Forming filter caps

[novosibir]

No this is not the same.
The image shows the situation inside an amp with the filter caps in series, not in parallel and shows also the bias filter caps, which must be excluded of the forming process.

Larry

[neikeel]

I think the pic that Mike has linked to is a 12 series cap board (with 100 x2 mains and 32 x2 screens in series (missing the link wire on the screens) as opposed to bias caps.

My suggestion would be to mount the spare 50w PT on a piece of board with mains input hooked up by a switch.

Take a piece of paxolin and make a rectifier with xs parallel strings of diodes as per Larry's post. Take the two HT wires to the ac in and the centre tap to ground,
Run off the recfified HT to the 100k resistor.
Take a scrap sheet of paxolin or whatever and mount turrets at intervals in pairs with short pieces of wire and croc clips. It would make sense to have a couple of easy points to put your DMM across the 100k too, and, crucially at the other end take a 5w Arcol 100k on a switch to ground so that you can bleed the caps once reformed.
You could do x5 of the 50uF caps at once with this rig. If they are lower rated (350v) and your PT is putting out 500v rectified don't forget to put them in series pairs.
Just my suggestion (been planning on making one of these for ages) I think Sparky made one to do all his 32/32 Eries?

[mrkrausman]

Hey guys I re capped a few amps but this is the first time I have run into a problem. Its 78 Marshall jmp 2203. I use the 100k resistor method. All wiring checks out. Forming starts at 280volts then gradually drops to 70 volts in an hour then spikes back up to 280v. I have a suspicion a cap is blown but no visible signs. But definitely there is a problem. Any help would be great. Thanks

[neikeel]

A dead cap!

Do them one at a time - see what happens.

[novosibir]

A dead cap!

Yep, something suddenly has startet to pull current and if there are no tubes in the amp, the bleeder resistors & the HT secondary center disconnected... must be a shorting cap.

Larry

[mrkrausman]

Well I swapped in the original known-good Daley caps and I am still getting a voltage drop of 280v across the 100k resistor connected beween the rectifier diodes CT is disconnected and the 56k bleeder resistors and tubes removed. I get 360v ac to the diodes and 450 vdc coming out. I am wondering if there could be something else drawing the current possibly on the board?

[mrkrausman]

A dead cap!

Yep, something suddenly has startet to pull current and if there are no tubes in the amp, the bleeder resistors & the HT secondary center disconnected... must be a shorting cap.

Larry[/quote
The caps all checked out with a multi meter. I then lifted the choke from the circuit and now the caps are forming properly. The choke has liquid leaking and the wires looked like they overheated. Hopefully this is the problem. I will have to order a replacement.

Its not the choke...something else is pulling too much current in the B+ line...10k2w coming out of choke overheats...back to the bench.

[jnew]

So I just finished a build ands want to do this. I haves RS and Erie caps that I hope will be good. My question is about the 100K resistor. I only have a 2/3 Watt. Can I still do this or does it have to be a 1 Watt or bigger?

Also, can I use Alligator Clips to make the connections to the resistor?

[novosibir]

I only have a 2/3 Watt. Can I still do this...
Also, can I use Alligator Clips...

Yes & YES

It's only the very first 5-10 seconds, where the voltage drop across the 100K might be beyond 300V and what actually would require a 1W resistor, but if the 2/3W resistor survives the very first seconds at overload (usually a carbon or metal film R will do), you then are below 200V voltage drop across the resistor, where even a 0.5W R would be ok.

Larry

[jnew]

Many thanks. On my way to the garage.

[jnew]

Ok. On my way now. In this 22 pages of info, maybe this was discussed but if we don't get to 5-10V's, how do we isolate which cap is bad?

[jnew]

15 minutes in and we're down to 165V's. It's ok for the PT to be getting warm throughout this process, right?

[jnew]

Ok. 3 hours in and I'm down to 60V's. However, the PT is pretty warm at this point. Bordering hot. Are we still good?

[danman]

To figure out which one is causing the trouble, you will need to start disconnecting each capacitor until you find the one that is causing the trouble. If you desolder a cap and the voltage suddenly drops down to the target range, you have found the bad cap. Your PT getting hot does seem odd. Are all of the tubes removed from the amp? If so, something must be drawing a fair bit of current and causing the PT to warm up.

[jnew]

Ok then. Do I need to interrupt this process to desolder the caps? I have a feeling it's the old 16uF Erie's in the preamp section.