St 70 channel out

[leadtower]

Hi All; I have one channel out on my ST 70. I can't figure it out.
Swapped tubes with known good one; stays in that channel.
Swapped output tubes.
Checked PS all voltages going into the board are good.
The output tubes bias up right to 500 ma on each tube and I get 0 mv by adjusting the balance resistor.
I do have a funny reading on one of the transistors; 28 v on one side of the right transistor; but I only get 18 v on the left transistor. I measured all possible links in the chain and that is one difference; the lower reading is on the bad side. I measured the resistance on the trimpots and they are all the same for each channel so that looks good.
I can get volt readings for the tubes if that would help.
The speaker jacks are good; I get sound out of both channels until the amp warms up; then it dies in one channel. I use the ST 70 transformers.
Thanks for any advice.

[Shannon Parks]

I do have a funny reading on one of the transistors; 28 v on one side of the right transistor; but I only get 18 v on the left transistor.

The CCS is blown. Replace the one reading 28V. I can mail you one if that helps.

This only the second one I know of blowing out. They have a 40V tolerance and some conditions (oscillation from phase swap in the case I know of) can kill them. Channel goes silent , but no other damage. Not as stout as tubes, eh?

[leadtower]

Pheew! Well it sure hones the troubleshooting skills. I forget where I got this; I guess mouser. Is that available through RS? I can just order it unless you have one in which case I'll buy from you.
Thanks again; it's the one with the high reading right?

[Shannon Parks]

Pheew! Well it sure hones the troubleshooting skills. I forget where I got this; I guess mouser. Is that available through RS? I can just order it unless you have one in which case I'll buy from you.
Thanks again; it's the one with the high reading right?

RS doesn't carry them - they're from Mouser. It should be the 28V one that has failed. I'll drop some in the mail to you tomorrow free of charge. Just email me your address.