How to test caps for leakage

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by texwest, Aug 3, 2008.

  1. texwest

    texwest Member

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    I was once told how to test a capacitor "in circuit" for leakage. It involved lifting one end and then testing for AC? Can you guys tell me how to do it? I've got a couple really old amps i want to refurbish. One is an old Ampeg Mercury i picked up for a couple hundred. Its kind of a poormans tweed tremolux.
    Thanks ahead of time

    Wes
     
  2. brad347

    brad347 Member

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    Almost.

    You measure DC voltage on each end of cap in-circuit. One end will have lower volts DC on it with respect to ground (often one end will be grounded and have zero volts).

    Then turn the amp off, drain the caps, lift that end. Turn the amp back on, and measure DC voltage at that (lifted) end with the amp running. Be extremely careful!

    If you measure more than half a volt or so DC leakage, I'd replace it.
     
  3. embot

    embot Member

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    Unless your amp has collectors value, it will probably be a lot simpler to just replace all the electrolytics in the amp.
     
  4. brad347

    brad347 Member

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    we're not talking about electrolytics (which I would recommend replacing even if your amp DOES have collector value-- it will have more collector value with replaced caps and original output transformer than it will with original, shorted caps and a blown OT).

    We're talking about non-polarized coupling and tone capacitors. These can also become leaky on occasion, and when they do, they can throw off the bias of the next stage in the amp, put DC on pots (making them scratchy), make loud static noises, and all sorts of other havoc. Some types are more prone to leakage than others (paper/wax kinds and yellow or red Astron paper/oil caps are especially prone to this type of failure).
     
  5. texwest

    texwest Member

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    thanks for the help, I just replaced the power cord and will be replacing the electrolytics. I'm wanting to know how to test all the red astrons. I've heard they are pretty leaky. What kind of caps do you guys recommend for getting the tweed sound? It seems like alot of people like the mallory150s for fenders. If I get all this done it should be in tip top shape.
     
  6. The Pup

    The Pup Supporting Member

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  7. WaltC

    WaltC Member

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    I like the 150s, but good orange drops will probably sound more "Ampeg" without having to spend crazy dollars...
     
  8. brad347

    brad347 Member

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    The red Astrons are great sounding caps. they do leak, sometimes. If they are not leaking, I would leave as many of them in as possible. They ARE the tweed sound.

    Mallory 150s are fine. For many years they were the best available replacement for the old PIO Astrons.

    Now I think the nod goes to the new Mojo Vitamin T oil-filled caps. They're a little more than a Mallory 150 but way less than "booteek" caps. That's what I would use in a tweed amp, now.
     
  9. Tone_Terrific

    Tone_Terrific Member

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    Lift the signal 'out' end of the cap, leaving it connected to the DC source and check with a meter for DC leakage. AC should pass DC should block.
     
  10. texwest

    texwest Member

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    Oh sh$% Most of the red Astrons are leaking around 10 volts or 18 volts. I was starting to think that I wasn't testing right. So I stuck in a mallory in two spots and I had no leaking. Boy this sucks. At this point I will have to replace six of these caps. The others might be as bad but they don't have much voltage on the other side so I guess it wouldn't matter if they leaked anyway. Is this normal for so many of these caps to be bad?


    Its interesting that when I tested for DC when the caps were fully installed I had like 160 volts on one side and then only .8 volts on the other. But when I lifted the .8 side it went up to 9 volts. Is this a normal result for this kind of test?
     
  11. texwest

    texwest Member

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    what tonal effects do these leaky caps cause?
     
  12. brad347

    brad347 Member

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    Lots of questions, I'll try to answer them all.

    1) Yes it is normal for the red astrons to be that leaky. They leak like sieves, especially when old--they are notorious for it. Before people figured out that inductance in different types of caps, etc caused them to sound subtly different, they used to just shotgun-replace them (and the yellow astrons too, which aren't quite as bad but are also prone to leakage). Many "EE" type techs still do this... the ones that refuse to believe one capacitor can sound different from another of equal value. But PLEASE let's not get into THAT debate... ;)

    2) It doesn't suck as bad as you think it does. Capacitors are cheap, and if they were leaking that much, the amp will sound much better with new, functioning caps than it did with leaky Astrons. The function of a coupling cap is to allow AC (audio) through to the next stage while blocking DC (i.e. plate voltage). Your caps were not doing this very well, and it's a critical function in an amplifier. We are fortunate to live in a time when very good sounding capacitors are being made at many different price points. It's not just "orange drops or mallory 150s" anymore.

    3) The reason that they had low voltages on one side in-circuit but higher voltages when you lifted that side is that the side with low voltage in-circuit was probably connected to ground (or very close to ground). Since ground is at zero potential, anything connected to it will be very close to zero volts. When you disconnected it from ground, that all changed. ;) And yes, it is normal. That's why you have to lift one side of the cap from the circuit.

    4) Lots of havoc can be wreaked by leaky caps. The main thing is that they throw off the bias of the next stage. This can really make things sound anemic and bad and in extreme cases can even cause damage to a tube. A coupling cap is between the plate (positive voltage) of one stage and the grid of the next. Bias voltage is a negative DC voltage applied to the grid. So if you apply a positive voltage from a previous stage to the grid by a leaking coupling cap, you're negating, to an extent, the bias voltage of that stage. Not good.

    For tone caps that are leaking DC, you will often hear it as crackly pots. Sometimes you see pots replaced in an amp from someone who didn't know to check the caps. I bet they were really frustrated when they still crackled! This is a minor nuisance, but if enough DC is getting onto a carbon-element pot, it will eventually destroy the pot. This is why bias pots are cermet-element pots, not carbon-element pots. Bias pots have DC voltage on them at all times.

    Replace them with some decent oil-filled caps. The Mojo Vitamin T caps are cheap and sound good. Give the new caps a few weeks to break in and you'll be in heaven. You're not losing any "mojo" with the red astrons. If they're broke they're broke, and there's a reason you don't see to many of them in amps anymore (very high failure rate).

    You're talking to a guy who is big time in favor of original parts, and I say your feelings do not need to be that hurt to see those go.
     
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  13. WaltC

    WaltC Member

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    Brad,

    I'm glad to hear some good words for the Mojo Vit. T's. I've not used any but I've got a Dabeck amp (mine, built by Dabeck in Texas, great amp, great guy) that had some noise problems that showed up when I installed a master vol. in it. I checked out the amp and the noise was coming from the couplers (three of the seven Vit. T's in the amp, either tone or coupling caps) and the majority from one that even hummed when you touched the outside shield of the cap with the amp running. Replaced the three and the problems went away. Two of the three were leaking around 32 volts and the third was not making noise that I could tell but was leaking around 22 volts so I replaced it anyway. Amp is quiet, stable and sounds great again. I put in two NOS sprague black beauties and one mallory .

    Pretty new amp (two years or so) so I was surprised to find the Vit. T's with problems.

    Glad to hear it's not common...
     
  14. texwest

    texwest Member

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    Thanks for all the help Brad. I just went to your website and saw that you graduated from North Texas. I got a degree in classical guitar from there in 1987. Studied with Tom Johnson. At the time Paul Leblanc was a grad student, but I found out he is a teacher there now. Just dabbled a bit in jazz at the time. I managed to get into one of the guitar big bands being led by Jack Peterson. I think he's retired now. Now I'm really into jazz and am dabbling in amp repair.
     
  15. brad347

    brad347 Member

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    WaltC:

    Wow, that's interesting! I haven't used them a ton but I've never had any problems with the ones I have used. I just started using them recently (they haven't been out that long).

    My sample size is small, I've just been using them in my own amps (half a dozen amps or so) when one of the originals go bad or if I just want to try something else. This is the first I've heard of any problems with them. Maybe there was just a bad run or something? It's a strange coincidence that the only bad ones I've ever heard of were all in one amp! :)
     
  16. brad347

    brad347 Member

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    I had to do my two required classical guitar juries for Tom. He seemed cool but I never studied with him.

    I'm just a dabbler in amp repair too, have read a few books and got my hands dirty in my own amps and those of my friends. Built a couple. Never did it professionally.

    If you still live in the Dallas area, you should hook up with Craig Wallace. I bet he could show you a thing or two about amps.
     
  17. scottl

    scottl Member

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    It warms my heart to see fellow jazzers soldering.... ;)

    I like the PS series polyester Orange Drops. Not the 715. The PS.
     
  18. Dr. Tweedbucket

    Dr. Tweedbucket Deluxe model available !!!11

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    Bump to read this later ... good info!! I bought a 1950s Valco that has the original wax caps ... it sounds great but I'm thinking it will need some service.
     
  19. ardpan

    ardpan Member

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    Never ever saw any type of oil cap in any vintage guitar amp, just in old lab, test, medical, and some hifi equipment. No reason not to use them, however. Go nuts.

    Any old wax cap is 99.9% guaranteed shot and needs to be replaced.

    These amps were made to be played, serviced, and played again not worshiped and collected. Again, not that you can't or shouldn't fetishize them but when parts go they go. You wouldn't bum about a new roof on an old house would you?

    If you can spare the scratch I'd replace the Red Astrons w/ Red Jupiters. Never read a bad thing about them, they look cool, they are the only red thing out there, and will make you at least half happy when you look inside.

    Save the old ones from when you sell the amp. Some people will pay more for an all orig non working or bad sounding amp than a good sounding rebuilt amp. Some can't even tell the difference. There are all types of collectors.
     
    Last edited: Jun 6, 2016
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