Capacitor testing

[Guitarzan]

OK guys, I did go thu the information pages and tried to find infpo on this subject and searched and everything but no luck.
The questions is, can you test a capacitor, like the filter caps from a tube amp or coupling caps or whatever, using a digital multimeter or do you need a special tool for testing caps?
The reason I ask is that I copped an old Bogen PA 100watt version with the weird power tubes that don't have a "micanol" base. Since no one decided to take me up on nabbing any of the old twin reverb parts I offered for free, I thought I might reuse them in the Bogen since they are of the same value but look a heckuva lot cleaner and more viable than the ones on the old Bogen. That Bogen amp is prolly at least 20 years older than the Twin was so I just wondered if there's a way to test the caps before swapping them out just to find out they're shot.
Please and thanks as always. I hope everyone is doing well and are all set for Christmas.

[PRR]

You really need a special rig with HIGH voltage. A fancy multimeter may tell you that you have 39uFd, but it probably tests with less than one Volt. OK, until you put 300V on it, and it blows-up.

While I could start describing a cap-tester, there's many pitfalls which may not be obvious until a few failures.

At present cap prices, it is probably best to just buy new. Save the maybe-good caps for emergency hail-Mary repairs.

[eleventeen]

Agree with PRR. I've been kind of tinkering with building a cap reformer/tester which has gone into "shelf" mode.

The basic format is: 450 volt power supply powering (in my case) a series stack of gas regulator tubes. OD3 (150 volt) 0C3 (105 volt) 0B3 (90 volt) and 0A3 (75 volt) One could just as easily use a variac or any other means of making a simple DC power supply. It only has to supply maybe 15 mils. Me, I wanted to use my pile of regulator tubes because they glow nicely. These tubes want between 5 and 30 mils of current and NO MORE. 20 mils is a good design-center. So current limiting resistors are essential. Anyway, the stacked-up tubes create a voltage divider of +75 volts, +180 volts, +285 volts, and +435 volts. They all have the same basing diagram so I can change them to get various voltages. Coming from the junctions of the tubes is yet another resistor calculated to limit current to 5 mils. These can be fed through a 10-ma full scale meter. This was completely a junk-box project, and I had minimal tools at the time so I wanted a project that didn't demand great neatness.

The idea is that one can hook up caps of various voltages and view the steady state current once they charge up. A good capacitor, I have read (but I have to check this) should charge up and in a perfect world would draw no current. In reality, an electrolytic will draw 2-3-4 mils. The less, the better. [Google: reforming electrolytic caps] It's of course also desirable to 1: place the cap in something or other (like a cut-off plastic juice bottle) such that, in case it blows up, it doesn't blow tar and pitch all over the place, and 2: provide a bleeder resistor so that you don't zap yourself. The bleeder current has to be figured into the steady state current of the cap-under-test, as well. You want to run the caps for a good number of hours, perhaps even days, in the reforming process. And you want to know they are good before you put them into anything you want to work and stay working.

I ought to get this finished, it's 90% done and I have a metric ton of caps to test and/or reform. 

[stratele52]

I used Heatkit IT-28 Capacitor Checker . It test coupling cap and electrolytics caps and it can go to 600 VDC

Many other ( if not all portable) meter are assumed to check the capacitors, but in reality only read their value. This does not mean that work . Electrolytics like filter caps need some high voltage to  discharge and leakage tests.

The Heatkit do all that.

I also have the portable Meterman LCR 55 wich only read vallue

[jjasilli]

I use an Eico 950B Resistance-Capacitance Comparitor.  IT goes to 500V

[stratele52]

I use an Eico 950B Resistance-Capacitance Comparitor.  IT goes to 500V

Look same or pretty close from the Heatkit. Those two companies made very good test equipment 50 years ago .

[eleventeen]

Are those Heath/EICO things capable of reforming? Those pieces are generally available for $10-$15 used. If they can hit the cap with big volts and stay on for  days without overheating, they should work.

[stratele52]

Are those Heath/EICO things capable of reforming? Those pieces are generally available for $10-$15 used. If they can hit the cap with big volts and stay on for  days without overheating, they should work.

Yes Heatkit can , I used some time. If you can buy it for $15 buy some.

[Guitarzan]

WOW!
Amazing stuff guys. Way over my head. If I can't stick a gator clip on each end and look at the readout, forgetaboutit.
So...PRR & Eleventeen get the cheroots.
It's just that those big old 100s that came out of the twin reverb filter cap board look so clean and are identical to the 100s that in the bogen. The caps in the bogen are obviously no good with crap oozing out of the ends or the paper all nasty and looking oil soaked. Some of the smaller ones are sort of bulging too and so I don't trust much of anything in it as it pertains to capacitors.
I suppose new caps are a lot cheaper in the long run than blowing up a suspect cap and having it take other components with it. Not to mention the time involved but I don't usually count the time tho since it's either dink around with my amps or drink beer & watch old movies on TCM. Of course if the movie happens to be Lawrence of Arabia or Gunga Din, Red River, To Have & Have Not, or a dozen other classics I could spit out then I'll take the movie every time but they run so many crappy ones that it's often completely unwatchable. Anyway, I need to get some of these older pieces I have piling up all over the place up and running so I can peddle them and generate funds for the twin. I still need legs, a reverb tank with cables, and a handle. I could also use a bvlackface era grill cloth. The tolex on the old cab is cleaning up so nicely, so much better than I ever expected, that it makes the grill cloth look out of place now. I guess there are worse problems to have eh? Very sincere thanks to all who responded. Thank you for taking the time guys. I truly appreciate it.
-dale {8^)

[DummyLoad]

i use a sencore lc75. digital readout with cap leak testing up to 600V. switch for electrolytic (10kuA) and coupling (100uA) types. 

--pete

[jjasilli]

If I can't stick a gator clip on each end and look at the readout, forgetaboutit.

The old, er, I mean vintage, test equipment forces one to take a step back and formulate at least a minimal depth of understanding of just what the heck it is that you're doing!  A lot of guys started there and may now take it for granted.  But for newcomers like me it's a valuable learning experience.  Basically our guitar amps are 1930's technology improved in the '40's.  

[PRR]

> at least a minimal depth of understanding....Basically our guitar amps are 1930's

[kagliostro]

Thanks PRR 

K

[stratele52]

i use a sencore lc75. digital readout with cap leak testing up to 600V. switch for electrolytic (10kuA) and coupling (100uA) types.  

--pete

I did not know this test equipment , look great . But very expensive ( $450 ) if ''eleventeen " write he can buy Heatkit or Eico for $15

[jjasilli]

Steve Bench shows how to build a DIY inductance tester.  I think it's here:  http://www.jacmusic.com/techcorner/SBENCH-PAGES/sbench101/#Inductance

[SoundmasterG]

Another option for testing caps for leakage is a Sprague Tel-Ohmike. I have one and it goes to 600v, can check value, ESR, leakage, etc. You can find them for not too bad on ebay,

To the OP, is your Bogen the CHB100? If so I may be interested in it if you are looking to sell it... I'll send a PM.

Greg

[stratele52]

Another option for testing caps for leakage is a Sprague Tel-Ohmike. I have one and it goes to 600v, can check value, ESR, leakage, etc. You can find them for not too bad on ebay,

 
Greg

I did not this test equipment , another like the Eico and heatkit .

Thank's

[SoundmasterG]

Another option for testing caps for leakage is a Sprague Tel-Ohmike. I have one and it goes to 600v, can check value, ESR, leakage, etc. You can find them for not too bad on ebay,

 
Greg

I did not this test equipment , another like the Eico and heatkit .

Thank's

Yeah, here is some info about them...

Greg

http://www.ohio.edu/people/postr/bapix../CapChkrs.htm