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northerner - 2012-09-22 08:50

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

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.

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

deliverance - 2012-09-22 08:54

i am sure retrodos will be along soon to help you .

northerner - 2012-09-22 09:03

Originally Posted by deliverance:

i am sure retrodos will be along soon to help you .

Thanks mate, I just really hope someone can help as I'm a bit gutted to be honest. Spent hours researching this, getting the gear together, and then working on it, everything went well so it was a bit of a crapper when it actually made the sound worse

northerner - 2012-09-22 09:06

Hey! I've just noticed its changed under my name to 'Advanced'...cool!

Shame my skills don't match the description!

driptip - 2012-09-22 09:09

i cant see the pics.

docs - 2012-09-22 09:23

Did you try audio inject at the amp IC ? On that IC there are 18 pins, pin 2 and 17 are your channel outputs. 7.5v should be being presented at those pins too so do a voltage check on those. I'm not convinced the volume slider is the problem.

deliverance - 2012-09-22 09:25

check the big brain on doc"s

northerner - 2012-09-22 09:44

Originally Posted by docs:

Sorry you lost me about 4 words in!...I know it says Advanced under my name but i think it needs changing to Dimwit

If you can tell me using words more suitable for someone without a clue about electronics please Dave as I'm dead keen to get this fixed but don't actually know anything about electronics! Thanks!

northerner - 2012-09-22 09:45

Originally Posted by driptip:

i cant see the pics.

Weird?...I can see them on the pc and my phone so not sure whats happening there...suffice to say it looks lovely!

docs - 2012-09-22 09:48

Originally Posted by Northerner:

Originally Posted by docs:

Sorry you lost me about 4 words in!...I know it says Advanced under my name but i think it needs changing to Dimwit

If you can tell me using words more suitable for someone without a clue about electronics please Dave as I'm dead keen to get this fixed but don't actually know anything about electronics! Thanks!

First of all I assume you have the schematic ?

If you do, locate the main Amp IC and check voltages on pins 2 and 17. The voltage To the IC is being supplied at pin 1 and should be 15v or thereabouts.

deliverance - 2012-09-22 09:51

pics man pics .

northerner - 2012-09-22 09:56

I do have the schematics but have never read one in my life! I'll have a look and see if I can work it out...that'll be another skill worked on!

I'll try it mate and report back with the results. Off for a few beers now though to console myself over my failure

If the voltage isn't as you say what does it signify?...nothing too serious I hope!

docs - 2012-09-22 09:58

Originally Posted by Northerner:

I do have the schematics but have never read one in my life! I'll have a look and see if I can work it out...that'll be another skill worked on!

I'll try it mate and report back with the results. Off for a few beers now though to console myself over my failure

If the voltage isn't as you say what does it signify?...nothing too serious I hope!

Well we have to start somewhere and since you say that the channel is low on every function we should start at a point that is common to all. If the voltages are good then we know that the power is being supplied is all. If the voltage is bad on the channel that is bad then you have to find out why

northerner - 2012-09-22 09:59

deliverance - 2012-09-22 10:01

i can see this box ending up in bury

deliverance - 2012-09-22 10:03

top job si looks great

docs - 2012-09-22 10:03

This is the diagram showing the board layout, I have circled the IC. It is near the bottom on the mainboard. You can just see that pin 1 is bottom left, pin 10 is top right and pin 18 is bottom right. The voltages are indicated a few pages above on the schematic and the IC is located on the top right hand side.

northerner - 2012-09-22 10:04

Originally Posted by deliverance:

i can see this box ending up in bury

Either that or buried

If its beyond me, which is looking likely, I'd happily pay for some fixing as it sounds so bloody good even on one channel!!!!!

northerner - 2012-09-22 10:05

Originally Posted by docs:

You are a star mate! Shall try my best!

docs - 2012-09-22 10:06

I can look at it but I'm worried that you said you made it worse by checking the volume slider. So if the fault was found it might not be 100% because of whatever happened in the slider because since it was all in good order I would suspect something else is at fault.

northerner - 2012-09-22 10:22

It did worry me that I'd make it worse by meddling but I'm always the optimist! I'm actually heading to Manchester at the end of October so I could drop it round then if im clearly out if my depth. Then at least I won't make it even worse!

lav.loo - 2012-09-22 10:57

hope you get this one sorted SI, when all said and done it looks pretty minty and after all it,s an m70

northerner - 2012-09-22 11:40

Cheers Azz. I'm determined to get this one a1 as its such a lovely machine

northerner - 2012-10-29 12:50

It's now lost some value as its no longer the far rarer mono version of the M70...its now the far more common stereo version

lav.loo - 2012-10-29 12:54

ha ha well in that case chuck it over to me for the price of a few beers ha ha

northerner - 2012-10-29 14:18

It would have to be a lot of beers! This M70 will never leave me, not after the effort that's gone into it!...someone will have to pry it from my cold dead hands LOL!

Think I might get bured with it...with a few dozen D's!!

lav.loo - 2012-10-29 14:22

ha ha there's nothing like pure devotion eh Si

radio.raheem - 2012-10-29 14:45

Originally Posted by Northerner:

It would have to be a lot of beers! This M70 will never leave me, not after the effort that's gone into it!...someone will have to pry it from my cold dead hands LOL!

Think I might get bured with it...with a few dozen D's!!

I'll trade it for an m90.....c ya m70 haha

least it's working now Si congrats mate

northerner - 2012-12-10 06:32

OK so the story so far is that I tried to fix it and failed.

Then Docs had a go at it and managed to get it going on both channels by repairing the volume control (thanks Dave!).

So I love it but the volume on the two channels are off whack (due to the nature of the way it had to be repaired rather than Dave's skills! ) and you have to fiddle about to get them matching (still sounds awesome tho). So now I knew it was the slider I thought I'd keep an eye out for a replacement.

When Bredgeo posted that he had some I jumped at it ...I PM'd him within minutes of the thread being posted! The replacement duly arrived but when I checked it I found it was the wrong one...for the record level, not volume, and they weren't interchangeable He did get me the replacement tape deck door tho, so it looks spot on now!

So I kept my eyes open again and discovered that RR had a scrap box in his shed that he didn't want. He agreed to send it me a month or so ago but he was then busy with doing deals to clear some of the better boxes from his collection so I was a patient boy. Trouble is he was going to send it me last week but since the 'keeping posts clean' issue he's gone to ground so I'm likely back where I started again

So what I'm trying to ask in a very long winded way is, does anyone any where have an M70 volume control?...I'm willing to pay, grovel, beg!!...whatever, I just want to get my best box spot on....please!!