Forte Model 3 Issues

koenigr

Member
2014-10-20 3:04 am
Hello community,
Having some issues with a Forte Model 3 amplifier and would love some input. The left channel was blown to smithereens; multiple output devices, both driver transistors, and 3x small MPS transistors (I replaced all failed devices with new parts).

Powering up on a variac, everything runs perfect up until about 90VAC, after which the bias jumps wildly out of control, quickly jumping up to >1V. If I push it any further the line fuse blows, but the rail fuses remain intact.

I've disassembled the left amp board multiple times and at this point have tested every single resistor, transistor, and diode out of circuit, all measure okay with a multi-function transistor tester. I've also re-flowed every joint with fresh solder, still no luck :(

Has anyone run into similar issues? Also worth noting: I already replaced the bias trimmer with a new multi-turn trimmer.

Thanks in advance!
 

koenigr

Member
2014-10-20 3:04 am
Here are some photos if it helps
 

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