Contact John Soderberg at "Vintage Amp Repair" assuming it still exists. He's the to go-to Threshold/Forte dude.
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Thank you for this response. I am thinking that the site no longer exists, but I am willing to look into it again. Simple as most power supplies are, compared to other circuitry, I will likely have the guy working on it trace out the design. Interestingly, this design is actually a voltage doubler, and both channels share the same transformer secondary. The goal will be to separate the secondaries so that each channel has it's own secondary.
Both good links. Unfortunately, the very diagram that I am looking for is no longer available on the DIY site. Makes me wince! The other vintage amp repair tends towards Threshold, but i bet I can call him and get the needed info.
Thanks both of you for helping. If you have ever done one of these searches, you come up with just about everything you don't want before it's over.
Just got an email from the engineer, and he is quite certain of the design, but would have liked to see it in a schematic. I know the feeling.