xratedpez
Super Member
Hi Folks. Restoring a pair of internally tri-amped speakers and I've run into an issue I'd like to get some opinions on. The schematic can be found here.
The midrange amplifier keeps blowing its 1.25A fuse. Through testing, I found that T634 and T635 were bad. There has been some work on this board before, and the pair that were in there were BD139 and BD140. I had those on hand, so I replaced them, but the short still persists and the DBT still burns bright. The voltage coming into the board (labeled 'v' on schematic) is pretty much dead on 60V.
Since there weren't too many other silicon devices in the Mid circuit, I worked my way through and tested them all out of circuit (all other transistors including outputs). I also tested diode D631, which tested good (and turned out to be germanium IIRC).
I've isolated the issue to the amp circuit by removing the DIN connector to the tone circuit and the short still persists.
Full disclosure-- I *did* recap the board, but this exact issue was present beforehand (and, frankly, I thought it was the MR driver-- I proved that to be false after the fact). Normally I'd have diagnosed and fixed this issue before recapping.
Thoughts? Physical short somewhere perhaps? Open to investigate anything. TIA!
The midrange amplifier keeps blowing its 1.25A fuse. Through testing, I found that T634 and T635 were bad. There has been some work on this board before, and the pair that were in there were BD139 and BD140. I had those on hand, so I replaced them, but the short still persists and the DBT still burns bright. The voltage coming into the board (labeled 'v' on schematic) is pretty much dead on 60V.
Since there weren't too many other silicon devices in the Mid circuit, I worked my way through and tested them all out of circuit (all other transistors including outputs). I also tested diode D631, which tested good (and turned out to be germanium IIRC).
I've isolated the issue to the amp circuit by removing the DIN connector to the tone circuit and the short still persists.
Full disclosure-- I *did* recap the board, but this exact issue was present beforehand (and, frankly, I thought it was the MR driver-- I proved that to be false after the fact). Normally I'd have diagnosed and fixed this issue before recapping.
Thoughts? Physical short somewhere perhaps? Open to investigate anything. TIA!