(Topic ID: 116454)

Bally MPU Locked On LED - been testing and stuck


By parkway

4 years ago



Topic Stats

  • 38 posts
  • 11 Pinsiders participating
  • Latest reply 3 years ago by balzofsteel
  • Topic is favorited by 3 Pinsiders

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#1 4 years ago

So was bored this weekend and decided to rebuild a solenoid driver board (SDB) I had lying around along with a spare MPU board with working Fathom roms (I tested it in my Fathom and it worked). MPU rebuild went great and tested good with a known, good working SDB. On to rebuild the spare SDB now. So, long story short, in my haste and while distracted on the phone with my brother, I installed the 5v regulator in backwards. Didn't realize this until I turned her on and *pop*, blew R50 on the SDB. Had my nicely rebuilt MPU in there too. Awesome. Stupid mistake.

So this is where I'm at with the MPU using a known good SDB (will have to figure out what I damaged on the rebuilt one later): LED locked on. Getting 5v at the two test points on the MPU. Pin 40 on U9 goes high as it should. Shorting pin 39/40 on U9 (or jumping r1/r3 junction to TP4) as suggested in the guides does NOT cause the LED to flicker or start flashing (LED still locked on). I also checked the voltages on the legs of a couple of other chips suggested in the guide and they seemed a little high (like 3.3 instead of the 2.5 like it should be). I swapped out what I am pretty sure are good U9 and U11's and LED still locked on.

Could I have fried something in the reset? I'm guessing with the voltage regulator in backwards I dumped some current into the MPU and blew at least a couple things up. Any suggestions? Again, I'm not horribly worried since these are spare boards, but it would be nice to have a spare set to fall back on just in case (without dropping $300+ on Alltek boards). Not to mention, a good learning experience (like don't install the regulator backwards a__hole!). Thanks.

#2 4 years ago

I should add that the MPU before I rebuilt it had zero acid damage (and was working fine after the rebuild anyway).

#3 4 years ago

No one? I've followed Clay's guide and have repaired these before, but something isn't adding up and just looking for a "fresh set of eyes".

#5 4 years ago

Not even a quick wink. And MPU is getting a nice, solid 5v at both test points that should be 5.

#7 4 years ago

Ok. Dumb question, but how do I check them? Anything a DMM can do?

#9 4 years ago

Great. Thanks very much. I just so happen to have some spares of all of those, so I guess I'll swap them in and see. Is there anything else I might have particularly blown up besides IC's?

#12 4 years ago

Yes it's on the bench; good boards are in the machine itself. I'll give pulling the ic's a try and go from there as long as this blizzard doesn't knock my power out.

Is Q2 the same scr as on the lamp driver board? Looks very similar.

#15 4 years ago
Quoted from barakandl:

no

it is a transistor. go to great plains electronics. search for a the bally mpu rebuild kit. download pdf with part layout.

Ok, thanks.

Quoted from ChrisHibler:

Some Bally/Stern MPUs will not boot on the bench when using a PC power supply. Give it a try in the game.

Good to know. I will try it in game.

#22 4 years ago

I'm going to dig into this more after work tonight. I actually do have some 2N3904's lying around (no idea why) so I'll replace that while I'm at it.

Clay has something about the Dallas on his guide and I found it interesting. I skimmed it before but maybe I'll give it a thorough read and try it out one day.

Yeah it is nice to bench test things instead of swap them in and out of the machine, but I heave heard others say that sometimes it doesn't work.

#24 4 years ago

Awesome, thanks. I like Ed so I will most likely grab it from him when I'm ready. I have another MPU that I have to fix (and this time hopefully not blow up), so maybe I'll go that route with it.

#27 4 years ago
Quoted from barakandl:

If pin 40 of the CPU is high than the reset section is probably working.
I think using the Dallas reset generator is not worth it unless you are already replacing all components in that area due to corrosion.

The Fathom board I've been talking about, pin 40 does go high so I'm not too worried about the reset. I think you guys are correct about one or more IC's being bad. I'm not going to go tinker with anything that doesn't need it.

The other spare board I have (out of a Centaur that was in a flood) has some acid damage and I bought it for a few bucks to basically just screw around with and work on for fun. That one I know has a shot reset section so I was thinking of trying the Dallas route on that.

#28 4 years ago
Quoted from JKnPA:

1. Don't replace the transistor unless it is bad.
@ I'm not saying it is bad; just an easy check.
2. Bench testing...... you test your know Good MPU. If it tests Ok, then you can compare it to your
Bad MPU.
* Just my opinion/just trying to Help!

I hear ya'. I appreciate it.

#30 4 years ago

Ok. I had some time to monkey around with this. I decided to rule out the chips so I took a known working (as in tested it tonight) board and swapped U9, 10, and 11 from the "bad" board into the working board. Using those chips from the bad board, the good board gave me a flicker and began to boot, so I know that those are good. So that narrows it down to the ROMs I guess. My reset circuit seemed to test ok, so maybe I isolated it to the ROMs themselves? Glad I at least didn't blow up U9 thru 11.

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