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Tales from Midibob's workshop MPU4

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#21 midibob


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Posted 12 January 2018 - 06:29 pm

Yep, those pesky tants do make a mess when they go!


I'm still having fun and games with this board. IC3 was swapped out and this enabled it to boot. It wasn't stable however and any flexing of the board would drive it potty. The alpha was also corrupt although it was now functioning.

So there was still a way to go and as it had already had remedial work done around the battery area it's likely there are still some tracks damaged somewhere. That can wait till tomorrow.


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Moving on I noticed one of the motor driver IC's had been changed and under one of the pins it looked black.

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Hmm, this had better come out and I'm not sure I'm going to like what's underneath!

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All that carbon has got to go otherwise it will eventually track across. For those of a nervous dispostion look away now. :lol:

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It all has to be dug out, luckily it hadn't reached the adjacent hole. Once that's done it needs to be filled with epoxy.

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When that's dry it can be drilled and riveted. A bit of the old green stuff and it's good to go. Won't need that strap wire again either. :)

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I'm also having fun and games with another board but more of that tomorrow if I get time.

#22 midibob


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Posted 14 January 2018 - 01:00 pm

Fixed!! :clap:


This was quite a bizarre fault in the way it exhibited itself. It appeared that flexing the board was driving it potty even though it wasn't booting correctly in the first place. As the initial triac lamp test wasn't running correctly I swapped out IC8. This didn't make any difference but what I did notice was just lightly touching the top of IC8 would stop it going bananas??


Knowing what I know now it was obvious what was happening but I couldn't see the wood for trees at the time. After a fair bit of head scratching and meter prodding I found that touching pin 23 of IC8 with my finger was enough to stop it going mad. Sure enough the track was broken from IC14 (74LS138) and therefore the 'enable' pin (23) of IC8 was floating in the breeze! Putting in a tack wire and all was good.


Another misprint on the diagram here which I think has been mentioned somewhere before but the outputs of IC14 are all one out compared with the designation of all the chips they feed ie Y1 should be IC2 (not IC1) Y2 should be IC3 (not IC2) etc.

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#23 sulzerned


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Posted 14 January 2018 - 01:22 pm

Well found bob nice work and what we've come to expect now !maybe a revamped diagram is on the cards at some point .

#24 booze.net



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Posted 14 January 2018 - 08:34 pm

Good work Bob. 


It shows how difficult some of these faults are to find.

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