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MPU4 board repair advice


Prototype_uk
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Happy New Year to you all.  I am currently in the process of getting an eBay board with battery damage up and running. I have repaired all 7 broken tracks under the battery and have tested for shorts and continuity end to end on each. All test ok now. I've not repaired the Battery track yet.

Before I order the bits I just want to check with you experts I have got the correct components to do the surrounding damage. This is my first attempt at this sort of board work so go easy on me please  :)

Parts list as follows:-

R115 https://www.rapidonline.com/truohm-cr-025-10k-carbon-film-resistor-0-25w-pack-of-100-62-0394

C9-C15 https://www.ebay.co.uk/itm/124536609131?epid=1874400900&hash=item1cfef5d96b:g:2dIAAMXQoiJRg2Z8

T12-T19 & T21 https://www.rapidonline.com/bc337-40-diotec-bipolar-npn-transistor-45v-50-3113

C47 https://www.rapidonline.com/suntan-ts170r1h104msbfb0r-0-1uf-20-50v-y5v-p-5-08mm-radial-ceramic-capacitor-11-3454

ZD22- ZD33 https://www.rapidonline.com/dc-components-bzx55c4v7-zener-diode-0-5w-do35-4-7v-47-3850

 

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Edited by Prototype_uk
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You should be OK with those parts. I would also swap out the BC327 (T20) unless you're confident it's OK. Have you also checked that the battery negative track does actually go to the ground plane? Sometimes the via gets eaten away. Also make sure it does go to the 0.1 cap as well even if it looks like it does!. Have you tested continuity on the other side of that cap to the resistor? Also check the battery positive connects to the resistor above IC19.

Has that PIO chip in the top left been swapped out? It looks a bit iffy to me although it's difficult to tell as I can't see it all.

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18 minutes ago, midibob said:

You should be OK with those parts. I would also swap out the BC327 (T20) unless you're confident it's OK. Have you also checked that the battery negative track does actually go to the ground plane? Sometimes the via gets eaten away. Also make sure it does go to the 0.1 cap as well even if it looks like it does!. Have you tested continuity on the other side of that cap to the resistor? Also check the battery positive connects to the resistor above IC19.

Has that PIO chip in the top left been swapped out? It looks a bit iffy to me although it's difficult to tell as I can't see it all.

Thank you for checking the parts. Yes the track is well and truly eaten to the 0.1 Cap but I haven't got round to repairing that bit yet lol. I've got some spare BC327's so I will be changing that too. I will check the other points you mention thank you. Do you think I need to strip the solder mask round these areas (photo in red) too as it seems to have discolored or would it be ok to leave it? 

I haven't change the PIO (IC8?) and it looks original. Looks iffy you mean as in poorly fitted?

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Hi, your trace repairs look fantastic... Nice work.  This is not a bad thing but id maybe do a few more of the traces too slightly above TR13. Appreciate that the traces might buzz out now but one of the most frustrating things you can have is tracks failing after you finish it and you have to keep re-repairing it 5mins, 5 days later or 5 months later.  It seems not to productive but it saves so many Ball aches later on.

I had a scorpion 4 a couple of years back and on the first test after repairs I could hear weak traces pinging like elastic bands and one by one rows of lamps going down. I'd scraped the culprit traces and checked them above the custom chip and they were slightly damaged but intact. Once I saw what could happen it was a huge eye opener and I switched to not caring or even checking what traces are broken anymore but just repair/reinforcing everything that's damaged. It takes slightly longer but you end up with a much more robust board and less headaches plus your success rate can only go up... hope that helps... top job though 👏 😎

Edited by MattyL
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Good advice there from MattyL.

IC8 had the looks of a fake chip going by the lettering. If it's original then it's likely OK so ignore that.

The solder mask on these boards is pretty resilient to contamination (not like Scorp 1) although the top surface can need a fair bit of cleaning if it's conductive. Stray voltages can leak from the triacs into the switch ports which causes some really weird faults. I find that sometimes IPA isn't powerful enough to get the film off so I use some industrial aircraft cleaner but if you do use something stronger you must make sure it doesn't attack the silkscreen! (or anything else)

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2 hours ago, midibob said:

Good advice there from MattyL.

IC8 had the looks of a fake chip going by the lettering. If it's original then it's likely OK so ignore that.

The solder mask on these boards is pretty resilient to contamination (not like Scorp 1) although the top surface can need a fair bit of cleaning if it's conductive. Stray voltages can leak from the triacs into the switch ports which causes some really weird faults. I find that sometimes IPA isn't powerful enough to get the film off so I use some industrial aircraft cleaner but if you do use something stronger you must make sure it doesn't attack the silkscreen! (or anything else)

Yes very good advice from MattyL, I will look at this further. Good spotting from you too so I have taken some better photos. The chip doesn't look like someone has previously ripped it out and installed a new one but I might be wrong. I don't have any strong cleaners I was just using 220 and 600 paper to rub off the solder mask. Should I do that on all other areas where the mask is discolored?

 

 

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I always check with a meter. Put the black lead on the ground pin and then just drag the red lead over the mask, you'll soon see if there's any contamination on the board surface. With any moisture in the air you can see quite a bit of voltage if it's bad. A clean board should be next to nothing.

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4 hours ago, midibob said:

I always check with a meter. Put the black lead on the ground pin and then just drag the red lead over the mask, you'll soon see if there's any contamination on the board surface. With any moisture in the air you can see quite a bit of voltage if it's bad. A clean board should be next to nothing.

Great tip buddy thank you I will give that a try. As this is an unknown spare board is there any danger of it blowing my Rom if there is a fault on it? I would hate to loss my ROM and render the machine useless :( I have however bought a second hand ROM off eBay to use for testing but it is for a different machine so is there any danger of doing damage to the machine, reels or loom itself or will it run regardless? Sorry for all the questions but I've never had to mess about with it before in the 20 years of owning it.

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Difficult to say but just for example if you were to use the diagnostic ROM on a machine it would probably burn out the solenoids! A different game it's unlikely to cause any damage but you never say never! If you're checking for a condutive board you don't need the ROM cart in anyway.

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On 12/30/2021 at 6:56 PM, midibob said:

Difficult to say but just for example if you were to use the diagnostic ROM on a machine it would probably burn out the solenoids! A different game it's unlikely to cause any damage but you never say never! If you're checking for a condutive board you don't need the ROM cart in anyway.

I have been doing all the repairs without a ROM installed :)

What about if I was to leave the solenoids and lamp looms unplugged would it still boot? It would give me an idea if it would fry mine or not lol.

 

 

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2 hours ago, MikeB said:

That’s starting to take shape nicely! Hopefully when it’s done it will fire into life 👌

Yes getting there slowly. Let's hope it does fire into life rather than burst into flames 😁😁😁

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On 1/7/2022 at 6:44 AM, MikeB said:

That’s starting to take shape nicely! Hopefully when it’s done it will fire into life 👌

Right all new components solder in. I have only put Solder Mask on traces under components and have left the rest exposed for the inevitable fault finding mission when it doesn't work :) Hopefully I will be able to try it on Tuesday. I'll get my thermal camera at the ready too to watch out for that fire 🔥 🔥 🔥 🚒

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Great job! I think you've got it all ready to go there... all the battery damage looks nicely repaired and new components used...  Fingers crossed that will be a winner. Looks like it's clean‐clean from grime and residue too. I think your confidence will improve with it the longer it's running. 

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On 1/9/2022 at 11:48 AM, MattyL said:

Great job! I think you've got it all ready to go there... all the battery damage looks nicely repaired and new components used...  Fingers crossed that will be a winner. Looks like it's clean‐clean from grime and residue too. I think your confidence will improve with it the longer it's running. 

Well it doesn't boot. I got the thermal camera on it and R64 - R57 are getting hot. See attached. Sorry I forgot to calibrate the camera before I did the test so the image is a bit off. Looks like these are the lamp pre driver resistors?

FLIR_20220111_115953.jpg

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I've not thought of using a thermal camera before, I'm wondering if it might be any good for locating duff chips as the old finger test can be somewhat haphazard. Some of those resistors do run quite warm.

If she won't boot you'll need to confirm it's coming out of RESET first. Next step is to make sure you have activity on the ADD and DATA busses, the enable lines from the 74LS138 and IC22 feeding the RAM (74LS22) especially if it's got corroded legs. Yours looks pretty clean. Also check the 5v feed to the RAM from T20.

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28 minutes ago, midibob said:

I've not thought of using a thermal camera before, I'm wondering if it might be any good for locating duff chips as the old finger test can be somewhat haphazard. Some of those resistors do run quite warm.

If she won't boot you'll need to confirm it's coming out of RESET first. Next step is to make sure you have activity on the ADD and DATA busses, the enable lines from the 74LS138 and IC22 feeding the RAM (74LS22) especially if it's got corroded legs. Yours looks pretty clean. Also check the 5v feed to the RAM from T20.

I've just tried this other ROM I bought off eBay and it appears to be dead. This is what I have tried using to test this board. I didn't want to install my Andy Capp Rom in case this board i've repaired blew it. What would be the chances of that happening? Obviously if this test ROM is dud then that's one of the reasons why it isn't booting however I have checked the resistors on my other MPU4 which does kinda work (Apart from not accepting coins) And they are not getting hot with the lamp loom unplugged like this one is. I'll try and make a video to explain in more detail.

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57 minutes ago, midibob said:

I've not thought of using a thermal camera before, I'm wondering if it might be any good for locating duff chips as the old finger test can be somewhat haphazard. Some of those resistors do run quite warm.

If she won't boot you'll need to confirm it's coming out of RESET first. Next step is to make sure you have activity on the ADD and DATA busses, the enable lines from the 74LS138 and IC22 feeding the RAM (74LS22) especially if it's got corroded legs. Yours looks pretty clean. Also check the 5v feed to the RAM from T20.

 

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Hmm, a lot of unknowns creeping into this which is not good. Probably best not to mess around with your good ROMs just in case. So did you buy a whole ROM cart or just the ROM chip?

I'm thinking that the hot resistors are just the chip that's failed (IC14) but it could also be the PIO (IC3) which also could affect the boot.

You must have a known good ROM cart for starters otherwise you'll be chasing your tail.

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6 minutes ago, midibob said:

Hmm, a lot of unknowns creeping into this which is not good. Probably best not to mess around with your good ROMs just in case. So did you buy a whole ROM cart or just the ROM chip?

I'm thinking that the hot resistors are just the chip that's failed (IC14) but it could also be the PIO (IC3) which also could affect the boot.

You must have a known good ROM cart for starters otherwise you'll be chasing your tail.

Yes too many unknowns I know. I bought the whole Rom Cart https://www.ebay.co.uk/itm/Barcrest-MPU4-Program-Module-Game-Card-Lucky-Strike-/294513486176?nma=true&si=lX7%2Ft0IsjN2F4KoHPwdIOszIPXU%3D&orig_cvip=true&nordt=true&rt=nc&_trksid=p2047675.l2557

I agree looking at the Circuit diagram it's either going to be IC14 or IC3 keeping the Mosfets powered. Do you know where I could get a working ROM cart for testing?

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