Space Invader PS Missing +12

troxel

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It's for a Midway 90400.

I'm measuring +0.5V for the +12 volts on the power supply. Switched power supplies and getting the same voltage. I started checking diodes on both boards and both appeared good. Moved on to resistors and found some discrepancies from the image. Odd thing is that both boards were measuring similar values. I have included the values in the image and if there are two numbers, they read differently. Is the original diagram correct on the resistor values?

I have not done a capkit yet, but wanted to see if anything else was wrong before I ordered parts.

Thanks,
Theron
 

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Did you test the resistors in or out of circuit? If they're in circuit and parallel they will not meter properly. They also tend to only fail open or higher in resistance IIRC.

Rebuild it and good luck.
 
You might just have a bad 7812.

Edit: Just reread and saw you had the same output from two different power supplies. Are you sure you're getting the proper INPUT to the power supply board? Check the output from the transformer, and make sure it's getting to the board. Check your fuses too.

-Ian
 
Removed the main PCB and now the +12 was 12.14. Now I assume something is wrong with the main PCB?

Theron
 
Removed the main PCB and now the +12 was 12.14. Now I assume something is wrong with the main PCB?

Theron

Fire it up with the motherboard plugged in, but remove the daughterboard. I've had a few where a cap in the audio circuit (which is on the daughterboard) was shorted....sucking down the line.

Edward
 
Removed daughterboard and the reading was back down to 0.6V. Got my ESR meter and tested all the caps in circuit and they appeared to be fine. Someone mentioned a white screen was from a bad cap near the RAM. Those all appear to be fine.

I appreciate the help with this game. It's frustrating since I played it for about 10 minutes before this problem appeared. Now I just get a repeated sound with a white screen.

Any other thoughts?

Thanks!
theron
 
Hmmm 12v dyin down...did you mess with the ram at all? Verify all your soldering connections at the rams if so, and/or see if any are getting really hot. Could have a bad ram going shorted. Also just look the board over for any other physical things like a solder blob, etc.

Was this working and just up and failed, or something else?

(sorry very tired tonight)

PS - 7812 could be bad yet - may just not be able to hack the load. I can't remember but I think its one of the 2 transistors on the black heat sink - could be 7815 too.
 
Here's a recap on what going on with the game:

1. Bad power supply = replaced 1k 1W resistor
2. Garbage on screen = replaced Rams 1, 7, G, and 6 (according to test rom)
3. Game worked great for 10 minutes then went to the 16 lines while I was watching the intro screen

Getting about 0.6V for the +12.

4. Replaced 8080A socket and socket H
5. Did the 2764 rom modification
6. Tried a different power supply
7. Checked all the Tant caps by the rams (out of circuit)
8. Double checked soldering of rams
9. TIP32A and TIP111 tested good (in circuit)
10. Replaced Test rom instead of 2764 mod rom
11. Rams or any other chips on the motherboard were not hot to touch

Steps 4 - 11 didn't change the +12 volts and still get a repeated sound with a white screen.

Should my focus be on the motherboard, daughterboard, or both?

Thanks again for the help.
Theron
 
Found a bad 10 uf 25V cap replaced it, but had no change. Also noticed a couple of the 74157 has a bit of corrosion on them, but all connections metered fine. Could these chips be part of the problem?

Any other suggestions or areas to look at on this board?

Thanks,
Theron
 
Last edited:
Any other suggestions on this problem? I am struggling on what to do next.

Thanks,
Theron
 
Had a chance to test this in another game, and found out the motherboard is what's giving me problems.

Wondering if someone has any other ideas on what to check that is shorting out the +12 volts.

Theron
 
So the daughterboard tested ok?

Looking over the schems..CPU, a 3245 @ C5, all the RAMS, and roms when jumpered that way all use +12v. Didn't see much else!

My bet is on a bad ram yet.
 
That's correct. The daughterboard is working 100%. Switched it with a working one, and narrowed the problem to the motherboard.

The +12 is still getting shorted to ground. I looked for solder splashed on the RAMs I replaced, but did not see any. How do I know if a RAM is bad without anything being displayed on the monitor? Seems like the test ROM and grounding pin 7 is dependent of the display on the screen.

Thanks again. Seems like the problem is getting closer to being solved, but just need some more advice.

Theron
 
Looked over the board again today, and I believe the problem was one of the tantalum caps. The ESR checked fine in circuit so I didn't bother with it. Unsolder it and checked the capacitance and it was close to zero.

It's the bigger blue one by the RAMS. It's marked +22 +25. I assume that's 22 uf 25 V. Is this correct? Also, can I replace this with an electrolytic cap that I have on hand, or do I need to order a tant for replacement?

Theron
 
Replaced two tant caps and now getting +12 back. I switched back to the 2732 test rom. I am still getting a white screen. Should the 2732 be getting +12, and if so, what pin? I tested pin 24 and 20, but not getting +12.

Had + 12 on all the RAM, CPU, and 3245. A couple other things I noticed was the 33 ohm resistor by RAM G15 looks burnt. I tested out of circuit and came out 33 ohms. Chip C4 is hot to the touch.

Any other suggestion on where to go next?

Thanks again,
Theron
 
When you say a white screen, are you testing this in the cab, and if so have you tried messing with the monitor at all?

2732 rom should not have 12v to it.

Im looking at this thru a phone, so im not sure if you had the bars without the rom in. If not, focus on leaving the rom out until you get the bars working.
 
It's been tested in the cab. The screen is all white, and it's getting raster. Just looks like a tv that's not on a station.

I don't believe the problem is with the monitor, but the motherboard. I tested the daughterboard on another machine and it worked fine, and the motherboard didn't.

I replaced the 33 ohm resistor, but not change. C4 is getting super hot when the game is on for a few seconds. Trying to figure out why. Any ideas on what to check?


Thanks,
Theron
 
Some chips get pretty hot, but if its almost burn hot id suspect a shorted trace somewhere. Ill take a look at a board in a little bit to see what ur lookin at.
 
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