Tuesday, January 18, 2011

Received my ON-Semi MJ21195/96 power transistors for the Soundcraftsmen power amplifier late last week and slapped them in over the weekend. In a wholly unscientific manner I opted on the MJ21195/96 because the response curves tapered in a gradual manner and the ramp up to roughly the 50% current output appeared peppier. Plus they were cheaper, by 57 cents each. If the amp sounds like shit when I'm done, exploring the other output devices is within my capabilities.

That would, naturally, only happen after a recap and some other tweaking the good people at audiokarma.org have spoken of.

This, and a pair of pliers to grab and twist the shaft, is how I reached the rear screws without tearing this thing completely down.

As I wrote a while back, the rebranded TO-3 transistors were less than informative in regards to determining which of the 2SB554/2SD424 pair carried which cryptic alpha-numeric stamp. Direct measurement to determine the orientation of "diodes" on surviving devices was the simplest route to identification, here we see the unobtanium "5A" PNP (2SB554) being replaced with current production devices.

Replacement surgery went well, next up looks to be replacing the differential pair to shed some DC offset on the outputs, but before I go that far I aim to secure a copy of the service literature on this amp to assist in avoiding any pitfalls. Why yes, I probably should have had that from the beginning.

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