Denon PMA-500v issues...

[drummr]

Hi, I'm having a problem with a vintage Denon receiver that up until a week ago worked wonderfully. The right channel randomly got severely distorted, while the left remained normal. Today I opened it up to see if I could see anything wrong, and I found 2 blown fuses that correlate to the right channel. As a test, I took the 2 functional fuses from the left channel (same specs) and put them in place of the blown ones. As soon as I hit the power button both fuses blew and a capacitor right next the the fuse on the right let out a little smoke. So now both channels sound horribly distorted.

Any idea why the right channel would blow the fuses instantly? From what I've read it might be something to do with shorted output transistors?
Here's a pic of the area in question: http://i240.photobucket.com/albums/f...n/IMG_0407.jpg

[drummr]

Re: Denon PMA-500v issues...

Quick update: Took both output transistors out for the right channel and both of them are shorted. Left channel ones check out. I also checked the resistor that smoked and it seems to be fine.

[999999999]

Re: Denon PMA-500v issues...

Yeah that's the usual cause, though sometimes it's a leaky or shorted transistor just prior to the output transistor pairs.

[drummr]

Re: Denon PMA-500v issues...

The receiver is actually my friends and he would occasionally turn it up too loud and it would make a horrible popping noise from the speakers(clipping?). As a precaution I am replacing both output transistors on both channels, the cap that smoked (It seems to work, but the top is bulged out), and the fuses (duh).

[drummr]

Re: Denon PMA-500v issues...

Ok, I replaced all 4 output transistors (including the 2 left channel ones, just in case), the blown cap, and the fuses and it worked great for about a day and a half. I had the receiver hooked up to my computer after testing it out, but not playing anything, before giving it back, when it randomly made a loud horrible electrical arcing hiss noise from the right channel speaker and it smelled like a toasted resistor. I quickly turned it off and unplugged the power. Both brand new right channel transistors are shot, and a nearby resistor looks burnt, but the fuses didn't blow (they are the right spec). With the right channel fuses removed, the left channel plays fine. With the fuses in place, the speaker relay doesn't engage.

What should I be looking for now? Or should I just give it back and say it's not worth the effort/cost to try and fix?

Thanks

[999999999]

Re: Denon PMA-500v issues...

Sounds like you had excessive DC on the output, you'll need to check that channel's output transistor pair again and start tracing back the circuit with no load connected, or at least a very high impedance load like a power resistor, not a speaker.

At the base of the output pair transistor(s) you should have an AC voltage with minimal DC offset. Compare against the left channel with an equal signal to each and equal load. Trace back to the component prior and see if it is also receiving AC with minimal DC offset.

It's a bit hard for me to trace something with so many components with no schematic from a pic, but it could be that you have a basic push-pull current gain stage after an opamp on the right channel that corresponds to "IC-5" for the left channel but is hidden in the picture. If this is the case, also measure the output pin of that to see if it has minimal DC voltage (offset) or it is already present at that point in the circuit.

Of course you should also measure the power rails, both at the output transistor collectors to be sure they are equivalent +/- values, and at the IC-5, again assuming it's a dual rail opamp). The values for the transistors and the opamp may not match each other but the power rail positive and negative values per each separate stage should be polar opposite equivalents of each other. For example it could be (wild guess, just an example) +12/-12V rails for the opamp, and +35V/-35V for the output transistor pair per channel.

There could be some other fault, for example I see some diodes one step before the output. Fortunately you have two channels which look to be laid out near identically so you can take multimeter measurements as described above and compare where readings from the faulty right channel deviate from the left channel. Wherever they begin to deviate, a component somewhere right before that in the signal path is probably to blame, or the power supply rail going to it.

Whether it is worth the effort or cost I cannot say. Since I suspect the prior trasistors which are probably less than $1 each if you use near equivalent parts instead of vintage identical parts, it could be cost effective, for that matter a lot of even less expensive components could be replaced to shotgun the problem for pennies a piece, but the time to do it on top of the time to disassemble and reassemble is something only you can place the right value on.

Since it's about 25 years old now, I'd probably pick between giving up or also committing to replacing all the electrolytic capacitors on the amp board while it's out, maybe spray some cleaner in the potentiometers too and work that around. On the other hand I don't know if the owner has other amps or how much of a rush this is. Since I have other amps I'd probably trash it and save the case, transformer and heatsink for a new chipamp build in it but that's a whole new topic.

[randtek]

Re: Denon PMA-500v issues...

I once had a Denon that I rebuilt, lasted for about a week, went out again. It turned out to be a grounding issue with the amplifier. The power amp grounded through the heat sink screws, and they had gotten corrosion over time between the screws and the copper traces, probably due to dissimilar metals and electric current. This might be something to check for. Another thing I have learned from many years of experience in this field is that when repairing a blown channel I ALWAYS replace at least the driver transistors too. I usually just order all of the transistors in the preamp and power amp stages. They are usually dirt cheap, and it saves me from a lot of repeat failures. The reason that I do this is simple. Modern amplifiers are DC coupled, and when a transistor fails in one, it throws biasing off throughout the whole channel. This can cause the semiconductors that have not failed to be stressed, and they will probably fail prematurely. This tip was given to me by a Pioneer engineer many years ago, and has probably saved me a lot of repeat fails.