capacitor multi can question

[R.G.]

I can't think of a good way to test high voltage electros for polarity that's guaranteed not to harm it. Maybe some kind of pulsed voltage-current test. The discharge curve timing and oscillator sensitivity tricks might work. I'll have to go look.

The problem is that the same current you'd sense for "oops, conducts that way!" is eating away at the insulating oxide while it flows. On a new, good cap, that might re-form in the weak spots that are conducting current to do the test. Limiting the current limits the damage, so the low-current ways like discharge curve timing and oscillator sensitivity (which ...have... to work by the varying capacitance and resistance of the thing while conducting) will do less damage to be repaired or ignored.

Enzo has the right idea: figure it out from the circuit itself. There's nearly always a diode somewhere making a great polarity indicator for you. It's just sometimes tough to work your way back to the diode.

I have seen some few of these kinds of caps. "Can com pos" means "can common positive".

[Randall]

So, did I do a lot of damage to the new caps when they got hot? They are a JJ dual 100/100 can and two F&Ts tied in to make it a quad. They all measure to with 3 % on my meter. No more ripple on the negative supply than I would expect. And the amp sounds fine.

And now I know about this "can com pos" thing too.

Steve, you don't know how much better that makes me feel!

[R.G.]

If they are no longer getting hot when you have them inserted correctly, and they are functioning correctly, then they will self-repair up to the limits of the voltage normally on them. This is probably good enough not to sweat it for new caps you've put in. I have the engineer's approach to this - if it works, it must be OK. The more hours you clock on them, the less you hurt them.

As an aside, the capacitance value for electros is probably the last thing you should worry about. Electrolytic caps are much better than they used to be; filter caps were often sold with +80/-20% tolerances. Within 3% is GREAT - but not too useful in the applications to which electros should normally be put. And point-source leakage issues don't affect enough of the cap's plate area to change the measured capacitance much, so measuring the capacitance doesn't help much in this little contretemps.

[Gingertube]

Steve Connor wrote:
"PS: Does anyone know what the can of a non-polar electrolytic is connected to? I think the last one I saw had rubber bungs in both ends like a 600V Atom."

Non Polar Electrolytics consist of 2 x "ordinary" polar electrolytics of twice the value wired back to back in series. It is generally the 2 positive connections which are connected together as the "centre" connection and it is the two negatives which are the external connections. It is unlikely that the can is connected to any of these 3 points.

Cheers,
Ian