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Forte Model 3 Issues
Forte Model 3 Issues
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Old 7th October 2019, 08:17 PM   #1
koenigr is offline koenigr
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Join Date: Oct 2014
Default Forte Model 3 Issues

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!
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Old 8th October 2019, 01:18 PM   #2
koenigr is offline koenigr
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Join Date: Oct 2014
Here are some photos if it helps
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File Type: jpg 20190926_160015.jpg (775.5 KB, 93 views)
File Type: jpg 20190926_160012.jpg (931.3 KB, 89 views)
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Old 8th October 2019, 03:08 PM   #3
AVWERK is offline AVWERK  United States
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Location: So Calif.
The A968 driver is non existent these days so hopefully it survived
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Old 9th October 2019, 12:35 PM   #4
koenigr is offline koenigr
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Join Date: Oct 2014
Thanks for the tip! The original 2SA968 was blown and I replaced it with parts supplied by the owner, come to find out they were counterfeits purchased on eBay that crap out at higher voltages. Replaced with MJE15032/MJE15033 and all is well!
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