Denon PMA-757 - Can it be fixed?

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pcspinheiro

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Joined 2005
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.
 
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Coconuts 500

Member
Joined 2010
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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pcspinheiro

Member
Joined 2005
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 µF. What's the alternative to that? Any non greasy contact cleaner?

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

Cheers,

Paulo.
 
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Coconuts 500

Member
Joined 2010
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?
 
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pcspinheiro

Member
Joined 2005
Just put in 5050 LED chips (because of their wide dispersion of light + they were just laying around in my house) with an appropriate resistor to reduce their intensity and it's working like a charm. Will go into the sonic parts when I have more time and get the components. I had the feeling the left channel is a bit weaker, could this be due to the volume pot? The seller did say the left and right were sort of on and off when changing volume and that it needed some fiddling with to get both playing sound...

Thanks all!
 
Karl vd Berg

Karl vd Berg

Member
Joined 2012
Oh, oh!

Another situation here...

I've got this model too, few days ago. Everything was fine... until some 10 minutes ago! :confused:

Left channel seems to be suddenly attenuated a bit at times. Channels are not equal for a brief moment. The right channel remains good and loud, but the left one, at times, is not "punchy" as it should be...

I tried now Tuner/Aux/DAD and it's the same problem.

Power caps?
 
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