Inaccessible filter caps - draining query

[Steve Blackdog]

Hi folks

so how do you suggest I go about draining filter caps that are sitting on the pcb with no wires showing above, legs trimmed below and sealed all round?

cheers

steve

[Steve Blackdog]

Thus

[Helmholtz]

Do you have a schematic?
If so, use it to identify accessible points/components where the caps connect to.

[Enzo]

Agree^^^

Those caps connect somewhere. In a tube amp, the plate pins of the power tubes are convenient. In a SS amp, the power transistors have legs. Though with values like 220uf 350v, I'd expect it is a tuber.

[g1]

I'd expect it is a tuber.

Finally we find ourselves face to face with the elusive 'rock n roll potato'.

[nickb]

Agree^^^

Those caps connect somewhere. In a tube amp, the plate pins of the power tubes are convenient. In a SS amp, the power transistors have legs. Though with values like 220uf 350v, I'd expect it is a tuber.

Sometimes the first set of filters caps are located before the standby switch so this may not work in all cases. Best to see the schematics first.

[drewl]

Got a Peavey on the bench with same situation.

The big old choke resistor is right there for draining the caps.

[Steve Blackdog]

Here’s the relevant bit of the schematic. I am wondering if c142 is faulty. I get 0.00vdc at the junction with the IN 4007 diode it joins up with, with or without power.

At the moment I am getting sound, but whisper quiet. Before I lash out on some new tubes, I am seeing if anything else looks wrong.

best

steve

[Helmholtz]

The big old choke resistor is right there for draining the caps.

What is a choke resistor?

[The Dude]

............ I am wondering if c142 is faulty. I get 0.00vdc at the junction with the IN 4007 diode it joins up with, with or without power......

What is the resistance from that point to ground?

[drewl]

What is a choke resistor?

R200 on your schematic, the big white power resistor in your picture.
for smoothing out high voltage ripple, some amps use a transformer "choke" other amps use a big resistor.
Check the resistance of R200, 203 and 204.
Check the rectifier diodes and if any filter caps are shorted.
And check continuity between all those parts and their solder connections.

[Helmholtz]

I am wondering if c142 is faulty. I get 0.00vdc at the junction with the IN 4007 diode it joins up with, with or without power.
steve

So you mean zero Volt at "HT PS"? If so, you have no PI and preamp supply. Check if R203 is open. Do the EL34s have screen voltage?

Why don't you post the complete schematic?
Not even clear what brand and model this amp is.

[Enzo]

Choke resistor is a description. Peavey tends to put a big 400 ohm resistor in the B+ string between first and second filter stages - where a choke would have been in amps that had chokes.

[Steve Blackdog]

So you mean zero Volt at "HT PS"? If so, you have no PI and preamp supply. Check if R203 is open. Do the EL34s have screen voltage?

Why don't you post the complete schematic?
Not even clear what brand and model this amp is.

thanks helm holtz
I have been cautious about sharing the schematic in full, as Blackstar gave it to me on the basis that I use it for my personal use only. But here it is in full.

cheers

steve

[Jon Snell]

R201/2/5 & 6 already drain the capacitors as well as balance the voltages.
Give it time to discharge before those angry pixies try and bite you.