Denon PMA-520 Blown Outputs Perhaps

[DreadPirate]

Here I go again, wrecking a perfectly good unit. I decided to make the adjustments on this very nice sounding integrated, per the service manual, which I have attached a link to. Before I realized that my meter was not working properly (its readings are affected when the battery is low), I had turned VR403 all the way over and, apparently, have blown the outputs, though I have yet to remove and test them. The 5A fuse blows on turn on. The outputs are 2SC3855/2SA1491 and drivers are 2SC3421/2SA1358. They appear to be readily available from bdent and I need to place an order anyway (trying to stay positive here, feeling real stupid and wondering why I fiddle with things altogether).

I do intend on doing some troubleshooting first before replacing them all around. My approach will be to pull and test all of them (outputs/drivers), as well as the diodes and transistors ahead of the bad channel I find.

I have a question, though. There are a number of ICs in this unit and I've had success doing the previous on all discrete layouts, but I don't want to get into a repair quagmire here. Should these be okay? Is there a way to check them prior to blowing another set of outputs if they are not? Is it possible to prove the transistors good/bad in place, or must all be removed?


4shared.com - document sharing - download pma520.pdf

[nigelwright7557]

I have found on my MOSFET amps overbiasing them usually just results in the output MOSFET's blowing leaving the driver intact.

If the output is BJT then you might find the drivers have gone as well.

You could always route the output from the VAS back into the LTP and check the driver is working OK. Thats what I usually do. If that is OK I risk one output pair. If taht is OK I put all the output transistors back.

Just dont forget to set minimum bias before powering it up !!!

I fell foul of a dodgy multimeter too, the rotary switch was making bad contact and the bias was always zero !