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(Topic ID: 232745)

System 11 - Dead MPU & High Power Supply

By WonderMellon

1 year ago

Topic Stats

  • 16 posts
  • 4 Pinsiders participating
  • Latest reply 1 year ago by WonderMellon
  • Topic is favorited by 1 Pinsider


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acid top (resized).JPG
acid bottom (resized).JPG
Acid Damage (resized).JPG

#1 1 year ago

I just picked up a High Speed. It has been sitting for quite a while and was very well loved/used during it's lifetime. Went to power it on and only get speaker hum and some GI lighting. There is some definite acide damage at the battery holder. At least 3 of the connectors corroded off. I don't see much downstream damage, but I also may not have the experience to notice all but the worst.

After some forum searches and some poking around on, I did a few tests.

All the fuses test good.

Power Supply Test Points:
1 = 9.4 v
2 = Ground
3 = 11.28 v
4 = -14.7v

TP 1 = Ground
TP 2 = 9.4 v
Pin 8 on UU15 - 8.9v

It looks like the PS is putting out too much power on TP1 and TP4, while slightly low at TP3.

I get no lights on the MPU at all. Since the game is only a few hours old to me, I haven't pulled the MPU and checked to see if supplying the correct 5v will power it on.

I am looking for suggestions on how to get the PS to supply the correct voltage. My experience has been mostly going from 0v to the correct voltage after replacing caps or such. Bringing down voltage is new to me.

Is there a way to tell if overpowering the MPU has blown something?

I am learning, but still very much a novice. Please don't assume I have done something obvious, as I may not know about it.

Thanks for any help you can provide.

Acid Damage (resized).JPG
#2 1 year ago

The 5V regulator has problems. Do not trouble shoot with the CPU board attached - may cause more problems as this exceeds the absolute maximum supply voltage for some of the parts.

First thing to look at is the 5V section on the power supply. My first suspects would be the small electrolytic caps C7 and C8.

Once 5V supply is back to normal - CPU board may need a bit more attention.
Battery corrosion seems to have been eating some traces around U41 so the damage may be worse than it appears.

#3 1 year ago

Acid above U44, looks like a little above u45. Some below SRC3, and to the right of SRC5. Maybe its just dirt, But L2, looks a little burnt.

#4 1 year ago

Thanks for the quick replies.

No worries about testing more, I wasn't planning on turning it on again. Looks like I will need to get a cap kit for the PS and a remote battery holder. Anything else I should pickup while I am ordering things to replace?

JR99svt, I am not sure if that is acid or dust either. My assumption was dust. It sure looks like dust on the caps (?) above the battery holder. If I take a few swipes with a soft brush and it comes off, would that indicate dust instead of acid? Does acid really travel up against gravity? Is there a way to test L2 to see if it is bad?

I'll post back when I get the it attached to 5V to see if I get any sign of life. Right now I am worried this may be beyond my skills.

#5 1 year ago

Once the battery starts leaking, the corrosion starts. It's not necessarily the battery acid that kills the board, it's the corrosion. I'm not 100% on the chemistry, but corrosion spreads on metal when electricity is passed through it. If it's eaten into the traces around any of the chips, it gets hard to repair. You may not know the extent of the damage till you pull parts.

#6 1 year ago

OK, Good News/Bad News

The good news is that when I removed the board and applied 5V, 12V and Ground to the J17 pins, the LED display gave me a big fat 0. Pressing the CPU switch changed it to an 8 for about 2 seconds, then went back to a 0. Based on my newly acquired limited knowledge that seems good.

The bad news. It looks like the battery corrosion indeed moved up instead of down. I removed the battery holder and, while it looks like mold, I am sure it is corrosion. The back of the board doesn't look too bad, but the front seems pretty bad. The resistor packs look like they have started corroding at the bottom as well. Is this something I can remedy myself?

acid top (resized).JPGacid bottom (resized).JPG
#7 1 year ago

WonderMellon Up is much better than down

#8 1 year ago

What do you mean?

#9 1 year ago

The components "up" are much easier to replace than "down" components

#10 1 year ago

Ah! Got ya. Thanks.

Ordering replacements.

Looking at the power supply now. Hoping replacing the caps get the voltages under control. They are too high right now.

1 week later
#11 1 year ago

Trying to work my way through troubleshooting from start to finish. That means power supply first.

I purchased a cap kit and replaced all the caps. That brought back my 100V (which I didn't find out I was missing until later) but is did not help my other readings. The next suspect was the transformer on the heat sink. Replaced that and I am still too high. Now I get

1 = 8v
2 = Ground
3 = 22 v
4 = -14v

The low voltage section is not that complex and I have now replaced almost all of the items. I am at a loss of where to search next. If a voltage was out, I could try to find bad resistor or cap. Any thoughts on what I should replace or what I can test to see where the out of whack voltage is coming?

#12 1 year ago

Did you replace the Bridge rectifier? If you haven't, that would be next for me, then IC1, if the transformer on the heat sink you were talking about was Q5, 2n6057.

#13 1 year ago

I have spent the past hour testing and measuring. After retesting, I think my replacement transistor may be suspect (It was a used one and I "assumed" it was good). Here is my latest information

I tested from the C, E, B points under the power supply. With the original transistor I am getting
C = 13
E = 13
B = 0

Give me 13V at test point1

With my replacement, I am getting
C = 13
E = 0
B = 0

Gives me 0V at test point 1

Testing the bridge rectifier gives me 14V at the + and -14 at the negative.

Would you be able to confirm that the correct values of the transistor should be
C = ~12
E = 5
B = ??? (I am assuming 0 for ground)

I am very new to schematics, but it looks like the Emitter of the transistor is what is supposed to provide the 5V.

I have brand new transistors in route, but won't get them until Monday or so.

Thanks for your help.

#14 1 year ago

I will measure when I get home from work

#15 1 year ago

C is 12v
E is 5.2V
B is 6.5V
You dont have voltage on B, it comes out of IC1, Pin 10. what voltage do you have on pins 10, 11 and 12 of IC1? (measure each pin to ground). Put the newer 2n6057 back in.

#16 1 year ago

I took a few minutes to remeasure before I left for work. It looks like the pad on the front of the board does not allow enough connection for the DMM probe on the base line so it just read 0. I turned the board over and measured from the pins on the back.

Here are the voltages:

IC pin 10 = 13v
IC pin 11 = 26v
IC in 12 = 26v

C = 13
E = 12
B = 13

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