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Proton preamp - problem with intermittent channel

Marantz 500

Marantz 500

Active Member
I have a very nice Proton 1100 preamp which I have been using for a few years. Recently, the left channel has started dropping out (becoming quiet) after a few minutes of listening. Turning the preamp off, then on again, with a signal coming from the source and the volume turned up, restores the sound from the left channel, but usually the sound disappears again in a few minutes. Sometimes I can listen a whole day without a problem, but usually not. I tried cleaning all switches and pots with Deoxit, but this didn't cure the problem. What could be causing the problem - any ideas?
 
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Are you certain the problem is in the preamp? If you swap channels on the cables at the input of the power amp, does the bad channel change to the right?
Did you try replacing the cables?
It is possible there is a muting relay in the preamp that may need to be replaced. Also possible culprits are bad solder joints, especially on the input and output jacks. If the problem is not in the preamp, it could be dirty or burnt contacts in the speaker relay in the amp.
 
Thanks, dr*audio! Yes, I have tried changing interconnects and also swapping channels - the problem is in the preamp. I suppose there is a muting relay in the preamp, since after turning it on it is quiet for a few seconds, then it lets the sound through.
 
I have the same problem on a D540. It turns out on my unit, as you pointed out, the protection relay is bad. The relay(5/8"W x 1"L x 1-1/8"D) is made by Matsushita; part # is JD2 DC24V. I don't know whether the exactly same part or not, but you might try wiggling the relay. The D540 is 20 years old, and the parts are hard to find. If you know where I can get the relay, please share it with me. Thanks, magpize, Colorado
 
protons are notorious for bad solder joints . i fixed mine by simply tagging every
connection w/ a well-tinned iron. you can't always 'see' a cold joint, especially
on a 20+yr old component.
 
Good point, karlo! The solders are solid on my unit. When I turn it on, I get no click. When I 'wiggle' the relay, on the other hand, the contacts are made and it sounds normal. Therefore, the relay is no good.
 
Hello out there

Does someone has the service manual of the Proton 1100,
it will be usefull by a repair or a modification.

Thanks a lot. Conny
 
I've been looking for one for years. I'm about at the point where I'm going to draw up my own. But I have so many irons in the fire right now. Maybe someday . . . soon. :sigh:
 
magpize said:
Good point, karlo! The solders are solid on my unit. When I turn it on, I get no click. When I 'wiggle' the relay, on the other hand, the contacts are made and it sounds normal. Therefore, the relay is no good.
Click to expand...

Maybe I'm not understanding, but if you wiggle the relay and contacts are made that definitely sounds like bad solder joints to me. I started having an issue with the balance knob on my D940 and lower sound on the R channel. I could turn it fully toward the right channel and still have sound coming out the left speaker. D-5'd the switches and pots and also followed with fader lube, but while I had it opened up, I went and hit the solder joints of the balance pot and some others (should probably redo the whole board). No problem now. Don't know it was the solder joints, but I did find on some that when the solder liquified, it opened up and it was like there was a clean hole and the lead poking thru, but no solder touching it at all. YMMV
 
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