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It's a Denon PMA-750. It sounds great... but then suddenly the left or right channel will die. If I bonk it on the side, it comes back OR the other channel also dies...if I bonk it again, both come back. Nothing is obviously loose inside...
Could this be output caps (I hope because that's easy) or something else that doesn't require desoldering two leads? (that would suck because then it's hard)
Follow Ups:
Thank so much for all the suggestions! I'm excited about working on the amp this weekend! I will post results as soon as I figure out what's what.Now all I have to do is figure out what a relay looks like. I'm only familiar with the really big ones on 16mm telecine islands. Do the tiny ones look similar? Watching all the videos now.
Edits: 08/19/16
Sometimes the relay has a clear plastic top.
Edits: 08/19/16
Or you just replace them, as needed?
Thanks!
In general, if they are acting up, it is wiser to replace the relay if available. Sometimes just cleaning the relay contacts will resolve the problem although it is often a temporary fix especially if the contacts are significantly pitted or damaged.
Can you take them apart and clean the offending, pitted points?
Thanks!
You can clean clean the pitted/heavily oxidized contacts by removing the cover if they have one. If totally sealed, it's best to replace.If badly pitted, you can try sanding them down with very fine emory cloth or the like but again usually best to replace.
Press the tone defeat button a few times and see if that clears it. If not, press the speaker selector switch buttons and see if that helps. Then it could be the switch or the speaker relay. The relay and switches as well as pots can be cleaned with Deoxit (video link). Often the relay and switches, however, may need to be replaced. Good luck!
Edits: 08/19/16
I have a lot of poking to do! And then I will fix the amp. :)
Input selector switchs, speaker switch, speaker relays if it has them. I'd bet input select for switchs. Look at the pic where someone wedges something in probably to get it to play properly. Those multiple switch's are much more of a pain in the ass to deal with than a single rotary switch.
I don't think its a coincidence that the pic I found shows someone having an issue with them. Welcome to the world of vintage needing repair. Age happens
ET
"If at first you don't succeed, keep on sucking till you do suck seed" - Curly Howard 1936
it's a turn-on relay. I use a NAD rceiver for my extension speakers and I leave it on for just that reason. In the past, I would sometimes have to lift the front a 1/2 inch or so and then drop it to the shelf. Don't like doing that so I leave it on
"Man is the only animal that blushes - or needs to" Mark Twain
Edits: 08/19/16
Nah, look at the pic in my post above. Common issue.
ET
"If at first you don't succeed, keep on sucking till you do suck seed" - Curly Howard 1936
If so, it could be that the relay contacts are going bad and have intermittent contact. The Carver "The Receiver" would do this. Once the relays were replaced it was like new. The relays are the speaker relays and there is two relays. These are there to prevent turn on/off Thumps, plus to protect against DC on the output of the amp preventing speaker damage.
Edits: 08/18/16
'tapping' around with WHATEVER 'probe' you end up using can also disclose the famous 'cold solder' which is easily fixed. Spraying with some 'cold stuff' will also disclose the bad joint.
Since it can be EITHER channel, or BOTH or NEITHER, I'd start with 'common' stuff to BOTH channels. That's the PS
How warm does the Denon normally run? Well ventilated? Any 'stacked' gear?
Too much is never enough
Bad connection, dirty contacts, cracked solder joint... You could buy some contact cleaner, DeOxit is very good and start by spraying/cleaning the RCA inputs, selector switch and/or if it has them speaker selection switches etc. When you spray a contact exercise it.
If that doesn't fix it and you are inclined to try to find the problem yourself, take the cover(s) off, use something non conductive and push/touch circuit boards, components etc., you will eventually find the problem.
If cleaning all the pots and contacts it still has the problem, check the speaker relay.
Check the pic in my post above. I found it on the net.
ET
"If at first you don't succeed, keep on sucking till you do suck seed" - Curly Howard 1936
Maybe drill a hole in relay shell and spray, then cover with tape?
You have some dirty contacts or one of the potentiometers is crapping out. It is a mechanical problem.
First buy a can of DeOxit - yes you are going to have to order it.
After that open up the cage and with a pencil push and probe the switches, connections, volume pot, etc... to replicate the source of problem. Then DeOxit.
NT
Pencil lead is conductive, as well as the metal shell that secures the eraser. Perhaps better to use the end of plastic writing pen, a wooden chopstick, or a wooden or plastic handle of a kitchen utility, etc.
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