(Topic ID: 261149)

Data East DE MPU low 5v on bench

By thedefog

4 years ago


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#1 4 years ago

***SOLVED - FAULTY ATX BENCH PSU

I've got a DE MPU that has a short pulling 5v to 4.1v. I've cleaned up previous work - poor solder & jumpers. I've buzzed traces on all the important sections/lines as usual 100%. My bench supply is good, Sys 11 MPU I have boots fine with it.

Is this low a drop consistent with a logic IC short, or should I focus on components like decoupling caps, SIP packs, etc.?

#4 4 years ago
Quoted from PinballManiac40:

Sounds like you have at least one short IC. Usually, I measure from 5 volts to the pins of 74xx parts first. A lot of times, I see a pin measuring 1 to 5 ohms to 5 volts. Could even be a pin shorted to ground.
Any history on the board, such as a coil locking on and shorting a transistor? Likely something like that would be the IC I would start with checking.

I found a few blown switch driver transistors, so I started with replacing them & the associated 74LS244 & SIP packs. Short is still present. Probably the PIA shorted here, but I hate removing PIAs until I am 1000% sure.

Quoted from Tuukka:

It should be easy to find the shorted IC. Just put your finger on them, the shorted one will be hot to touch.

I use this method with the PIAs usually, but this time it didn't work! I have an IR temperature gauge I use to compare like ICs for that purpose, and it didn't help here. Nice cat!

Thanks for the input

#5 4 years ago

Just checked the rest of all the logic & all other active components. No shorts. Checked switch PIA, doesn't appear to be shorted. Checked pins for gnd continuity as well, appears to be fine.
Flipped board, started looking for micro bridges. Buzzed out a lot of stuff and checked for nicked traces by previous repair work.... All good. This is driving me nuts.

#6 4 years ago

Bumping this. Hoping someone can offer short hunting advice for MPUs more specific than remove ICs based on educated guesses. My experience with divide and conquer is I rip up 1/4 of the board in the process of finding something dumb.

Based on what I've done so far:
-I'm assuming I have a larger sized physical bridge somewhere as ICs/components/traces are not heating up, freeze spray did no reveal anything abnormally warm, and not a direct rail to rail bridge brcause it powers and lights LEDs for PIA and +5v. It is behaving just like an accidental solder bridge would. I have about 410 ohms between 5v & gnd.
-I'm dropping a perfect diode drop amount from 4.9v on my supply to 4.2v, which should answer questions, but in reality has made things more confusing because of the behavior of the heat check. So I focused on physical short/bridge as the culprit.
-I've gone over both sides under the magnifier for about 3 hours and confirmed suspect areas with the schematics.
-Buzzed all traces originating from the CPU and complete D & A section, Ohmed traces and ICs, mv tested for vcc shorts, diode tested all logic...

Can PIA ICs short in a way that would drop the 5v rail like this? I want to believe the switching PIA is bad because of the shorted transistors I found, but the upstream 74LS244 from the 2n3904s was fine when I desoldered it and tested it out of circuit. Not possible to roach PIA and not that logic IC too, right?

#8 4 years ago
Quoted from GRUMPY:

I start with U-17 thru U-20 and U-45, U-50. I snip the 5 volt input lead one at a time and retest the ohms. If that doesn't get it then I go to the PIAs. These are Williams numbers, you will need to use the Data east corresponding numbers.

So all of the coil-related ICs (controlled and special). Thanks man. I'm running out of less destructive ideas, that helps.

#11 4 years ago
Quoted from GRUMPY:

I have also seen some techs (not me) cut the 5 volt trace in key spots to isolate sections of the board off for quicker testing.

I will try this. So far I've removed about 6 logic ICs, socket for CPU, and it is already getting close to where I don't want it to, as far as guesswork is concerned.
I also removed the blanking and POR sections out of concern of corrosion. Oddly my voltage dropped further down below 4v when I did this. I'm guessing by removing it, some of those inverting inputs that are normally low are high now, so they've activated other parts of the board to draw current.

I just bought another less messy used board off fleabay to rework simultaneously so I can have some comparison point for this one. Not sure what I'll do with two DE boards, but it wasn't too $$$, and the time invested in this so far is way more than I was expecting already.

#13 4 years ago
Quoted from barakandl:

Have you used this power supply on DE MPU before? The load may to be great and it pulls down the power supply voltage.

I haven't. But it is a 550 watt ATX supply that apparently delivers 30 amps @ 5v. It could be failing, who knows. It isn't a crap brand though, but perhaps this PSU has been overly stressed by bench work. I'll definitely investigate that angle tonight. Thanks!

#16 4 years ago

Thanks for your help guys, this has been a great thread info wise. Hope it helps others as well.
I moved onto other stuff to give my brain a break from it, but I'll try what was suggested with the power supply, the logic input snipping, and the vcc probing and report back.
I did check the sys 11 mpu 5v to compare and there was no significant drop.

#17 4 years ago
Quoted from Billc479:

If you have a spare board, compare current flows on the 5 volts. That should tell you more than just the voltage, and will let you know if your power supply can handle the load.

going back to this, drawing .79A on the DE board with no cpu, rom, ram, which was less than I had expected if I have a short. Going to test the sys 11 to compare, but I have to pull it again from the game to do it.. I thought these boards were pretty hungry and expected at least 1 amp (with CPU).

#18 4 years ago

What do ohms look like across 5v & gnd typically on these boards? Both this DE one and a sys 11 are roughly 550 ohms.

#19 4 years ago

***SOLUTION

Got an interesting update.

So, my converted ATX psu was problematic, but in a more complex way that I never would have guessed if you guys hadn't suggested I check it out.

So the way the ATX bench supply goes, you need a 10W 10ohm power resistor loading it to trick it to provide more than roughly 1/2 an amp of juice, like if a mobo was connected.

The power resistor in my supply failed by disconnection (I didn't zip tie it to the chasis, and it wiggled itself loose), BUT the load of the sys 11 MPU was just enough extra draw that it turned on the higher amperage output on the psu and prevented the voltage drop. So that is why it worked with sys 11, and not with DE.

I doubt anyone else will ever run into this issue.

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