M70 good and bad!

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Northerner

Northerner

Boomus Fidelis
So I got myself an M70 that needed a bit of work but as it was my first really good box I figured I'd give it a go :-D

THE GOOD - given it a good clean inside and out, cleaned switches and heads, repainted the grills, repainted the speakers to match the blue at the rear of the speakers, and fabricated a new cassette door as the original was cracked all the way across the middle. Radio works on all channels, tape deck plays perfectly, just need new aerials which I'm gonna pinch from my JVC RC656. :thumbsup:

Here it is so far
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THE BAD - the right channel was really weak so after reading every M70 thread ever I decided it was a dodgy slider. Got myself the manual and some more tools and set to work. Got the tonal board out ok and managed to remove the slider and dismantle without too much difficulty. Visually couldn't see anything wrong; no cracks and the feelers were in good condition. Used my new multimeter (never used one in my life before!) and the resistance read steadily from 0 to 50k ohms which I understand to be the right rating for this slider (I did this very carefully so as not to damage the strip). So I figured there was no break along the strip so must be at the weak points at either end.

So I used silver conductive paint on the sections marked at either end of the strip (note - the picture was taken before I thoroughly cleaned it with isopropyl alcohol). Only a few square mm was applied and very lightly.
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I carefully reassembled and its actually made it quieter on that channel to the point you can barely hear it at all :annoyed: . As working on the slider has clearly made a difference to the sound do you think I've just done a weak solder joint somewhere or is it something else? Please bear in mind this is the first time I've attempted anything like this so don't go too techie on me please!

Please help me out guys as I'm desperate to get this beauty booming in stereo not mono!!

Thanks.

Simon
 
BoomboxLover48

BoomboxLover48

Boomus Fidelis
With switch assemly disconnected from the PC board, measure the resistance when you move the sliders on each of VR702 and VR802. You should see the resistance value lowering from 50 ohms to 0.

Try checking the slider that was working and repeat it on the one that had issues. :yes:

On one of my M70s the hairline break was near the end connector to the carbon resistance strip. See the white conductive paint I put there in the attached pic. Norm helped me identify the problem and bad Boy Bill with the schematics. It was so much fun to correct the problem and was such a great experience when an almost completely dead channel coming up alive with full power. Let me look for my post on the repairs.

No worries! you will fix it soon!
In the worst case you will need donor tone control board to fix the issue. :yes: :yes:
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Northerner

Northerner

Boomus Fidelis
I actually used that thread as my guide :-D , even printed it out, couldn't have managed without it mate so you've helped me alot already! Will run those tests when I take her apart again, thanks! :thumbsup:
 
Northerner

Northerner

Boomus Fidelis
baddboybill said:
Does cassette and line in have same issue with the stereo separation being not equal?


Bad Boy Bill
Click to expand...
Yes it's the same on all. It's the same through the headphone socket as well :sadno:
 
baddboybill

baddboybill

Boomus Fidelis
Well you can always switch volume sliders to see if there's any effect. But you may also want to check Bass and Treble pots as these also can be an issue.


Bad Boy Bill
 
Litfan

Litfan

Requiem Æternam
Reading all this, reminds me, i must get mine in action, hasn`t been used for around 6 months. It worked ok then, but as you all know, these things, are so un-predictable.
 
Northerner

Northerner

Boomus Fidelis
The opinion now is that it's not actually the slider, but this leaves me pretty much without a clue so I'm likely to have to seek more expert help on this one and see if good old Docs will work his magic on it. Tho for now it's just parked back on my shelf...looking awesome but sounding half arsed! :(
 
Northerner

Northerner

Boomus Fidelis
docs said:
Drop it off mate, I'll see what I can do.
Click to expand...
Thanks Dave! With it looking so good and all working bar that channel I'm a bit scared of ballsing it up. Reckon it's better to accept its beyond me and get someone who actually knows what they're doing have a go at it! :-D if its ok with you I'll drop it by towards the end of the month as I'm coming to Manchester then anyway. Thanks again Dave :-D
 
baddboybill

baddboybill

Boomus Fidelis
Just a suggestion. Try using line out into another box's line in. This will tell you if it's in amp or preamp section ;-)


Bad Boy Bill
 
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