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Repair Denon PMA-280

R

Rinkert

New Member
I am trying to repair my Denon PMA-280. The fuse blew, and it keeps blowing after replacement. I measured the transistors attached to the heatsink (C3853 and A1489), and one of them shorted on all combinations of the legs. This is the C3853 at place TR319 (schematic), for speaker channel B.

I removed the transistor, and while waiting for a new one, I wanted to test whether speaker channel A would work. The fuse does not blow anymore, but the relay does not switch. So still no sound.

Do you know if I can check whether there are more components that need replacement. Otherwise my new transistor will break again, and I am back to square one. Thank you guys.
 
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You have to completely remove the power from the defective B channel then the relay should click for the good channel.

Also, when replacing shorted output transistors, it is always a good idea to replace them in pairs, and also check the emitter resistors and the driver transistor associated with the bad channel or when you power up again, you're just going to blow the fuse again.

If you replaced the blown fuse several times, it's quite possible that you did more damage than a shorted output transistor.
 
Allright, thx for your reply. Fuse blew 3 times total now, hopefully I did not damage anything else.
I understand that the division is not in speakers A and B, but in channel left vs right.

Just removed the other output transistor for the right channel, and now the relay switches again. Tested, and indeed I now only have sound from the left speaker output. Hope that testing the device like this is not harmful...
I measured all the other transistors on the board, but none of them short like the bad output transistors. Does this mean that the other components seem fine? And replacement will solve the issue, or are there still some other things I can check? Thx
 
Nope, it's not harmful.

I am currently using a fisher stereo amp where one channel is blown (it was given to me this way) and it does have both the output transistors and driver transistor shorted for that channel so I just removed them and am now just using the amp as a mono monitor. Been doing so for years without any problems.

Got any pics?
 
Allright, good to know, thx. Here the current status, no output transistors for right channel.

20200428_234728.jpg


20200428_231605.jpg


And the power section
20200428_213350.jpg
 
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