(Topic ID: 264810)

Stern M-200 MPU doesn't always reset on power up


By Fytr

13 days ago



Topic Stats

  • 10 posts
  • 5 Pinsiders participating
  • Latest reply 8 days ago by barakandl
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#1 13 days ago

So I have an M-200 MPU in my Star Gazer that doesn't always reset/boot reliably when I power the game on.

It works fine about 2/3rds of the time. When it fails the MPU has a solid LED. Pretty sure it's not resetting consistently.

I power up my games with a wall switch that turns on the plug circuits. Often Star Gazer won't reset when the wall switch is turned on and I'll have to use the cabinet switch to power it off and on again. It almost always boots fine when I do this.

I completely rebuilt the reset section on this board after repairing alkaline damage on it a few years ago, so every component is newish.

I also saw a tip on http://techniek.flipperwinkel.nl/ballyss/rep/index2.htm#flash to use a capacitor to make flaky reset circuits more reliable:

"It could be the +5 volts is not stablizing within the "window" the CPU expects (about 50ms). To make the reset window longer, try putting a 470 mfd electrolytic capactor in the reset section (on the solder side of the MPU board). Solder the positive leg of this cap to the collector (top leg) of Q1. Then solder the negative leg of the cap to ground (the emitter of Q1). This will increase the reset timing length. Now test the MPU board. If the MPU board works and continues with the flashes, the +5 volts was just not getting "stable" in the 50ms window (there could be a power supply problem, probably with the large 5 volt rectifying capacitor on the driver board). "

I tried adding a 470uf cap as described but it made it worse. In fact it only booted once in about 10 tries with the cap installed. So I removed the cap and now I'm back to it booting about 2/3rds of the time with the wall switch.

Oh, the powerboard is using one of those new fangled solid-state voltage regulators also (voltages are fine once things are booted).

Any thoughts or suggestions?

#2 13 days ago

Are you talking about the 5V voltage regulator on the solenoid driver board? What voltage do you have on TP1 and TP3 (have you tied them together?) on the SDB?

#3 13 days ago

Got any other mpu200's you can swap in there and see if the issue follows the board that would tell you right away where to concentrate your repair efforts.

Also, try turning off the other machines on that circuit first and powering up - if it works then the sudden voltage demand with the other machines just might be enough to drop out the voltage. I don't have any experience with those +5 switching regulators, but I do know that they produce more high frequency noise that the machine was never designed to deal with. If you have a spare solenoid driver board with the old linear regulator on it try that as well.

#4 12 days ago
Quoted from dothedoo:

Are you talking about the 5V voltage regulator on the solenoid driver board? What voltage do you have on TP1 and TP3 (have you tied them together?) on the SDB?

Yes, on the SDB.

TP1 = 4.79vdc
TP3 = 4.65vdc

I also noticed that I replace R50 with a jumper wire. Been awhile so I don't recall the reasoning, maybe related to using the EzSBC.com PsU5 voltage regulator?

Quoted from slochar:

try turning off the other machines on that circuit first and powering up

Tried this, no improvement.

Quoted from slochar:

Got any other mpu200's you can swap in there and see if the issue follows the board that would tell you right away where to concentrate your repair efforts.

I don't, but I do have some other working Bally SDBs that I could potentially swap in to this game. Worth it?

#5 12 days ago
Quoted from Fytr:

I don't, but I do have some other working Bally SDBs that I could potentially swap in to this game. Worth it?

I would just to take another board out of the equation. They're pretty easy swaps especially if you have the cheapie plastic circuit board removal tool that Marco and others sells - I always lose mine so I usually have to order a couple on each order.

#6 11 days ago
Quoted from slochar:

I would just to take another board out of the equation. They're pretty easy swaps especially if you have the cheapie plastic circuit board removal tool that Marco and others sells - I always lose mine so I usually have to order a couple on each order.

Okay, so I swapped the SDB from my Flash Gordon. Problem gone. Interestingly, the problem doesn't occur on Flash Gordon either, even with the EzSBC.com PsU5 voltage regulator.

I won't keep it this way though, I like my games to have the original Stern or Bally boards in them.

I guess I could try swapping just the voltage regulators between the boards, a bit of work though. At least I know it's probably not the MPU board at all. I can live with re-booting the game manually once in a while.

#7 11 days ago

Start by adjusting the regulator to 5V under load. There’s a tiny adjustment pot on the EzSBC.

The jumper in place of R50 is indeed because of the EzSBC. It will never regulate to 5V with the resistor in circuit.

#8 10 days ago
Quoted from dothedoo:

Start by adjusting the regulator to 5V under load. There’s a tiny adjustment pot on the EzSBC.
The jumper in place of R50 is indeed because of the EzSBC. It will never regulate to 5V with the resistor in circuit.

I either didn't realize that was there or had forgotten about it since installing it.

So I adjusted it while it was installed in Flash Gordon to be an even 5.02vdc on the SDB. Then I moved the SDB back over to Star Gazer and it booted fine.

Here's where we enter "couldn't just leave well enough alone" territory...

So I figured I'd check to see if it could use any further adjustment on Star Gazer, and sure enough it was reading 4.85vdc on the SDB. So I adjust the pot up a bit and booted the game. Alternating solid red LED or flicker with no flashes on the MPU, and a fairly horrible static sound coming out of the speakers. Multiple reboot attempts, same thing. Solid red and single flash seem to alternate with power cycle attempts. Ah crud.

So not exactly sure what happened but it seems like I either under or over voltaged my MPU board with that last adjustment, or maybe all the power cycling damaged something.

So my voltages now are:
SDB: 5.15vdc
MPU: 5.05vdc / 13.88vdc.

Since I was getting a flicker but no flashes I replaced the 4 ROM chips with an older set that I had and it will now boot to 2 flashes. 5101 memory must have also taken a hit. I installed a couple spares I had but same thing, 2 flashes.

So it's roughly alternating between solid red and booting to 2 flashes now.

I guess sometimes best is the enemy of good-enough.

#9 8 days ago

I recently brought a Stars to life that had an MPU that had been exposed to 12vdc instead of the 5vdc it's supposed to get... It fried quite a few of the chips that are not able to tolerate more than 5vdc. In your case (mine was an mpu-100, yours is an mpu-200 which is a little different) that would appear to (possibly) include the ram chips at U7, U8, and U13 (2x 5101 and 1x 6810), the clock chips at U15+U16 (9602 & 3459). My ROMs survived, and it's possible your roms, pias, and cpu/6800 *might* have survived, though they have low voltage tolerances like those other chips. Not to say you're gonna have to replace all those chips, but that's what it took in my case.

Using a logic probe, it's possible to test the chips and determine what might be fried.

#10 8 days ago

You mentioned that R50 was jumped over but R49 also needs cut out of the driver board for the replacement switching regulator.

Adjusting logic voltage with all the boards connected can be dangerous. You can get a situation where the wiper pin of the adjustment pot goes briefly goes open as its moving it can cause the voltage to shoot up damaging ICs. Like when a speaker pop while adjusting a volume pot. Like frunch said if you sent over voltage down the 5v rail i'd expect lots of stuff to be bad and zero flashes. Usually U15 is the first to go. Ram and ROM fail. PIAs usually survive, sometimes CPU survives. I've had to repair replacement MPUs a couple times where every single chip is bad except 4000 series stuff after 12v goes down the 5v rail.

The original problem could be not related to power supply but the reset circuit too. The thing about the 470uF cap to extend the reset has to be a typo. That capacitor value is way to much microfarads. 470nF maybe. Investigate the reset circuit works properly. Low when first powered up and then quickly goes to high. Make sure you see the low to high and not no signal then high or always no signal. Sometimes when the reset circuit is blown it will still boot... sometimes.

Assuming you have tried known good 5101 chips check Pin 22 of each 5101 ram is near 5v. Check 5101 ram address/data continuity back to the cpu chip. Make sure the RW OD and CE1 pins of the 5101s pulse during its turn in power on self test. CE2 always high.

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