Denon POA-2400 repair

Ezric

Member
2018-08-17 4:01 am
I would greatly appreciate any help anyone can give. Its got a fault that is driving me crazy. No self diagnostics are lit, all caps have been replaced. I have a schematic if anyone would care to take a peek at what you may think i can look at to see whats wrong. The 2 resistors highlighted burn up as soon as the amp is turned on, TR512 and 514 Emitters appear to be shorted together, albeit I cannot seem to locate where. I'm not wanting someone to fix this for me, I would like to myself and I hope someone can lead me or just give guidance. Thanks for reading and looking forward to getting this amp fixed.
 

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Be warned this this could be nontrivial.

Clearly the current mirror is doing its job but current is excessive. This in turn could be due to excessive current in the previous stage, caused by too little current in the input stage, in turn caused by a failed -16V supply due to a shorted 16.2 V zener D504. So maybe check that one (along with the 5k6 droppers for good measure, as well as connections to them, so that there is no good reason for the voltage not to be present).

And that's just one possibility. Troubleshooting complex DC-coupled circuits is not necessarily easy.

R548/550 are flameproof resistors, so they are clearly intended to burn up (or rather, go open safely) when something goes wrong.
 

Ezric

Member
2018-08-17 4:01 am
Clearly the current mirror is doing its job but current is excessive. This in turn could be due to excessive current in the previous stage, caused by too little current in the input stage, in turn caused by a failed -16V supply due to a shorted 16.2 V zener D504. So maybe check that one (along with the 5k6 droppers for good measure, as well as connections to them, so that there is no good reason for the voltage not to be present).


I thought about the zeners, however ive got 16.3v. I went ahead and replaced them none the less. The 5.6k droppers test good as well. Forgive my ignorance but why would a low input voltage swing the following transistors so high?