Problem Pinball CPU 5v ground short

[Ronnie Dent]

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! Could a tiny decoupling cap take out a 4 amp fuse? I thought it would just burn up!
Board was running prior to this but started resetting and the longer it was on it would go into a reset loop and now it will instantly take out the 4 amp logic fuse.

Things I have tried
doing an ohm check around the board to try and see if I can get close to the short area but don't think my fluke is sensitive enough, getting 0.3 everywhere.
Lifted the 2 tant caps, removed every socketed chip, removed the 2 electro caps in the 5v rail, lifted one end of a few decoupling caps that I thought might look suspect!

The board is a sega whitestar and the schematics are available here link

Would welcome any advice?

Thanks Ronnie

 

[Ronnie Dent]

Here is a pic of the board

 

[philmurr]

In the absence of a short circuit detector my first check would be any tantalums. There looks to be one on the 5V rail just above the crystal in the centre of the board, pull one leg and recheck.

 

[Lurch666]

Have you inspected the underside of the PCB to see if any legs are bent and touching something they shouldn't?

 

[Ronnie Dent]

In the absence of a short circuit detector my first check would be any tantalums. There looks to be one on the 5V rail just above the crystal in the centre of the board, pull one leg and recheck.

Thanks for the reply Phil, that’s a decoupling cap I just checked it anyway, there are only 2 tants and have had both out
Thanks Ronnie

 

[Ronnie Dent]

Have you inspected the underside of the PCB to see if any legs are bent and touching something they shouldn't?

Yes I have been over it with a magnifier a few times but found nothing! only thing I found was a fractured decoupling cap and I just broke it open for now.
thanks Ronnie

 

[philmurr]

Thanks for the reply Phil, that’s a decoupling cap I just checked it anyway, there are only 2 tants and have had both out
Thanks Ronnie
View attachment 24445

Sorry I wasn't clear I meant the rant on the original photo above the 40 pin chip above the crystal but it sounds like you've checked that anyway.

Remove the rest of the socketed chips then try again but other than that there's not really an easy way to find it ☹️

 

[Ronnie Dent]

Sorry I wasn't clear I meant the rant on the original photo above the 40 pin chip above the crystal but it sounds like you've checked that anyway.

Remove the rest of the socketed chips then try again but other than that there's not really an easy way to find it ☹️

Ah ok sorry I did have all the socketed chips out and still had the short! it would be fairly easy to lift a leg of each decoupling cap I guess, but is it more likely to be a chip at fault?

Thanks Ronnie

 

[philmurr]

Ah ok sorry I did have all the socketed chips out and still had the short! it would be fairly easy to lift a leg of each decoupling cap I guess, but is it more likely to be a chip at fault?

Thanks Ronnie

It could be either unfortunately.

I've had a bit of success using a bench power supply with variable current limit. Power it up and slowly crank up the current limit and whatever is faulty goes pop or burns up.

That wouldn't work though with a standard switcher that just shuts down on a short.

 

[Georgian2]

What is the resistenve between 5V and GND? I alwas do it like philmurr said. If nothing pops, it will get warmer. I usually set 3V on the power supply and maximum amps.

 

[Ronnie Dent]

I’m getting 0.3 ohms to ground at the 5v input pins
I do have a bench psu but it does cut out for some reason, I can pulse the power by dabbing the ground and I get a brief flick of the 5v led but nothing is popping

 

[Lurch666]

If those are power regulator chips on the heatsinks at the top of your picture I would try those next.
No idea if they are prone to fail but they are on the power rails so might be the cause.

 

[Ronnie Dent]

If those are power regulator chips on the heatsinks at the top of your picture I would try those next.
No idea if they are prone to fail but they are on the power rails so might be the cause.

I think they are the 12v audio amps? Could not find anything on them to do with the 5v but maybe I should look closer at them

Thanks Ronnie

 

[Ronnie Dent]

I managed to run the psu at 3v 5 amp which powered the leds, nothing popping but 4 chips getting up to 100 degrees! I have a thermal camera and I'm getting a hot smell!
so U204 U207 and U208 are all 74HC245's ( I already pulled U204 as I suspected it earlier but it's not to blame)

And the pal chip U213 beside the cpu chip also reaching 100 degrees (not that either as it's been pulled as in socket)

Not sure what's going on? Is this progress lol? Should I crank up the amps more to potentially cook these chips?

None of these chips run hot on a working board

Thanks Ronnie

 

[Lurch666]

Just spotted what might actually be a power regulator in the top left of your picture.
It might again be for higher voltage but if it's on the +5V line then it's worth a check.

 

[Ronnie Dent]

Also the pal chip U213 off the board only reads 4 ohm across it's Vcc to neg, that's not right? but it's not the board issue

 

[Georgian2]

Worst case you can cut the vcc pin on the hot ICs and see if it solves the issue. You can cut them near the package so you can resolder it. This is a very last solution.

 

[Ronnie Dent]

Worst case you can cut the vcc pin on the hot ICs and see if it solves the issue. You can cut them near the package so you can resolder it. This is a very last solution.

Thank you, I did think about that and it's worth a try

 

[Ronnie Dent]

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

 

[Georgian2]

This area looks bad with a lot of acid damage.
You have a tough board there. I had an arkanoid with the same problem. All srams were shorted but only 2 got really hot, one did not get hot at all. Took me some time to find it. Myabe somehow an higher voltage managed to get in the 5V line and mess up everything.