether the unit is + or - earthed.

So far I have replaced it with a 10mf Elec rated at 450 volts...so will find out if that's OK at power-up!

Trevor
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Old 16th Jan 2017, 8:06 pm   #6
TonyDuell
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Default Re: Reversible capacitor

Can you give a model number for the radio, please. I have all the early R&TVS books and
could look up the unit to see why the capacitor should be non-polarised.
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