5050 with 2700/01 speaker protection problem

leifer

New Member
Hi guys. 5050 had speaker protection problem. Replaced burnt relay and most of the caps on driver board and the 2700 board. The 22 ohms measured out ok, 1 was 225 ohms. Replaced 6.8 ohms on 2701 board. Not replaced the nonpolar caps on those 2, or the 6800uf 50v power caps ...yet. It powers up, relays energized, and 20 seconds later drops out. I tested the diodes and transistors, and seem ok? Finding the replacements sounds like too much fun.. power supplies seem ok. Any ideas?
 
Update:
No expert here, just my 3rd Sansui receiver I've ever worked on.
Mostly work on tube guitar amps, some solid state amps, and have a 9090 I bought new in Korea, and restored a few years ago.

Finally got test signals to run for a few seconds.
Relay RL601, voltage doesn't get below 2.8 volts from TR603 collector, before it opens again, about 15-20 seconds.

One channel is OK, one is static.

So, looking into the driver and output areas, on the F-2700/2701 boards.

Very few voltage references on schematic I have, doesn't have the board layouts or parts list, just the redone schematics.

Any decent parts places for these diodes and transistors besides UTsource.net?
 
The protection circuit might be working fine. Have you checked/adjusted DC offset? If it's high, it will trigger the protection circuit. There may also be a thermistor on the output transistor heat sink. If it gets too hot, or if the thermister is out of range for some reason, it will trigger protection.

Please post the serial number for the database. Click on the link in my signature block.

- Pete
 
Hi Pete,
Thank you for any advice!
I did adjust the offsets. 15 mv on each channel. I work on these amps after work, so will post the rest tonight.
Went for the obvious problem areas first.
Right side is noise, left seems to put out test signals. Just getting into the testing, figured it was working both sides, so customer said... and finding the F-2700/01 schematics helps. :) No solid state expert, but have 30+ years as prototyping engineering technician building many circuits and equipment. Transistors aren't my specialty but relearning again. And I love the Sansui's!
 
It's very rare for a protection circuit to trip spontaneously with no fault in the amp. Check the service manual, it often has a description of the operation of the protection circuit. Be sure that none of the failures monitored by the circuit are occurring.

- Pete
 
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