Sunday, January 6, 2013

Resurrecting a Peavey Ultra 60, Part 1

A Needed Fix; Repairing an Ultra 60

Peavey Ultra 60, sans case
Keeping with what's apparently becoming a minor obsession with Peavey gear, I picked up a Peavey Ultra 60 amp (91B, made in 1991) a couple weeks ago for $100. The seller said it worked fine, but for the price I had my doubts. Still, I've done well with malfunctioning multichannel amps. Usually the fix is fairly simple; some bad caps or a disconnected wire, cracked resistor, etc.

While missing power tubes, it came with a full compliment of working preamp tubes (four 12ax7's). So I popped in a pair of 6L6's and down the road I go...

OK, it works, but there's a problem. At low volume levels the amp behaves. Push it a little, and one power tube starts to redplate, big-time. Swapped tubes, and it's definitely not the tube itself.

After my misadventures with a Peavey Triumph, I'm gratified to see that the Ultra 60 has only three circuit boards (OK four, if you count the footswitch and send/return board, but that board would be easy to eliminate). A main board with the preamp, power supply, etc., a board for the triodes (preamp, PI) and a power amp board. It's a PAG design, very similar to my Peavey Bravo (head conversion), with all the 12ax7 triodes inside the chassis. But there are WAY fewer inter-board connectors, MOLEX and otherwise, than the Triumph.

So I pull the output board--easy, yes? NO. The output sockets are riveted to the chassis. An application of Dremel tool and a power drill, and the board is free.

And the problem is immediately evident. A torched resistor has burned carbon deposits all over the board, and that's a ticket to redplate city. The Ultra 60 schematic is a bit harder to find in the wild than most other PV schematics. But it is out there. The toasted resistor is part R80 (10K) and there's also part CR12, a 1N4003 diode right beside it, covered in carbon.

From the inspector's marks,this board was assembled in April, 1991.

Arcing is bad

Carbon deposits on the chassis from the arcing

I removed the fried components first. Then the whole output board was submerged for a good wash-up in my ultrasonic cleaner. After that, a thorough scrapping and sanding removed all the remaining carbon deposits. I replaced the 10K resistor with a 1/2 watter, the diode with a 1N4007; both replacements somewhat more robust than the originals.

Test it once, looks OK. Reinsert the output board, and try to bolt it in place... No go. Not enough clearance; the screws and nuts required prevent the tubes from seating well. In frustration, overnight I research all sorts of speed nuts, etc. that might work. In the end, I simply re-rivet the sockets to the chassis, just like the factory.

The Power Amp board repaired

In the photo above it's easy to see the larger 1/2 watt resistor near the center of the board. The new diode sits right beside. The little pale green thingies are 100pF caps masquerading as resistors. Also visible are the screws & nuts I tried in my failed attempt to replace the rivets.

With the board in place, and the connectors reattached, it was time to fire it up, give it some juice....

Success! (and WOW--sounds really good!)

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