Amplifier Circuit Blowing Fuses

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!
 
Any chance the drivers you found bad/replaced were reversed (installed on the opposite position) by someone ahead of you?

John
 
dont overlook a shorted speaker out line . those are 3 amp devices .
i cannot download schem now as low on internet credit
if they are used as drivers check outputs voltages . if drivers and doing all the work then this may well happen .
 
Any chance the drivers you found bad/replaced were reversed (installed on the opposite position) by someone ahead of you?

John
Ding ding ding! John is the winner!

Didn't think to check someone's work on that level-- unreal! The transistors were swapped. To think someone took the time to work on these and the midrange never worked. Thanks for the help!!
 
No worries, glad it was no more complicated than that. I probably did that to myself at some point in the past, sometimes it's not always the previous guy through that gets the blame :)

John
 
Back
Top Bottom