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Can BP caps be used for polarized caps?

N

namvet

New Member
Just finishing up re-capping an old Marantz 2238B. But I ran into two apparent errors on the board markings and component location pictures in the SM - which led me to wonder if non-polarized (BP) caps can be used to replace polarized caps?

C701 on the main amp board is marked for polarity one way, but the original cap was installed opposite. I noticed this reversal while removing the original cap, and confirmed it by checking a digital photo I'd taken of the board before starting (the neg. side was on the positive marking). It's twin at C702 was installed opposite, with the positive side of the cap aligned with the "+" mark. I installed the new cap at C701 the way the old cap was aligned - reversed from the marking.

My question: Could I have simply used new BP caps in both C701 and C702 as substitutes for the original polarized caps?

Also, at C307 on the tuner board, I found an original BP cap in that spot which was marked with a "+" for a polarized cap on the board (and in the SM). I left the original BP cap in that spot for now until I can get a BP replacement, but wondered what would happen if I tried a polarized cap in that spot at C307?
 
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C701 and C702 should really be BP caps because there is really no DC voltage on either end. There is a possibility of a small voltage on the base of Q701 but it could be either negative or positive, depending on the input offset current on the diff amp. Since it is not known what voltage, if any would be there it is better to have a non-polar cap there. C307 can be a polarized cap with the + side connected to pin 11 of Q302, since the voltage is more positive on that pin than pin 10.
 
It's generally OK to replace a polarized electrolytic with a bipolar electrolytic ONLY if there is no DC component present across the cap. If DC is present, the bipolar cap will be short-lived.
 
Is it feasible (and safe) to touch the probes of a volt meter to both legs of a cap while the unit is powered on to check both the polarity and the actual voltage across those two points?
 
namvet said:
Is it feasible (and safe) to touch the probes of a volt meter to both legs of a cap while the unit is powered on to check both the polarity and the actual voltage across those two points?
Click to expand...

That depends on the voltage in the circuit at the measurement point, but you are not sure of the voltage is why it is being measured.

That and your own body capacitance/ resistance can affect the reading.

It is safest to use the hook-on style probes that way your hands are nowhere near the circuits and there is less risk of slipping and shorting something out.
 
namvet said:
Is it feasible (and safe) to touch the probes of a volt meter to both legs of a cap while the unit is powered on to check both the polarity and the actual voltage across those two points?
Click to expand...

The measurement of interest is the DC voltage on the input leg referenced to ground (i.e., +ve probe on cap lead and -ve probe on chassis).

Note that you can use film capacitors to replace polarized capacitors in the signal path.
 
hirscwi said:
It's generally OK to replace a polarized electrolytic with a bipolar electrolytic ONLY if there is no DC component present across the cap. If DC is present, the bipolar cap will be short-lived.
Click to expand...

Not true. A NP electrolytic cap is really 2 polarized caps of opposite polarity, in series. Because they are in series, no DC is present across either cap so they can't be damaged.
 
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Generally yes you can use a non-polar, as long as there is space. They're often physically larger than a polar cap.

For what it's worth, I've "made" NP caps for AC use out of a pair of polarized wired back to back, negatives together. Its the motor run cap in an old radio I own that uses a small clockwork mechanism to operate the volume control. Its been fine for several years now. A regular polarized likely would have lasted about 2 minutes in that job.
 
I put a 0.22 mfd. 50V Nichicon FG into that C307 spot with the + leg to pin 11 as per Dr*audio's suggestion. It's working fine so far. I've got two BP caps coming for C701 and C702.

Thanks to all for the good information and advice.
 
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