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CPS2 A Board - No Video/Audio - Fixed!

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tika

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Hi all, I have a CPS2 A board (v4 revision) that worked a few weeks ago, and now when booting produces no video or audio output and I'm trying to debug/fix it so I can use it again. I have this faulty A board and another working A board next to each other and am using a logic probe to test points and compare between both boards.

Initially I thought it was a PAL chip which had died, so I transplanted the PAL ICs, to no avail, so I transplanted all of the socketed IC's (PALs 1-5, Z80) from the working A board to the non-working A board, but it made no difference either. I've also tested the original ICs from the broken A board into the good A board, and can confirm that they are working in the working A board.

I've probed around the boards and have noticed that with the failed board, the RESET line is always low, and the ADDR lines (both A and B lines) are all LOW also, whereas on the working A board, both the RESET and ADDR lines are HIGH, so it makes me wonder is there something that's pulling these lines LOW and preventing the board from starting/displaying output - any thoughts on where to start looking? I've also had a look at the CPS2 reveng project and schematics, and I can't find where the RESET line is generated from

Additionally, on the failed board the custom B-21 IC is and the LS157s to the right of the B-21 (near CN4), are quite hot to the touch. Compared to the working A board, the B-21 there (and LS157's), they're not hot at all. Would it make sense that the B-21 has failed in this scenario and bringing the RESET line to LOW (and preventing the rest of the ICs from starting)?

Any help/guidance would be appreciated!
 
I believe the RESET signal originates from the M51953B IC that is between 11L and 13L on the A board. I'm not sure what path it takes to get where it needs to go. Try comparing pin 5 of the IC between your working/non-working A boards. If you find on your non-working never goes high it can be an indication the capacitor to the lower right of the IC is bad.
 
ack said:
I believe the RESET signal originates from the M51953B IC that is between 11L and 13L on the A board. I'm not sure what path it takes to get where it needs to go. Try comparing pin 5 of the IC between your working/non-working A boards. If you find on your non-working never goes high it can be an indication the capacitor to the lower right of the IC is bad.
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Thanks for the insight and direction, it has been quite useful and I ended up getting the board working! Below is a list of things I tried on the faulty A board and eventually figured out it was a fault cap at CC29:

I checked the schematics and noticed that the M51953B IC had a cap and resistor on the output pin #5, so I originally thought that either the resistor or cap had shorted which made the RESET line always stay LOW. I tried testing both of the R41 resistor (4.7k) and C26 (.1uF) cap in circuit but couldn't get a reliable read, so I pulled them off the board and they checked out OK.

I noticed the JP1 and JP2 jumpers near the 13K IC (74LS04), and on both A boards, the JP2 jumper is factory connected/bridged. Probing both of these jumpers on the boards produced inverse outputs - on the working A board, the probe output was LOW on JP2, but on the faulty board the JP2 was always HIGH.

With the JP1 jumper, when probing the top half of the JP1 jumper (closest to the number "54" above JP1 and close to R29), it read LOW also, but on the faulty board it was HIGH. The second half of the JP1 jumper (near C26) was the inverse of the top half of JP1 jumper for both the working and fault A boards.

I also tested bridging the JP1 jumper on the faulty A board to bypass the 13K IC , which surprisingly, stopped the B-21 IC from heating up but still didn't boot a game. It did however show a blank white screen when booting the A board up so there was a bit of progress there (previously there was no output at all). I tried to bridge the JP1 on the working A board, but doing that made the board not boot at all (no video output at all). This lead me to think the 13K IC was at fault as when bridging JP1, it would bypass the 13K IC, this turned out to be somewhat of a red herring.

Finally, this lead me to the CC29 1uF capacitor as looking at the schematic and the M51953B IC, which uses the capacitor on the DELAY line, so I thought if that cap is borked and always pulling LOW, there'd be an indefinite delay in starting the IC and producing output to the RESET line - I believe this was the issue.

I tested the cap in-circuit and got weird results on the faulty board, but on the working A board it showed consistent results by using the continuity mode on the multimeter, so I pulled it out and replaced it with another cap, and viola, the board works and boots properly!

Any idea how common do these Tantalum caps die?

On a different note, I've left the C26 and R41 components off the faulty A board and replaced the CC29 cap and to board came to life/works - I'm curious, will leaving these components off be a problem, or should I get replacements and put them back on the board?
 
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I want to call out @ack for the guidance/pointer above - it really helped me isolate where the issue could be - thanks mate!
 
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