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Sega whitestar cpu 5v ground short?

ronnie63

ronnie63

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Looking for some help please! I'm trying to track down a 5v rail short to ground, the problem is on the cpu board, short of desoldering every chip and decoupling cap how can I narrow it down? tried running it on my bench psu at 5v 1 amp to see what gets hot but my psu drops down to less than 1v due to the short!

Would welcome any advice?

Thanks Ronnie
 
short?
so you can actually measure low resistance with a meter between 5V & GND
any socketed chips you could remove and see if it clears
 
pinballtoys said:
short?
so you can actually measure low resistance with a meter between 5V & GND
any socketed chips you could remove and see if it clears
Click to expand...

Yes I'm getting zero ohms across the 5v rail, I did remove all the socketed chips and the 2 tantalum caps but no change, have been over the board with a magnifier and only found a broken decoupling cap, prior to it blowning the logic fuse it was resetting which were ever inreasing the longer it had been on and the then it would go into a reset loop! So maybe that was a sign of something starting to fail?

Thanks Ronnie
 
myPinballs said:
Short to ground on 5v is the worst to fix. It will be a 100nf decoupling cap somewhere but which one…

Good luck..
Click to expand...
Thanks Jim I'm finding that to be true! I did run up the board on the bench with 3v 5 amp and I 'm getting some hot chips at around 100 degrees! that being the pal chip U213 also u204, u208 and u207 all 74hc245's

Also the pal chip is reading 4 ohms across it's Vcc to neg but that's not the short culprit on the board

Cheers Ronnie
 
Hot chips is bad as they are most probably broken. You can measure voltage drop across the five volt rail vs ground plane on the board with a sufficiently high res meter, that works well usually. Thermal camera also helps. Would start with pulling the hot chips and take it from there. And current limit your power supply so you don't generate more damage by burning the board.
 
drhex said:
Hot chips is bad as they are most probably broken. You can measure voltage drop across the five volt rail vs ground plane on the board with a sufficiently high res meter, that works well usually. Thermal camera also helps. Would start with pulling the hot chips and take it from there. And current limit your power supply so you don't generate more damage by burning the board.
Click to expand...
Thanks drhex, I do have a thermal camera I thought about cutting the vcc pin on the soldered hot chips to see if they are the cause of the short? desoldering these chips is not easy even with a desolder station, all the pins are bent over and solder top side of board is hard to remove, I think easiest safest way is to cut all the legs and remove each pin. I also have a fluke that reads milivolts if that might help
 
So I snipped the vcc pin of the 3 245's I'm reading 4 ohms across their vcc to ground, that's not right? now other chips are warming up but not getting hot.

I also lifted a leg of each and every decoupling cap but the short is still there! Could one of these 245's shorted in another way that's still pulling the 5v down?

Thanks Ronnie
 
Its possible, if a chip has failed anything could be shorted inside, but if you've remove Vcc leg then it's doubtful because there wont be a connection to Vcc, UNLESS another pin is tied high direct to Vcc?
 
AlanJ said:
Its possible, if a chip has failed anything could be shorted inside, but if you've remove Vcc leg then it's doubtful because there wont be a connection to Vcc, UNLESS another pin is tied high direct to Vcc?
Click to expand...

Thanks Alan, I'm chasing my tail with this! after snipping those pins I'm seeing other chips got hotter than they should! Am I correct in thinking some of the one's I have snipped are reading 4 ohms across the vcc and neg are bad? This includes the pal chip at U213 is the image available to burn to a new gal chip? But that's the least of my worries lol

Thanks Ronnie
 
Yes, snipping all legs and removing one by one is the safe course of action. You should be able to follow along the 5V rail with your meter and see the voltage drops happening at the respective chips. If a lot are shot this will be of limited value ofc...
 
drhex said:
Yes, snipping all legs and removing one by one is the safe course of action. You should be able to follow along the 5V rail with your meter and see the voltage drops happening at the respective chips. If a lot are shot this will be of limited value ofc...
Click to expand...
Thank you drhex

I think I have found it ! 12 chips snipped of the board and many vcc pins lifted but u8 a 74an is responsible for the short, not sure why as metering across its vcc to ground pin it does not show as a short?

I’m not sure this is the only issue until I re populate the board with new chips

Big thanks to all that replied

Cheers Ronnie


IMG_4369.webp
 
So without its Vcc pin connected powering the board doesn't show a short? Thought you measured a short without power?
 
drhex said:
So without its Vcc pin connected powering the board doesn't show a short? Thought you measured a short without power?
Click to expand...

It shows a short without power but if I read the snipped vcc pin to ground the chip is not shorted, but vcc pin connected to board reads 58 ohms, so the short has reduced with removing all the suspect chips. With this chip removed I'm reading 480 ohms 5v to ground
 
480 Ohms is 10 mA which sounds like ballpark. Any idea what happened to the board?
 
drhex said:
480 Ohms is 10 mA which sounds like ballpark. Any idea what happened to the board?
Click to expand...
I have no clue, only thing I can think is maybe a fake ramtron chip as that's the only thing that was changed recently, but not sure that would cause all the damage! for sure the gal chip at U213 is bad also and the likes of the cpu chips the BMST chip I don't know if they survived!

Thanks Ronnie
 
Do you still have the reset and clock circuits working? If so, you should be able to see activity on ADR and DATA bus at reset.
 
drhex said:
Do you still have the reset and clock circuits working? If so, you should be able to see activity on ADR and DATA bus at reset.
Click to expand...

Not sure but I will check that first, I will put back all the lifted legs of the decoupling caps and fit sockets where the chips have been removed, what needs to be fitted to check the ADR bus and reset? ram rom and cpu?

Thanks Ronnie
 
Found a further 4 bad chips! These being some of the one’s I had lifted the vcc pin, they are reading around 58 ohms on the vcc and ground, 2 of them are the gals, I think I have found the gal files online just hope they are the right one’s

Thanks Ronnie
 
I have all the bad chips I know about ordered inc 3 gal's, will no doubt have issues further on! Will keep the thread updated

Thanks Ronnie
 
Can you burn the PALs?
For checking ADR you need nothing but the CPU, clock and reset. Should put boot address on the ADR lines to get the boot code from the EPROM. Think it starts getting the first address from FFFE.
 
Moonraker said:
PAL is normally game specific. I had a headache getting one for goldeneye.
Click to expand...
Is it really! so would me putting jurrassic park roms on an x files board cause this destruction? I can only see the files for the chip locations not any specific type?
 
drhex said:
Can you burn the PALs?
For checking ADR you need nothing but the CPU, clock and reset. Should put boot address on the ADR lines to get the boot code from the EPROM. Think it starts getting the first address from FFFE.
Click to expand...

I should be able to burn these I have a Wellon programmer Thanks Ronnie
 
AlanJ said:
Sounds like that board got a good shot of the wrong voltage at some point!
Click to expand...
I don't see how that's possibe? I removed a cpu board from a working game then fitted this one and it popped the fuse! Nothing was plugged wrong
 
A programmable array logic chip, I would have thought it unlikely to cause the destruction you've experienced but perhaps it's not inconceivable. I can certainly say that the game won't work until it has the correctly programmed PAL fitted.

@myPinballs might know better...
 
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