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CR-800 Main amp problem

M

mikkoa79

New Member
I tested main amp with dummy 4.4ohm load with 1kHz signal and after positive signal clipping few seconds then fuses (F801-804) burned. Then I removed all main amp transistors TR717-720 (4pcs) and measured them, all was ok.
Then I tested main amp board without main transistors and output voltages were -39V (TP3) and -26V (TP4), also R744 (820R) smoke came out.
I would guess output voltages should have been around 0V also without main transistors, is that correct? I just want to test 1st main amp circuit to work correctly that I do not burn main transistors.
I have not done any modification to this amplifier. This amp feedback circuit is not familiar to me but it seems that feedback is working without main transistors.
 
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Welcome to AK.

A few questions might make for better replies. Does the relay click? How were you testing, speaker? Dummy load? What procedure? Had you done other repairs or mods before doing the test?

The short answer is that you can test without the output transistors, but you will get no voltage at the speaker terminals.

Do you suspect shorted outputs? Have you pulled and tested them.

A DBT may be in order.
 
Thanks Semanon!
I modified text to be more clear about situation.
This is my 1st post in AK, is Yamaha forum best for this kind of post or is there some other forum better?
 
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