Denon PMA-757 - Can it be fixed? - diyAudio
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Old 20th March 2012, 02:45 PM   #1
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Default Denon PMA-757 - Can it be fixed?

Hi guys,

congrats on the excellent forum! I mostly read stuff around here that sorts out my doubts/problems but this time I have to ask so sorry for bothering... I just bought a 1984 Denon PMA-757 because it was super cheap (30 bucks) and I saw some great opinions about it. The guy did say it had a problem with the volume control that sounded like an easy fix; you basically needed to fiddle with the volume knob a little because it would not always give sound out of both channels. As it turns out this is not the problem at all... it will give sound on both channels but it is very tiny even at full and while turning the knob the speakers do a loud "whooom".

My knowledge of the internals of these things is rather limited but could it be burned mosfets? It has these huge heatsinks that take up considerable space inside so I sort of assumed that is how it works. Do forgive my ignorance in electronics...

Anyways, I just thought it would be a fun project, although not exactly DIY, but you guys know tons about these things and that is why I asked here... The volume pot is an easy swap, if it is indeed the culprit, looks like an Alps blue velvet or similar but if it is something else, unless I can source the parts easy and cheaply (I'm half good with a soldering iron and love DIY projects) I will write back to the seller and appeal to his honesty.

Anxiously awaiting your wise thoughts!

Regards,

Paulo.

Last edited by pcspinheiro; 20th March 2012 at 02:47 PM. Reason: forgot details
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Old 20th March 2012, 03:20 PM   #2
pdul is offline pdul  Denmark
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You should start by going online, and search for a service manual, or at least a schematic, that vil make it much easier for folks here to give you advise, otherwise it will just be a lot of guesswork
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Old 20th March 2012, 03:37 PM   #3
pdul is offline pdul  Denmark
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Since it is that old, you can start with replacing all the electrolytic capacitors with new ones, and then clean all contacts and pots, no harm done with that, and ofcoarse clean as much as possiple everywhere inside the amp
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Old 20th March 2012, 08:45 PM   #4
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If you get sound on both channels, there is nothing wrong with the output transistors. This is one of the best commercial amplifiers in the world, and really, really worth your time and effort.

The volume pot should be fine.

Electrolytic capacitors could be a culprit. Maybe start with those, they need replacing, anyway.
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Old 20th March 2012, 08:58 PM   #5
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Start by cleaning all pots and switches (except power) with Deoxit. If you can't find the manual for it, the PMA-777 might be similar enough to be useful.

Last edited by DreadPirate; 20th March 2012 at 09:02 PM.
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Old 21st March 2012, 03:24 PM   #6
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Default SOLVED!!!!

Hi guys,

thank you so much for all the replies. It turns out that it must have been dirt accumulated all over the inside of the amp, including source selector buttons and possibly a "rusty" pot due to inactivity. After repeatedly using all the buttons and knobs back and forth the sound is there and I don't even have the problem that the seller reported. It is missing a few bulbs that light up the selected source button and I guess speaker set but I'll look into that with more attention now. I just needed to know if it could be salvaged or if it was dead...

I will try to get the Deoxit stuff (don't even know if it exists in Europe...) and also look into the capacitors, there are two humongous 15.000 F. What's the alternative to that? Any non greasy contact cleaner?

It is so old that the CD input was still called DAD.

Cheers,

Paulo.
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Old 21st March 2012, 03:48 PM   #7
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There are plenty of alternatives to Deoxit. Plain alcohol will do the trick very well. Pour a cap full onto each pot and switch, then work them good. Wait for it to evaporate before turning it on. If you can find something to apply it with, like an eye drop container, that will be excellent.

You can use 10,000uF smoothing caps, if you cannot find 15,000uF variants (can be hard to come by). Either that, or 22,000uF. Should be fine.

It's not old enough, if it even has a cd input.

By the way - this amplifier came in two variants - one with a toroidal transformer and one with a regular EI one. Which one do you have?

Last edited by Coconuts 500; 21st March 2012 at 03:55 PM.
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Old 21st March 2012, 04:06 PM   #8
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Hi, the one I got does not have the toroidal transformer. Same as this one but in silver:
Denon PMA-757 integrated nudies - AudioKarma.org Home Audio Stereo Discussion Forums
It was really so cheap that I will invest a little time and money into getting it just right.

Cheers!
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Old 21st March 2012, 04:24 PM   #9
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Another quick annoying question: Most of the pilot lamps are gone as I mentioned and it is probably impossible to find those 16v suckers... would putting a small LED with the appropriate resistor in there be OK?
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Old 21st March 2012, 04:31 PM   #10
pdul is offline pdul  Denmark
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Why not? I already did that in my tuner
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