Testing SS amp after Fixing

[Prattacaster]

I have a yamaha G100 on my bench scheme here.. Yamaha :: Yamaha_G100_series_I2.jpg picture by Prattacaster - Photobucket
I have will have replaced all but two transistors on the output board along with some resistors, electrolytics, and a trimmer. I am replacing the Output transistors which have a plug in socket on a heat sink. I am wondering when I first fire it up should I do so without the Output transistors in, just like I would fire up a tube amp that I fixed without the power tubes in? or does this present some major hazard? Im new to SS amp fixing so excuse the dumb question.

[km6xz]

There are at least 3 versions of that amp, each with a different output topography. Which is yours? The oldest 2 are not using a bipolar power supply so it uses a large output coupling cap. The MkIII has a more conventional bipolar supplies so the speaker terminals are direct coupled and should have very low offset, with or without the power transistors in place. The G100 original uses cascaded power transistors which should sit at +45 volts at the input of the output coupling cap with the transistors in place but might vary a lot with them out of circuit. Only place a load on a SS amp after it has been proven to be stable, idle current adjusted and there is no offset.

It must have been a massive burn up failure to need so many parts, what was the cause of the failures? With SS power amp, the best way to fire it up is monitoring current and offset as you slowly increase the line voltage with a metered variac. I do not suggest working on one without a variac, they are a cheap but necessary piece of bench gear for working on any mains operated equipment.

[Prattacaster]

It was indeed a massive burn up, some of the resistors were just carbon skeletons of their past form. I found that the mounting screws for the output transistors had worn thru the green protective coating on the PCB and into the copper traces. Sending B+ voltage to ground, there was also a 5 amp fuse instead of a 3amp for the power board fusing, I'll be shortening the mounted screws.
The output section is coupled thru a 2200uF cap to the speaker. I have two different schematics of the G100. Neither of them are perfect matches but both are extremely close. The only major variance is an 2sc484 with its collector connected to PIN 6. It looks like this transistor replaces the 1.5k 2w resistor shown on the schematic, which my amp doesnt have.