Gameplan Sharpshooter VCU Repair

modessitt

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Here's the schematic:

http://www.pinrepair.com/gp/manuals/gp-sch-vcu.pdf

I have diagnosed my failure to boot to be a lack of -5vdc at connector J8. Found a burnt .68-ohm diode on the VCU board which I have replaced. However, I still have no -5vdc.

Traced voltage through and have voltage on one side of the capacitor leading to the -5 output, but nothing coming out. As the cap is in series, I should be reading something coming out, right? All the diodes read as good, but I have no voltage at either side of that first 1N4004.

I pulled the cap and there is some very small crud around one axial leg that leads me to believe it may have blown. However, the schematic calls for a 22uF 16v cap, and the part in the board is a 150uF 25v cap (just like the other two electrolytic caps on the board. Should I put a 22uF in? I don't have a 150uF handy.

Another thing that might be causing the problem is that I am reading voltage into pin 10 of the TL497 chip, but nothing on pin 8. Now, it might not have anything there, but the documentation for these games isn't the greatest (as noticed by no parts designations).

What do y'all think? Should I throw a 22uF 25v cap on there and see what happens?
 
Mod,

Haven't looked at the schems yet, but I can tell you a GP MPU doesn't use -5vdc. Missing -5 won't keep the board from booting.

Obviously the -5 needs repairing, but if the MPU is down, there's something else going on.

D
 
Personally I would test the old cap to see if it's actually open before replacing it.

That said... if I were messing with this thing I would probably replace that supply with a switcher.
 
You can try and get the -5V supply working, there are not that many components to replace, OR you can modify the MPU board so that it can use standard 2716 PROMS instead of the TMS2716 PROMS that require the -5V, and eliminate that VCU board entirely.
 
Mod,

Haven't looked at the schems yet, but I can tell you a GP MPU doesn't use -5vdc. Missing -5 won't keep the board from booting.

Obviously the -5 needs repairing, but if the MPU is down, there's something else going on.

D

It needs -5 if using the TMS2716 PROMs as Geese has mentioned. I'm going to replace the cap and see what happens.

Lindsey said:
That said... if I were messing with this thing I would probably replace that supply with a switcher.

I might go that route if I have to, although the MPU would still need the original PS as it needs the +24 for the solenoids to be detected before it'll boot...
 
Replaced the cap and now have my -5, however when I hook up J8 all my voltages get pulled down and it still doesn't boot. Did some further checking and found one of the disc caps in the reset circuit is shorted. I'll get some replacements and see where that leads me...
 
Replaced the shorted ceramic cap and I'm now booting up and playing just fine. I just have a sound issue now, but I'm going to start a new thread about it...
 
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