This amp came to me via a web contact. It had a bad channel and typical grime from the years of use. Over all, the unit was in good physical
condition. Upon inspection, I found it had been worked on in the past in a rather hap-hazard and rushed manner. Most likely by a commercial
shop where time is money. Technique, workmanship, and neatness were way down on the priority list. I found replacement parts soldered onto
the leads of the old components (they did not want to bother with removing the board from the unit), miss-matched output transistors, and
sloppy application of heat sync compound (you can note this sloppy workmanship in the photos). It is no wonder the unit failed again.
Also, I had to rebuild this twice!!! After spending about two weeks on the unit and doing an initial power on tests, I was ready to pass a 1Khz
signal and check the unit for gain and distortion. I had my scope connected and was watching the wave form. I ramped the power up, and after
just a few seconds, one channel began to distort. I reach over to shut down the unit, but before I could… poof!!! The channel went up in smoke.
Wow!!!! What did I miss? I spent the next few hours pulling the board and checking components, but I could not find a cause for the failure. On a
hunch I cut open the output transistors and, to my disbelief… counterfeit transistors. I purchased these from a well known online supplier of
transistors (and whom I had dealt with many time in the past). I contacted the supplier via email with photos and received no response. I have
not purchased from this source since.
So I spent more $$$ for a new set of transistors (form Mouser this time), as well as several resistors that went up with the outputs. I set the unit
on the shelf while I waited to receive the new parts. In the mean time, I started work on a different project… then another, and another. A year
later, I finally got this unit back on the bench. Here’s the happy ending… a full set of new ON Semi transistors have been installed, the channel
has been rebuilt, and the unit is now in top notch working order.
I have arranged this project write-up a little differently then usual. Instead of a chronological step-by-step, I have arranged this in a "before
and after" format. So much time has passed, so many photo opportunities missed, I deemed this the best way to present the results of the
work and provide this information as a reference.