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M70/M75 channel issue - mono = good, stereo = bad


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#1 jlafleur

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Posted 01 October 2017 - 02:40 PM

I have an M70 and an M75 exhibiting a similar problem.

he left channel on the M75 is very weak/M70 is weaker(than the right) on all input modes when using it in stereo. On Mono there is no problem.

Sliders seem fine?

 

I'm thinking either a amp chip or filter capacitor, but want to know if someone has a stronger opinion.



#2 jimmyjimmy19702010

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Posted 01 October 2017 - 05:36 PM

Most probably an amp issue.

#3 Superduper

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Posted 01 October 2017 - 08:56 PM

Most likely degraded contacts in switches such as record bar.  Would not be the main amp chip since it is playing fine in mono.  The amp doesn't care whether mono or stereo -- it reproduces exactly what is fed to it.



#4 jlafleur

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Posted 05 October 2017 - 04:58 PM

Most likely degraded contacts in switches such as record bar.  Would not be the main amp chip since it is playing fine in mono.  The amp doesn't care whether mono or stereo -- it reproduces exactly what is fed to it.

 

Advice? Just deoxit the heck out of those switches? I'm happy to clean out whatever switches, but I would love some guidance as to which ones given this specific problem :(



#5 caution

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Posted 05 October 2017 - 06:54 PM

What's your skill level? Can you do more than spray switches if it comes to it? With a vague description like weak output, you'll probably have to get in there and investigate it a little bit to pinpoint the bad part.



#6 jimmyjimmy19702010

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Posted 05 October 2017 - 08:29 PM

The M70 has two main amp chips so it could possibly be a dead/dying amp. The mono switch maybe just bringing the output of the healthy amp into the other channel

#7 Superduper

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Posted 06 October 2017 - 12:06 AM

Advice? Just deoxit the heck out of those switches? I'm happy to clean out whatever switches, but I would love some guidance as to which ones given this specific problem :(

As caution said, cleaning contacts is frequently challenging enough and luckily, is the most frequent cause of old stereo problems. Anything more such as failed components in circuitry is something that requires investigation that is beyond the ability of most diy'ers that don't have at least a little background in electronics. Back to cleaning, you already know about deoxit... yes, if you decide to clean, you should clean every switch aggressively and it's not all that easy because in order to do a thorough and effective cleaning, the internal chassis needs to be removed from the cabinet to access the switches, especially the record/playback switch which is hard to find and clean even with the chassis out.

The M70 has two main amp chips so it could possibly be a dead/dying amp. The mono switch maybe just bringing the output of the healthy amp into the other channel

Jimmy, the mono/stereo modes don't function like that. Any mixing between channels is always done and injected in the input side of the output amplifiers. The outputs of the amplifier modules, whether single 2-channel modules, dual discrete mono amplifier modules, or dual 2-channel amps in BTL mode, always goes directly to the speakers without hi-level mixing. In fact on BTL amps such as on the M70, you can not bridge the outputs to mix the 2 signals because the two channels do not share a common ground.

#8 jlafleur

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Posted 20 January 2018 - 06:10 PM

NOPE, I didn't fix it... read below...



#9 jlafleur

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Posted 30 January 2018 - 02:41 PM

AHHH :(

 

So it did NOT fix it, I just had the switch on mono (you'd think I would have been more thorough...)

 

So I've used a good slider, a good amp chip, I've changed most the electrolytics on the preamp and power amp boards, I've swapped a couple diodes...

 

nothing is working... Can anyone give me some pointers? :(



#10 the_hun

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Posted 01 February 2018 - 10:25 PM

All I can say is I know my selector switch (tape, radio, line in, phonograph) needs to be cleaned. As when I put pressure on it from line in to radio I get the left speaker cut out and faint. But if I manipulate that switch I can get it just right to where both speakers come in nice and clear. Might be worth a shot.