Forming filter caps

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jnew
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Re: Forming filter caps

Post by jnew » Sun Jul 02, 2017 11:24 pm

And by the way, are we measuring AC or DC voltage. I have a new meter and I think maybe I'm setting it up wrong. :palm:
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Re: Forming filter caps

Post by danman » Mon Jul 03, 2017 8:45 pm

You are measuring voltage after the rectifier diodes so it will be DC. The only way to find the problem if the voltage never dropped down to the target range is to shut it down and start isolating caps. Did the voltage ever drop down or did it stay where it was?

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Re: Forming filter caps

Post by jnew » Mon Jul 03, 2017 10:45 pm

Voltage did drop. Power transformer got pretty hot. Then when it was all done, I put everything back to the way it should be, put the tubes in and now I can't take it off standby without blowing a main fuse. There's obviously a short somewhere which would explain why the power transformer got hot. I hate to think that the power transformer was damage but at least when I turn on the amp with the tubes in, the tubes will get power and glow like they should. I'm completely stumped on this one.
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Re: Forming filter caps

Post by novosibir » Tue Jul 04, 2017 12:44 am

jnew wrote:Ok then. Do I need to interrupt this process to desolder the caps?
jnew wrote:And by the way, are we measuring AC or DC voltage.
Sorry for speaking so, but someone who doesn't have basically knowledge about electronics as well as about amp service & repair shouldn't try trouble shooting on a tube amp, to save his own life, as well as a pharmacist shouldn't try doing a heart transplant surgery himself.

viewtopic.php?p=229425#p229425

Warning: If anybody can't read instructions, or doesn't understand my instruction about forming caps absolutely 100% and/or after reading it isn't shure absolutely 100% about that, what he's supposed to do in the amp - then he/she better shouldt give the amp to an experienced amp tech to have him doing this! Always keep in mind: Inside the amp occur lethal voltages, which might kill you, if you're doing ONLY ONE MISTAKE !!! And the voltage doesn't care about, whether the mistake has happened due to accident or due to misunderstanding!
The fault almost always is sitting in front of the amp :wink:

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jnew
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Re: Forming filter caps

Post by jnew » Tue Jul 04, 2017 1:12 am

I was being mindful, yet trying minimize interrupting the process. Thanks. 8)
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Re: Forming filter caps

Post by axeman » Mon Jul 10, 2017 2:51 pm

No and use a fan on the pt

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Re: Forming filter caps

Post by mrkrausman » Tue Jul 11, 2017 4:37 pm

I think it would be beneficial at this point for the pros to weigh in on how to narrow down a short in an amp assuming tubes have been removed and we are properly set up for cap forming. Lets say we have a situation where its not the e caps but something else is drawing excess current measured across the forming resistor. In my situation the problem goes away when i lift the first 10k resistor feeding B+ to the preamp can and i read low voltage to the V1 plates. V2 and 3 seem fine. The two 10k resistors in series read fine but bringing up the variac i heard sizzling and saw smoke coming from the V1 side of the pcb. Any help would be great. Thanks
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Re: Forming filter caps

Post by axeman » Tue Jul 11, 2017 4:57 pm

Are the capacitors on the board rated for ?

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Re: Forming filter caps

Post by jnew » Tue Jul 11, 2017 6:29 pm

Turns out my preamp cans and PI can are leaking too much. Just the risk you take when buying old vintage stuff. Put good caps in and it's off and running. After forming the caps and getting got under 5V's, what I thought was a short only turned out to be two wires swapped on V2's pins 2 and 3. Amp is all good now. 8)
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Re: Forming filter caps

Post by mrkrausman » Tue Jul 11, 2017 8:55 pm

jnew wrote:Turns out my preamp cans and PI can are leaking too much. Just the risk you take when buying old vintage stuff. Put good caps in and it's off and running. After forming the caps and getting got under 5V's, what I thought was a short only turned out to be two wires swapped on V2's pins 2 and 3. Amp is all good now. 8)
Great! Im done fooling around. Im ordering new cans and starting all over if i have to take it apart piece by wire i will. I have learned to love the smell of frying resistors.
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Re: Forming filter caps

Post by jnew » Tue Jul 11, 2017 9:57 pm

Yeah, I was afraid I had done some damage when that PT got pretty hot but it's all good. When you get the new filter caps, double and triple check everything. You might be surprised at how the simplest things get overlooked.

Oh, and I kept blowing mains fuses too. Which also made me think there was a short but I had a bogus rectifier diode.
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mrkrausman
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Re: Forming filter caps

Post by mrkrausman » Tue Jul 11, 2017 11:14 pm

jnew wrote:Yeah, I was afraid I had done some damage when that PT got pretty hot but it's all good. When you get the new filter caps, double and triple check everything. You might be surprised at how the simplest things get overlooked.

Oh, and I kept blowing mains fuses too. Which also made me think there was a short but I had a bogus rectifier diode.
Thats very good advice... will do.
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Re: Forming filter caps

Post by syscokid » Thu Jul 27, 2017 8:45 pm

I'm in the middle of replacing two filter caps on my 2204, because I want to try going back to stock values. The Preamp filter cap and the PI/Screen cap. I started the forming process with a 100K 2watt resistor between the rectifier and the Main filter cap. Voltage drop started at around 230 and within the first 30 seconds the voltage dropped to 159. The voltage drop remains at 159 after 3 hours.
Normal?
Should I isolate the caps?
Can I isolate the caps by disconnecting their respective ground? Even the Main cap?
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Re: Forming filter caps

Post by neikeel » Thu Jul 27, 2017 9:14 pm

That is odd.

Did you remove all of the tubes? (sorry to ask but it has been done)

What you can do is desolder the caps and jumper them + to - with the last one - to ground on the chassis, that way you know they are isolated form the rest of the circuit.
Usually if the caps are no good they bottom out at 20v and then start to climb again.
Neil

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Re: Forming filter caps

Post by syscokid » Thu Jul 27, 2017 9:32 pm

neikeel wrote:That is odd.

Did you remove all of the tubes? (sorry to ask but it has been done)

What you can do is desolder the caps and jumper them + to - with the last one - to ground on the chassis, that way you know they are isolated form the rest of the circuit.
Usually if the caps are no good they bottom out at 20v and then start to climb again.
Thanks.
The tubes are all removed.
I'll try the method you described.
I'm still going to be looking for the same positive final results of around a 5v drop at the resistor, right?
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