How Do You Test A 6000uF cap?

[Paul Marossy]

Someone gave me an old Heath SO-4541 oscilloscope from 1975 which is kind of cool.... but it hasn't been turned on at least since 2005 and it's been in a storage shed here in Vegas since probably about the same time. So I am wondering if the Nichicon 6000uf 16V caps and the Mallory 1uf 1600V caps on the power supply board are any good. I tested the 1uF caps with multi-meter and they seem to be OK as far as resistance is concerned, but the 6000uF caps test as a dead short, which I assume is because rectifier diodes and/or zener diodes are in parallel with them?

Am I on the right path or are those things dry as a bone inside?

[Rob Strand]

and they seem to be OK as far as resistance is concerned, but the 6000uF caps test as a dead short, which I assume is because rectifier diodes and/or zener diodes are in parallel with them?

Hard to know.  It can take a while for a 6000uF cap to charge-up with the DMM.  If it's really shorted maybe it's quicker to pull it out and test it separately. With that out, you could check for other shorts on the board.

[Paul Marossy]

and they seem to be OK as far as resistance is concerned, but the 6000uF caps test as a dead short, which I assume is because rectifier diodes and/or zener diodes are in parallel with them?

Hard to know.  It can take a while for a 6000uF cap to charge-up with the DMM.  If it's really shorted maybe it's quicker to pull it out and test it separately. With that out, you could check for other shorts on the board.

Yeah I thought of the charge time thing. I could also use a 9V battery to charge it up and then measure it with an analog MM and see what the needle does. Aside from simple tests that I can do with a DMM, there is this cool way I discovered this morning (linked below) to check ESR with a function generator, and an o'scope. All these tests require the cap to be out of circuit, so I guess I'll have to pull out at least one cap and see what is up with it and go from there.

It will be interesting to see if caps that have sat for 25+ years with no power applied can still be good. Looking at the construction of the caps it's hard to imagine that they dried out but I guess chemically inside they could have gone bad. Which makes me then wonder if such a cap could be reformed...

[Rob Strand]

Yeah I thought of the charge time thing. I could also use a 9V battery to charge it up and then measure it with an analog MM and see what the needle does.

Worth trying.

there is this cool way I discovered this morning (linked below) to check ESR with a function generator, and an o'scope. All these tests require the cap to be out of circuit, so I guess I'll have to pull out at least one cap and see what is up with it and go from there.

ESR is a good indicator for bad caps.   When the ESR is high it is clear the cap is stuffed.  The issue comes when you get intermediate values.  Some circuits care and some circuit don't.   Then you have to look up the cap ESR specs.  The ESR specs for some caps are no longer or were never published.

I've spent hours trying to put together ESR tables and comparing my tables with the tables on ESR meters.  There's no "right" answer only very broad ball-parks unless you use the spec from that exact cap model and brand.

It will be interesting to see if caps that have sat for 25+ years with no power applied can still be good. Looking at the construction of the caps it's hard to imagine that they dried out but I guess chemically inside they could have gone bad

I have a lot of faith in pre 80's caps.   Some companies seem to put a lot of effort into making sure the seal was good and they last for ages.

[Paul Marossy]

ESR is a good indicator for bad caps.   When the ESR is high it is clear the cap is stuffed.  The issue comes when you get intermediate values.  Some circuits care and some circuit don't.   Then you have to look up the cap ESR specs.  The ESR specs for some caps are no longer or were never published.

So what range do you consider "good" for an electrolytic cap? 5-6 ohms seems to be in the bad range from what I gather.

I have a lot of faith in pre 80's caps.   Some companies seem to put a lot of effort into making sure the seal was good and they last for ages.

Ha ha, yeah... I have an old Tek 453 scope that I think is from late 60s and all the caps in there still seem to still be going strong. OTOH, I'm not sure about those 1600V 1uF caps in this Heath scope. Not sure what they are exactly but they don't seem to be polarized. I think they are paper caps but they have a plastic shell. Not sure how well they have kept their seals. They seem so fragile.

That reminds me a dude on YouTube (shango066) that finds TVs dumped in the desert 40 years ago that have baked in the sun & been exposed to the elements and resurrects them, often with not that much trouble - maybe replacing a few leaky caps or a broken pot or something. Amazing what kind of abuse some of those old TVs could take. The Zeniths seem to have fared the best.

[Rob Strand]

So what range do you consider "good" for an electrolytic cap? 5-6 ohms seems to be in the bad range from what I gather.

It depends on the voltage and the capacitance value.  It also depends if the cap is low ESR or not.

These tables can be used as a *guide*.   They apply to *normal caps* more than low ESR types.  One shows good, bad, and grey regions. The other shows a voltage dependency; I believe the values  represent the upper limits for good (hence the green coding).

I've probably got 15 such tables and they have disagreements.  You shouldn't read much too much into them.   It's virtually impossible to create an ideal table.  You end-up with bad values being ridiculously high and good values ridiculously low with a massive grey area separating the two - technically correct but practically useless!.

I have an old Tek 453 scope that I think is from late 60s and all the caps in there still seem to still be going strong.

They are really great units.

1600V 1uF caps in this Heath scope. Not sure what they are exactly but they don't seem to be polarized. I think they are paper caps but they have a plastic shell.

I'd expect maybe 0.1uF's?  Maybe the dot is hard to read?  You get all types of caps in that part of the circuit so I'd only be guessing.   Electro's at that voltage were normally made-up from a series connection of 350V to 450V units.

That reminds me a dude on YouTube (shango066) that finds TVs dumped in the desert 40 years ago that have baked in the sun & been exposed to the elements and resurrects them, often with not that much trouble - maybe replacing a few leaky caps or a broken pot or something. Amazing what kind of abuse some of those old TVs could take. The Zeniths seem to have fared the best.

Yes, it is pretty amazing.     In other cases someone drops something in water and it never works again despite putting it in rice and all that stuff.

[EDIT: One caveat,  the tables come from meters which test at say 100kHz (Bob Parker's unit might have used pulses) for the smaller caps (<1uF)  the increased value can be caused by the caps impedance XC instead of the actual ESR.]

[Paul Marossy]

You're right, those 1600V caps are 0.1uF, so I probably missed the dot on the one I looked at (and I didn't notice that on the schematic either).

In any case, I pulled the big electros from the power supply board to check them and they hold a charge so I think they will actually be OK. Thanks for the ESR chart. I still need to check the ESRs on those big caps but I don't expect them to be way off, or at least I hope they aren't. I'm not sure what type those 0.1uF caps are, but I'm guessing that they're probably OK too. Or at least they don't appear to be shorted.

[Rob Strand]

In any case, I pulled the big electros from the power supply board to check them and they hold a charge so I think they will actually be OK. Thanks for the ESR chart. I still need to check the ESRs on those big caps but I don't expect them to be way off, or at least I hope they aren't.

That's a good sign so they are probably doing something.     The capacitance can shift.  You can see how long it takes to discharge through your multimeter to estimate the capacitance.

BTW,  in some circuits even though the ESR is technically high the circuit still works.     In other cases where people have used the wrong caps you can have in-spec ESR but the circuit doesn't work - this is more for switchmodes, pretty much never occurs on 70's designs.

[reddesert]

How Do You Test A 6000uF cap?

With your tongue?

[Paul Marossy]

How Do You Test A 6000uF cap?

With your tongue?

The darn leads are too far apart 

[Rob Strand]

The darn leads are too far apart 

Where there's a will there's a way 

Wasn't there a short clip on MTV about tasting electricity?

[Paul Marossy]

The darn leads are too far apart 

Where there's a will there's a way 

Wasn't there a short clip on MTV about tasting electricity?

Don't know about the MTV thing but I just thought of how to "tongue test" - clip leads! I'll get back to you on that