Reverb in my Blues Jr doesn't work.

[dadydg]

I recently bought a used Blues Jr and the reverb doesn't work. I going to try to repair it. I've built a number of amps, but never one with reverb.

The reverb tanks seems to be good since the resistance at the input and output jacks are within specification (~63 and ~220 ohms, respectively). Also, with the Master volume and Reverb controls turned all the way up, a hum is heard if the center pin of the black RCA reverb cable is touched. I was told this indicates the reverb chip is fine (doesn't that sound correct?).

Any hints/thoughts/suggestions on diagnosing the problem would be appreciated.

Thanks.

[Don Symes]

If you get hum by touching the return cable's conductor, it sounds like the recovery stage is working. Start with the simplest things - try a known-good send cable. You may have to break the ground at one end to make it stop humming - that's a normal thing.

Also - check the supply voltage at the op-amp's pins 4 (-15V) and 8 (+15V). The op-amp _can_ work at lower voltages, but it may not have enough juice to drive the tank that way. These guys have a common problem with the +/-15V supply - R57 and R58 are under-rated at 750 ohms and 1W - going to a 2W part with leads bent into a sort of 'J' away from the board and back down helps.

If you wind up having to replace the op-amp: clip off the leads, heat 'em up to remove them from the board, clean out the holes and solder in a socket, then use a ty-wrap to keep the replacement in the socket.

Hope this helps!

[DrNo]

These things are known for failed reverb. In the tank the connection wire breaks off, Just inspect and reattach.

[Mars Amp Repair]

note he mentioned that he has measured the resistances of the tank...g