how to test high voltage capacitors

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AusTex64

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Jun 3, 2013
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509
I'm chasing a "ghost note" problem in a Marshall Super Lead  Model 1959 100W amp. "Ghost note" referring to 120hz getting into the audio path somewhere and intermodulating with the fundamental note. This is real interesting topic to read up about. Some people say "all 100W Marshalls do it to some extent". Yet my buddy has a 100W  Model 1959 that doesn't do it at all, that I can hear. So the obvious questions is  "if they all have the ghosting problem, why doesn't my buddy's amp have it too?"

The first advice across the board  is to change out all the big filter caps. Another suggestion is to increase the first stage filtering from 100uf to 200uf. So we changed out the first two to 200uf. It did help the problem, but it's still there, and it did change the feel of the amp a bit. I just keep coming back to "why does my buddy's amp not have this problem, with the stock filtering?" Guessing I'm gonna have to try to talk him into letting me study his amp.

Back to my question. I checked the two LCR 50uf + 50uf 500VDC caps commonly used by Marshall that were replaced. Here's an interesting quote about LCR caps - "Certain capacitor brands like F+T and LCR have high leakage current." I used our LCR meter, who's power supply consists of a 9V battery. It showed all sections were within 10% of 50uf. But it was pointed out that this isn't at the 400VDC that these Marshall run at, and it only takes one little spot between foils to conduct at that voltage and render the cap no-go.

SO, is there any method for testing high voltage caps that is reliable other than just substituting them in the amp and hoping for the best?

Sorry for the long winded post.
 

Audio1Man

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Nov 19, 2012
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226
Location
Thousand Oaks CA, USA
Hi Aus Tex 64
I posted this in another post and it may help you find the problem.

There is no magic value for decoupling, only good engineering practices and verification.

Power supply decoupling is needed to reduce the HUM, RIPPLE & or SIGNAL from detreating the performance of the product. In the old days a scope was used to check for the problems and with today’s tools we have improved methods. Electrolytic capacitors have typical tolerances that are wide +20%, -50% is not uncommon. They sometime only work @ low frequencies and require additional HF bypassing.

I verify my designs or repairs with a scope and my Audio Precision analyzer to check for noise /signals on the power supply nodes. The Scope has high voltage input tolerance and has a 1 volt output that can connect to and analyzer for Analog or FFT measurements for debugging.

I also use a signal injection method to check the product for correct functions. Using a signal source I drive a current into the node and see what comes out of the product.
Duke
 

scott2000

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Feb 21, 2015
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2,565
Location
Sunny...Sometimes Florida- USA
I have a post  in the Chamber asking about a build of a new leakage tester some guy came up with...

https://groupdiy.com/index.php?topic=69021.0

The usual checkers are the old tube based ones like the Eicos, heathkit IT-11 and 28.....and MJ uses a Sencor. (not tube) ..but all of these are kinda pricey....Not sure there's another way......

I've read you can disconnect one leg of the cap  then power up  and check for dc  on the lifted leg.....but I can't remember where I've read it or how reliable it is....I know it's dangerous.....and that was for coupling caps....
 

Audio1Man

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Nov 19, 2012
Messages
226
Location
Thousand Oaks CA, USA
I believe that what you are calling “LEAKAGE” is the fact that the electrolytic capacitors are not working well @ all frequencies.  DC LEAKAGE in the capacitor causes heating and lowers the DC level and can alter the capacitance value @ different frequencies.  If the capacitor value is not large enough to reduce or lower the AC HUM & SIGNAL then it will be inject it into other stages and this causes what you are calling “GHOSTING / IMD”. You may need to add a HF bypass to the PS.

What I said is to measure the power supply with a scope and other instruments to see what hum and signals are riding on the power supply.
Duke
 
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