Gents,
I got to thinking about it, and I am pretty sure I caused this problem. Of course, that's obvious - I am the only person that has been in this amp.
I mean, I am pretty sure i
directly caused this problem.
I was trying to remember what all I had done and I recalled that a week or so ago, I was trying to figure out why TR171 and TR 172 were mounted the way they were. I knew that they were part of the idle current circuit, but I did not understand why they were mounted the way they were, on ribbon cables and all that.
I tried straightening their mounting orientation, making them perpendicular to the heat-sink, as this is the way they are mounted in the Japanese A-1000, which is a slightly different unit. I found that this was okay, but the transistors bent over way too easily when working on the main amp board.
So I decided to orient them parallel with the narrow side of the heat sink, creating more metal to metal contact between the heat-sink and transistors . This worked, but I still had my DMM's hooked up to the bias test points and I noticed that the voltages had lowered once I turned the unit back on. That got me to thinking: maybe these transistors were some kind of temperature regulators, that would lower the bias voltage as they got hotter.
So, I decided to experiment. I sprayed a little freeze spray (upside down duster) and the bias voltage shot up. I put the soldering iron near one, and watched the idle current fall. I figured since a lot of folks troubleshoot transistors like this that it was OK to do and, technically, it was.
I figured from my little experiment that the original orientations were the best, as they provided physical stability for the transistors and let the heat from air convection tell the transistors how hot or cold they were.
For the record, I still don't know exactly what TR171 and TR172 do, but I figure, if they regulate the bias current somehow, then that would backup Zaibatsu's bias runaway theory.
The TLDR/short version of this story is that i played with the TR171 and 172 transistors a few weeks ago and probably fatigued the copper legs on those transistors. Subsequent power cycles caused thermal expansion and contraction on those now weakened legs and cause TR172 to fail.
And while I am one who knows when to admit screwing something up royally, that fact has not made me feel like any less of an idiot.
I am on a budget; this whole ordeal was so I could swap this unit with my main and only system that has been out of order since January. This was a high-stakes operation and I decided to play Mr. Wizard and failed miserably and spectacularly.
This was totally preventable and yet here I am with an amp that now has two fried resistors, two blown output transistors, a blown driver transistor, and a failed whatever kind of transistor TR172 is. God knows what else.
At least
@zaibatsu and
@avionic will know what happened.