I've been working on repairing a Stern MPU-200 board that came with my project Star Gazer for the last couple of months. Up until now I've tracked progress on my https://pinside.com/pinball/forum/topic/star-gazer-rising/ thread, but I think it's time to get more experienced eyes on this.

I am trying to learn as I go here, which is why I haven't just bought a replacement board or sent it off for repair, so bear with me please.

The board had some mild battery damage along the bottom edge and below the 5101 socket.
IMG_0196 (resized).JPG
IMG_0202 (resized).JPG

I cleaned up the MPU board, cut out the bottom 1/2 of the U8 socket, and removed C8 and C23. These were the only components that seemed to have any contact with the alkaline. I used the toilet bowl cleaner method to neutralize the alkalinity on the board.

I replaced the bottom 1/2 of the U8 socket, and put in new capacitors for C23 and C8. The board trace for U8, pin 8 (ground) was shot so I used a small jumper wire there to work around the missing trace.

Then I added a new large capacitor to replace the orignal battery so the volatile memory will be preserved so long a the game gets turned on relatively often, which is fine for my needs/usage.

Finally, I covered the exposed ground traces with solder to avoid future corrosion.

IMG_0639 (resized).JPG

IMG_0640 (resized).JPG

On my workbench, using my ATX powersupply, the MPU comes up with a locked on red LED.

I've been following the steps from the PinWiki article (http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#LED_locked_on):

Using my logic probe on U9, pin 40, it stays [EDIT] high on power-up, with no blip to high at all.

Shorting U9, pin 40 and 39 doesn't do anything (possible it could jump start the boot sequence).

So I worked though the various tests as described in PinWiki using my multi-meter and logic probe...

Removed all ICs except U6, U9, U11, and the roms.

All of the following power readings are correct:
U6, U2 (ROM): +5 = pin 24, GND = pin 12
U9 (CPU): +5 = pin 8, GND = pin 1 or 21
U10,U11 (PIA): +5 = pin 20, GND = pin 1
U8 (5101): +5 = pin 22, GND = pin 8
U7 (6810): +5 = pin 24, GND = pin 1"

U9:
- P3, P36, P37=Pulsing (good clock)
- P2, P40 = 5vdc (good)

Except for this one:
- P5 = No pulse, low. (BAD)

According to PinWiki: "Check U9 Pin 5. This is the VMA line. It is a pulsing signal and reads about 2.8v with a DMM. If incorrect first try a new U9. Next check U14D, U15C, U19B. U15 is the most likely IC to have failed."

So I went ahead and ordered a new 6800 chip, and also the updated v9 roms for Star Gazer while I was at it (and some new 5101s in case those turn out to be bad too).

Using the new 6800 and roms give precisely the same readings and results.

Next, I tried replacing Q1 and Q2 as I had the transistors, still no change.

So, I could really use some advice on where to test next to further diagnose this board. Any help is appreciated.

Okay that makes sense re: no VMA.

I messed up in my post, U9 pin 40 stays high, not low.

Looking through the schematics for the reset circuit now...

I think I have most of the components listed there, but not Q5, which is the one I want to replace next.

Based on findings while working on the SDB that was paired with this board I have reason to believe that it might have received more than 5v at some point (failed voltage regulator), so besides the transistors I guess any diodes in that circuit should also be replaced?

Nothing looks fried to look at it.

Installed a new Q5, no change.

Tested voltages at each IC:

U9 p40: 5.29v
U10 p40: 4.98v (empty socket)
U11 p40 : 4.36v populated / 4.45v @ startup - this seems like it could be a problem, no??

U13 p24: 5.27v empty
U8 p24: 5.27v empty
U14 - U20: 4.97v

Any thoughts? Assuming U11 @ 4.36v is a problem, suggestions on how to debug that circuit?

As far as I know this is the original board but this game was hacked up pretty good when I got it so could be from another game.

Good eye on the dip switches (?), but they only affect settings that wouldn't affect the board booting up AFAIK.

Really? Where are they I can't find any?

Yeah, I managed to find them and they seem correct.

Thanks for the link.

So does anyone else think that 4.36v on pin 40 if U11 might be a problem?

I need to work back from there in the schematics to see what components might be in play.

Thanks for the link! These look more detailed, I will run through them.

I previously used the steps in PinWiki and documented my results in https://pinside.com/pinball/forum/topic/help-me-fix-this-stern-mpu-200-board#post-3594414. To answers your question, no, shorting the pins on U9 doesn't cause any change in the LED.

Okay, started going through the procedure, here's what I've found:

>SYMPTOM IC: LED "ON" continuously

U9, Pin 3: Pass = 2.4VDC, reads 0.7VDC (fail)
U9, Pin 36: Pass = 2.6VDC, reads 2.5VDC (pass)

Since Pin 3 fails, next test is U16, pin 4, 5, and 10.
Says to check against voltages in schematic, but my schematic reading skills are poorish, doesn't list the voltages, seems like you'd have to calculate it based on the components in the circuit from a known voltage, so no idea what they should be..
Pin 4: reads 2.3VDC
Pin 5: reads 5.1 VDC
Pin 10: reads 2.14 VDC

These at least seem "reasonable", hopefully someone can confirm if they are the expected values or not. (?)

Assuming the U16 tests above are "pass", next test is to remove U9 and retest.
U9 socket only, pin 3: 0.7VDC (fail)

Solution: Replace U15.

So I also captured readings from each pin on U15, maybe someone here can comment if they look good/bad/ugly?

U15 (VDC)
P1: 2.3
P2: 2.3
P3: 2.2
P4: 5.2
P5: 2.1
P6: 2.0
P7: 2.4
P8: 0.03
P9: 0.7
P10: 2.2
P11: 2.2
P12: 2.5
P13: 2.2
P14: 5.4

These seem odd enough that I'll go ahead and replace U15, unless I hear otherwise.

Yeah, retested mine and I'm getting these on U9:
P4 4.9
P5 0.28
P10 0.28

Not sure if my MM can check frequencies, I don't think so, it's a Fluke 73III:
IMG_0901 (resized).JPG

But I do have a logic probe. Here's what I get using that on U9:
P3 low & pulsing on probe, 0.7VDC on MM
P37 low & pulsing on probe, 2.4 VDC on MM

So likely at least a bad U15.

You're right, transcription error! The correct readings from U16 are:
P1 4.85 P8 0.7
P2 4.85 P9 2.2
P3 0.19 P10 2.2
P4 0.18 P11 2.5
P5 2.0 P12 2.2
P6 4.85 P13 2.2
P7 0.01 P14 4.98

So it looks like my U15 is bad at least, if these chips aren't too expensive I'd like to order the whole set that I might need to avoid additional shipping costs and delays. Any suggestions on where to acquire them all in one place?

Well, I was basically gifted that MM so didn't really go through a vigorous purchasing evaluation or anything.

So that chip is $0.90 at Great Plains. I should just get all the chips that might be bad on the board in case something else is wrong after I replace u15, no?

I found your advice from another older thread:

"If you want to cover all your bases. Get a 4049 x2, 4052, 4011, 7400, 9062 as well. Your CPU is probably good, but it doesn't hurt to have a replacement. Having an extra PIA or two is a good idea as well."

So I've got these in my cart and will be ordering soon. The shipping is almost as much as the rest of the order so I'll just get them all in case I need them, and if not, there is always the "next" project.

Parts arrived and I replaced U15. After replacement no change in behavior, LED doesn't even flicker.

U15 MM voltage readings - original bad reading followed by new (good?) reading:

P1 4.85v 2.34v
P2 4.85v 2.34v
P3 0.19v 2.13v
P4 0.18v 3.74v
P5 2.0v 2.08v
P6 4.85v 2.34v
P7 0.01v 0.01v (ground, I believe)
P8 0.7v 2.34v
P9 2.2v 2.25v
P10 2.2v 2.25v
P11 2.5v 2.11v
P12 2.2v 2.35v
P13 2.2v 2.27v
P14 4.98v 5.11v

So those look better I think, maybe someone can confirm if these are consistent with a known good U15?

Since it didn't improve the LED lock on issue, I went ahead and replaced U16 as well. Unfortunately, I managed to damage the R26 11K resistor in the process, so need to pick one of these up before I can repair it and try the board again.

IMG_0992 (resized).JPG

It's been a long time but I've finally gotten around to looking at this board again.

Since I last worked on it I've added a desolding station to my workbench which makes replacing components a lot faster/easier. I replaced R26 and decided to shotgun replace all the components included in the Bally -35 Battery corrosion repair kit which also covers the Stern SB-200.

I'm super pleased to report that it worked! The board is booting fine again and I've put it back into the game.

Many thanks for all the help!