Help on Vs board repair/repair log

[Xeno_Dragon]

I just pick up 12 Vs board in need of repair. My 1st question which I think is an easy one.
- About half are missing 4 caps in the top right (control side) Any reason for this? --this turn out to be a mod to get the light gun working
- Some are missing sound (waiting on ic, to test)
- Some are reset (is there a watchdog cct?)
- Some blue screen
- Some white screen

Most have one side working. Any insights on these problem would be appreciated

[Jumpman]

Hi,
I suggest to look in the schematic: http://www.johnsarcade.com/vs_downloads/misc/VS_Schematic_%28MDS-02-CPU%29.pdf .
If you follow the reset line of the cpus you can see it goes in the watchdog circuit (G2 and G3). With your logic probe check if the logic components (the flip flops, inverter and the logic gate) are ok.
The S sound is on the J line from 8 to 10, the M sound E7 and F7, check the capacitor in this section of the circuit if the capacitor are all ok check the OPAMPs.
The RGB lines are all driven by the PPU check if the traces are ok, check if the video pots are ok.

[Xeno_Dragon]

Update on board repair. Sorry for the lack of info as I fixed these before documenting things down.
Missing sound
-on both cases it was a bad LM3900 (5N,5R)

Missing red
-bad LM324 @1P (floating signal) after replacing that the red was a little wash out. Replacing the 4066 (2P) fix that problem

Blue screen
-bad CPU socket
-bad ram (1E,6E)
-broken Ram trace

Missing player 1 input
-bad LS240

Thanks for the info jumpman, ill keep that inmind for future repair. The resetting issue turn out to be because of my test rig (psone monitor). I have one board here that is still giving me a blue screen even after checking/replacing the CPU and ppu socket, 6116 ram, and 5533p-a ram. Next up is the EPROM socket and 8416 ram I guess. Is there a test EPROM for the Vs. Board? I'm slowly picking away at these board, on what little free time I have. I do have to say tho that troubleshooting is some what addictive.

[Jumpman]

Have you checked if the logic components used to interface the various memories are ok? There are a couple of LS138, LS373 and some "and" and "not" gates.
Check if you have a logic probe the signals CE, OE, WE (all negated) of the memories.
Reflow the solder of the sockets.

[Xeno_Dragon]

Symptom
 - Blue screen on sub side of pcb

Repair
 - Address line was giving odd reading (depending on when I turn the board on). All the eprom socket had continuity to the address line. Replacing the 6116 ram did not repair the issue. After checking all the logic it turn out to be a short on the PCB between address 11 and 12. I trace the trace between the CPU and socket 6D and it look intack on both surface of the board. My guess is its some how conducting between the board? I cut the trace and ran 2 jumpers. The board run now.

[Xeno_Dragon]

Symptom
 main/sub colour look faded. Adjusting the colour pot has little change

Repair
 Replace the analog switch CMOS 4066 @ 1p/3p fix the problem