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Andrew96_

Project PC92 mpu repair

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So I decided to have a go at a pc92 board bought in a pile of boards some time ago off ebay, this one did not look too bad! but of course bad enough for it not to work! upon testing it would not produce a reset pulse correctly, and I always remove the 53v power to the board as the via's (through board connections) in red on the backside of the board would most certainly burn up and wreck the board!

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why they decided it was a good idea to route the 53v under the battery close to the 0v line is a mystery to me! not very good practice in my opinion!

so next I take all the reset circuit components off the board and solder the holes up of the parts I won''t be putting back, then I know I don't have to bother fixing the tracks around those bits!! just clean them up!

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quite a few bits to not have to replace!

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this board has this rom on it! any ideas what game it is for?

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Edited by Andrew96_
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As well as the usual track fixes If.  it were me there would be many more transistors in the cup ;). I replace every transistor in the vacinity of the battery by default. All the way to the 555 & pio even if they look good. Only cost pence so I never risk it.

 

Imo there is nothing wrong with the project coin reset circuit or the board it's made from as I've never had a board back with reset issues.... Just the original transistors let you down with green Hiden at the top of the legs. And I've worked some pretty rotten baors.

 

Also need to go round trim all the legs on the solder side as they can short to adjacent tracks or components... Especially after a trip with royal mail.

 

Lastly mpu off battery current is a good indication of the health of the mpu.

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R2 R3 and R4 form a voltage divider, I have found that underneath is the 53v line, the battery vapours affect the pcb I believe this causes a leakage within the board somehow to the resistors mentioned, upsetting the 2.4v voltage and thus a reset is a sliding curve instead of a right angled waveform even after all the transistors in the reset circuit had been changed with new. when the 53v is not present as in bench testing all is fine.... well that was my findings, if you found different then great!! I can only tell you what I found on some boards I did back in 2014. All these boards are for myself, I just document how I repair them. the other transistors in the area will be replaced later on so I don't remove them at this stage unless the tracks are rotted around them, lastly the ram I change with a batteryless version so this board is unlikely to ever see another battery, as I say I do boards for my own enjoyment in bringing them back to life, they don't ever get sold on!

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TBH I am surprised you have not come across leakage due to battery vapours on these boards, when it gets into the 53v track this can happen

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when cleaned up it has big holes in the board, definitely the 53v track!

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takes out so much blowing holes in chips and everything!

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all down to battery vapours!

 

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Ive seen Nothing that dramatic. Wrong fuse in the PSU to allow so much current to flow & create so much heat?

 

I've declined one with a much smaller scotch mark on the 55v line. Maybe good for spares.

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42 minutes ago, Louie Bee said:

Ive seen Nothing that dramatic. Wrong fuse in the PSU to allow so much current to flow & create so much heat?

 

 

nope fuse was correct rating, the burn started in the place I highlighted in the first post I am actually repairing, its fairly common for this to happen and it is down to battery vapours penetrating the board layers through the via's causing the 54v to do this! this is another reason I suspect the vapours have altered the boards dielectric propertied around the reset circuit, but I am not mega testing it at 50v to try and find out! I just don't use the original reset circuit! I found the reset worked fine if it was cleaned with IPA, but leave it a day so it dried out and reset was this bad rising curve! spray it again with IPA reset was perfect back to a great low rise time waveform! leave a day and no go again loop! so I can only presume dielectric conductivity because of the way the reset reacts, but how to prove it!! hence why I never decided to repair anyone's PC92 boards (and sent them back at my cost so there owner did not foot the postage bill) as could not guarantee the reset would always work 100% and all the time back then

Edited by Andrew96_

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This is why I don't power the 54v line when first testing a board!

What fall time do you get on the reset line on your boards when they are working?

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I don't think I have the kit to measure one shot rise of not reset (NRST) at power up or the fall at power down.. it's much less than a second as it seems almost instant between power up and the CPU pin 26 going high.

 

Then again I don't use alcohol to clean boards. I use something way more magical (and common).

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Z80 reset tends to be far more picky than normal, that's why a simple r/c network is not used, the processor has to run at least 3 clock cycles before the reset is released and has to be a very sharp edged transition to 0v.

IPA removes all the water from a board, so if a board has got damp over the years IPA removes it.

 

anyways if you have never had any troubles with the reset then great! I hope it stays that way, cause if you do it is a great pain to sort out, believe me!! I will stick with what I know on these boards for sure

as I have said this is how I fix these boards for myself

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No probs I'm not saying I'm doing it right, just trying to share some experience and an alternative view.

 

Project coin boards don't all need a special reset circuit to make them run satisfactory for the best judges (the owner). I've never noticed z80 CPU being pickey on the standard reset circuit maybe I'm just lucky.

 

Here is one with the magic (fairy) all over it & it's very damp in the picture, but it's running fine in the machine now.

 

I'd bet many will cringe seeing an mpu in the sink being scrubbed with pot & fingernail brushes, then blasted dry with a hair dryer. Maybe I should do my own threads but never have time.  Edit) it gets off al the dirt really well all the years of nicotine & dust gone. Makes them nicer to work with too.

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Edited by Louie Bee

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42 minutes ago, fruit58 said:

so long you dry the board thoroughly after should be good to go 

That's why IPA is good as it not only cleans off flux from the board from rework but it is very good as dispelling any remaining water, and the one thing I was always told is you don't want any water getting into the pcb where it has been damaged by vapours, but if that works for Louie then great

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at work we have to bake the boards at 80 deg c for 2 hours before fitting some chips, its a requirement

Edited by Andrew96_

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Anyway wire links over the damaged tracks done both sides

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and solder resist applied

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bits put back T5 and t6 in the ram supply changed, meter transistors changed, board works fine now!

 

that's as far as I will go with this thread, I get the feeling no one wants to know anymore and I am not trying to sell anything to do with pc92 bits, so I will stop there, I get the feeling I am treading on someones toes with a reset problem that doesn't exist! well who knows... perhaps I am making it all up for the fun of it........

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I tried it on a moldey MPS2 one day as I didn't fancy breathing in the spores... nothing to loose and it worked fine after fixing what was probabley the original problem that lead the MPU to be dumped.

I'm sure it''s far from ideal on life dependant circuit boards, but with few fold admitting sucess on PCL / PC90 / PC92 tec I thought anyone having a go may benifit. Sorry if I've hijacked your thread somewhat with may be seen as "alternative facts".

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Thanks for this Andrew and Louie Bee, nice to see repairs on PC92 boards, certainly I find these posts very interesting, have a few boards with exactly the same issue with the acid damage.

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1 minute ago, Andrew96_ said:

that's as far as I will go with this thread, I get the feeling no one wants to know anymore and I am not trying to sell anything to do with pc92 bits, so I will stop there, I get the feeling I am treading on someones toes with a reset problem that doesn't exist! well who knows... perhaps I am making it all up for the fun of it........

I'm not saying you've fabricated a reset issue you have seen and solved, pointing out that the mistry next day reset problem could possibley be avoided by other methods. I've been lucky enough to not have seen any issues (yet) on any project coin board.

If you were doing a pile of them - just give it a go... you never know...

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I did 13 that came from project coins repair department as is, all had battery rot and most had this reset issue,  perhaps these were ones they stored because they could not find the issues, I don't know, all I know is a lot had this issue which was not solved by replacing all the transistors, indeed some boards got them changed twice and it made no difference, guess I was unlucky but if YOU can repair them ok with everyone scrambling to give you coin then great! enjoy, just watch the moisture and that 53v line as that is a killer and destroys roms, its a high voltage to be affected by battery rot and I have seen a few boards burnt up by it.

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14 hours ago, Louie Bee said:

No probs I'm not saying I'm doing it right, just trying to share some experience and an alternative view.

 

Project coin boards don't all need a special reset circuit to make them run satisfactory for the best judges (the owner). I've never noticed z80 CPU being pickey on the standard reset circuit maybe I'm just lucky.

 

Here is one with the magic (fairy) all over it & it's very damp in the picture, but it's running fine in the machine now.

 

I'd bet many will cringe seeing an mpu in the sink being scrubbed with pot & fingernail brushes, then blasted dry with a hair dryer. Maybe I should do my own threads but never have time.  Edit) it gets off al the dirt really well all the years of nicotine & dust gone. Makes them nicer to work with too.

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Just for the record ive done.this to a few mpu4 boards and not had issues afterwards .its all about letting them dry properly. I put em on top of the radiators which works fine and leave the boards for a couple of days .

 

Andrew dont be disheartened as this is a very good interesting thread so keep up the tidy work and this thread .and thanks for sharing what you do .

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Hi Andrew that looks good. I dont have the knowledge on how to fix a board. I managed to take the battery of the board, but thats about it. Would you have a look at some pics I took from my PC92 and tell me whether it can be fixed ?

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Posted (edited)

That's burned up around the battery area. You'll be hard pushed to find someone to take it on who can get it running right.

What game is it from? Looks like one with the large display like bullseye as it's got most of the components found on later games so even harder to find a replacement... The unless it's been backfitted to a lowtec like 777 heaven.

Edited by Louie Bee

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