Peavey Classic 50 2x12 mishap

[Teleman55]

I had added adjustable bias and was testing with the transformer shunt method when my probe slipped and caused a short.The result is the 2-22ohm resistors between the filter caps in the -27v supply burned up,I replaced them and they burned again with in a couple of minutes.HELP!

[Teleman55]

The resistors are R61 and R62.Its a 27v supply.Sorry my touchpad acts quite nuerotic and the cursor twitches all over the place.

[Enzo]

If I had to wager, I'd say your probe connected B+ to the -27v rail and blew out C43. Power off and measure resistance across C43. Does it seem shorted? Try unsoldering C43 and see if the amp will power up.

Get a light bulb limiter going.

[Teleman55]

Thanks Enzo,I will give that a try tonite or tomorrow.I have a LB limiter.I was worried about the OT,so I measured the primary resistance-one side is 75 ohms the other is 58.Should I be worried about this?

[Enzo]

No, not until we have some reason to think it matters.

[Teleman55]

I tested the cap in circuit and found a short,removed the cap and tested it with an IT28 tester and an Atlas ESR meter and it tested ok.Out of circuit I measured the resistance of the cap and got no reading,other caps read a steadily decreasing resistance so I don't know what that means.I tested the C43 termminals on the board and its a dead short.

[Enzo]

If C43 is shorted even when it is not there, then it looks like the -27v rail is shorted to ground. One end of C43 is ground after all. Leave the cap out for now. May I assume you can measure the short to ground from the hot end of C43? I know the cap is out, but it is easier to type C43 than to type "the connections where C43 used to be."

SO what is connectd to -27v? For one, that 4558 IC that runs the reverb. I guarantee it wan;t happy if high voltage hit it. Does it show visual signs of damage, like a crack through it? Remove it from the board anyway. STill shorted C43 or not?

[Teleman55]

I checked the 4558 IC and that is where the short is.I replaced it(I have a dead PA amp loaded with them) and the short is gone.I hope to get to a power on test in the next couple of days-damn day job-and will post my results.How critical is the value of the R61,R62 resistors,in parallel they will be 11 ohms,I am out of 22ohm and only have R-Shack local to me.

[Enzo]

Its not critical, but you don;t want to wander far off. They drops a little violtage before that supply gets to the tube heaters.

[Teleman55]

Could the switching relay also have been damaged?

[Enzo]

DOn't know, you'll have to find out.

[Teleman55]

I reassembled the amp today,plugged in to a light bulb limiter and all seems ok.Thanks Enzo,I have been struggling lately with some health issues and haven't been able to do much amp work so I really needed some help.You folks here always come through.