RC-M70 RIGHT CHANNEL HUM AND LOW VOLUME

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kevink0000

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Jun 24, 2014
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Just picked up a not-so-clean m70. VU meters show equal levels when volume is advanced, but R channel is very low volume and distorted. It is full range, can hear bass, treble, and mids, but sounds almost like clipping at very low volume. Also, when running off AC power, as soon as unit is powered up, low 60hz hum is heard from R speaker only. Hum is overpowered by signal when volume is raised, but is ever-present. Problem does not go away with switching inputs.Moving input switches and sliders does not change effect, so not likely dirty controls. Any help would be appreciated.

Thanks,

Kevin
 

hemiguy2006

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May 5, 2009
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kevink0000 said:
Just picked up a not-so-clean m70.
I would start by opening it up and cleaning it out for sure.
Problem may be a dirty function switch and it cannot be accessed without opining it up
and tearing it down.
Definitely wont hurt to do this either way.
Also may be a bad slider as these radios are notorious for this issue.
slider goes bad and its resistance will change and cause a variety of problems.
What has me confused is the fact that you have equal level on the VU's
the humming noise may be a cap issue as stated by Cpl but I would venture to say that both channels would probably be emitting a hum if that were the case.
Good news is that there is a service manual available in the manuals section of the site.
I would also Hit Superduper up with a PM, as he is a pro at rebuilding the sliders on these.
He did mine and now mine is singing better than new.
Best of luck getting it fixed!!!
The M-70 is a great box
 

kevink0000

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Jun 24, 2014
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Thanks for the replies. I will be digging in to it soon. I own one of these from original from 1981, and its mint, but wanted one that is not so mint for general use. Worse case is I can run side by side with a voltmeter and the covers off, and compare pin voltages.

I just hoped someone had seen this before.
 

kevink0000

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Jun 24, 2014
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FYI for everyone's info, SuperDuper PM'ed me and said he had seen this before and it was one of the amp chips going bad. If the temperature is different between the two, then that is what it is. The amps are obsolete, but I just ordered thru Pac Parts. We will see if I actually get them. Mouser and Digikey did not list them. The part # is AN7145K(H).
 

kevink0000

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Jun 24, 2014
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Finally go to work on this, and found out the problem. The power amp IC is OK. The box had took a hit at the jack pack, and cracked the main pc board at one of the mounting screw holes, and took some PC board foil connection with it, near bias adjust. Fixed that, but problem persisted. Did some exploration with soldering gun trying to find bad/cracked solder joints due to the impact, and the heat found a bad mica capacitor that had been damaged from the impact. (.22uf, was value, I think c406) Will order new from Digi-Key, but heating the cap produced a fix to problem until cap cooled again, so I think once I replace it problem will be solved. Thanks for all of your help with this. Sorry I was so tardy in getting to work on this, but my real job got in the way.
 

koleloi

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Apr 17, 2014
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Hochiminh city, Viet Nam
May I sorry for digging up this old thread.

I just bought a JVC M70 in good cosmetics but have some problems that need your helps to deal with.

First, sound level is a little bit higher on left speaker than right, although it's hard to recognize at times. VU meter level also is a bit higher on left channel. However, I can hear good bass, treble, mids from both speakers.
Volume sliders are working ok I think. They are hard to slide but they just did their job, the sound increases normally as I slide up them.

And bigger problems are:
- when I turn up the box, it makes a short blast sound on left speakers (woofer and tweeter) but not quite loud, and occasionally from the right speaker too (in that case, the right speaker makes much more louder blast sound). And at the same time, the two VU needles move a lot, especially the RIGHT one - it move full to the end.
- when the box is powered up, on all functions, at all volume level, a hum/drone sound is heard from the LEFT speaker. It's quite loud, enough to heard from 4-5 meters if the music is off.
- when I turn off the box, the blast sound happens again on LEFT speaker, much louder than it's when turn on. I can see the left woofer jumping very hard, and a big noise also come from left tweeter. The sound is not very hard, I mean it's no hurt to the ears but is so annoying.
Also, the bass and treble control do nothing to sound quality. I have already opened the box and spray some contact cleaner to those sliders and function switch but nothing improved.

Edit: I do notice now that the Hum sound when box is turn on is actually come from both speakers. Much quieter on the Right side. And the sound is actually not retain the same level all the time, for each speaker, the noise varies if I slide the volume control, it's not necessary louder as volume is slided up, and become quite loud if volume is at top. I guest it's something more technically with the control board than just dirty.

Any help would be much appreciated.

Thank you very much!
 
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