Making 1 good 4-slot from two bottom and one top board

[lithy]

Looking for a little advice on best way forward.

I have a 4 slot that came with a project cab. It is doing a watchdog error clicking even with the diagnostic bios. When I got it, the board itself looked like it was in very good physical shape on top.



It did have a replacement style battery already in place with decent voltage but it does look like there had been some mild battery damage previously. I don't see any repairs though.



The bottom isn't as nice, there seems to be lots of what I think is wear to the masking along traces? I'm not sure if that's what I'm seeing or if it's a sign of a problem, new to this. There is also a big corroded spot toward the middle of the board, also a smaller one (upper right comnector of the photo but I seem to have continuity across the two pins)

 

[lithy]

The second board is a little rougher but still not bad. Had the original battery, I didn't take a photo before removing it but had just 0.1x volts left.

With the exception of the corroded fasteners, the bottom looks better overall than the other one to me.

Obviously the mess in the audio amp area catches your eye, there are definitely some leaking caps, the rusty fasteners and the volume sliders.

This one, using the top game slot board from the other 4 slot, passes all diagnosis bios tests and boots games. It has no sound and also a small graphical issue (fix layer?). I'll see if I can get some more example photos if also shows up on stuff like the flashing Press Start during attract mode.



So let's say I clean up the mess, recap it and figure out how to clean or replace the volume sliders. Could the graphics issue also be coming from these issues or am I looking somewhere else? And if I am, why would it be passing the diagnostic bios tests. Just asking because I assumed all the caps stuff had to do with the audio only but maybe there is some crossover somewhere.

 

[lithy]

Ideally, I would like to get both of these boards working 100%, but I'll settle for fixing the easier of the two and keeping the other around for spare parts.

One last thing, does anyone know what this resistor is for on LS161? It seems original, my working 4 slot has it, the watchdogging board in post 1 has it, but the board in post 2 does not. Just something I noticed in case it means anything for the graphical issue.

 

[GohanX]

Bold of you to make a thread about bottoms and tops in this forum.

 

[ack]

Ideally, I would like to get both of these boards working 100%, but I'll settle for fixing the easier of the two and keeping the other around for spare parts.

One last thing, does anyone know what this resistor is for on LS161? It seems original, my working 4 slot has it, the watchdogging board in post 1 has it, but the board in post 2 does not. Just something I noticed in case it means anything for the graphical issue.

View attachment 82094

There are different versions of the board. The one with the hand soldered resistor is older then your the board in your 2nd post. If you compare that section on the 2 boards you will notice your 2nd post board has a extra resister integrated into the board to top-left of the 74LS161 IC. The resistor is unrelated to graphics.

 

[lithy]

Took some more photos of what I think is the better board at this point after cleaning it up some more.

Leaning toward recapping it first to see if it fixes the no audio issue and then trying to figure out the fix layer vertical lines if anyone has suggestions there I'd appreciate it.

On the recap, is anyone out there selling ready to go cap kits still or do I just need to spend the time looking up parts on Digikey?

Board cleaned up



Battery area traces look great



The audio section, not so great, any cleanup advice for the rusty stuff?



Bottom looks pretty good too



Besides the rusty hardware which I'll either find replacements for or swap from the other board that has clean hardware.



A couple scuffed traces

 

[BIG BEAR]

On the recap, is anyone out there selling ready to go cap kits still or do I just need to spend the time looking up parts on Digikey

SNK Neo Geo MVS MV4 Cap Kit

<p>This kit contains all electrolytic capacitors for the 4 slot SNK Neo Geo MVS MV4 system boards (both main and cartridge PCBs).</p> <p>For a list of included parts, please see our <a href="/wiki/Neo_Geo_MVS">MVS wiki</a> page</p>

console5.com

BB

 

[lithy]

SNK Neo Geo MVS MV4 Cap Kit

<p>This kit contains all electrolytic capacitors for the 4 slot SNK Neo Geo MVS MV4 system boards (both main and cartridge PCBs).</p> <p>For a list of included parts, please see our <a href="/wiki/Neo_Geo_MVS">MVS wiki</a> page</p>

console5.com

BB

Perfect thanks!

 

[channelmaniac]

Obviously the mess in the audio amp area catches your eye, there are definitely some leaking caps, the rusty fasteners and the volume sliders.

Neo Geo caps don't leak. That board has been wet.

 

[lithy]

Neo Geo caps don't leak. That board has been wet.

Thanks, I assumed some white crusty/fuzzies underneath were from failed caps leaking fluid. I'm not near the board at the moment, will add a photo when I can. It resembled some similar caps when looking around online but that's not evidence of a cap failure for these?

I did figure it had seen moisture just from the rusty hardware and ack was able to give me a different troubleshooting tip to run the Z80 test using a regular cart with the diagnostic bios. I get this error on all four slots, but haven't really followed up since this testing, it didn't exactly match the good/bad result ack told me to look for, but looks like it could mean a Z80 issue?



If it's the Z80 itself as opposed to traces, I think I'd trust myself to swap it and I bought a cap kit for it as well figuring it would be something to do (my soldering is...fine, but long out of practice on PCBs since I did a rom swap to make a Ghostlop decades ago).

In any case, as always, I appreciate all the input and free advice from the experts!

 

[ack]

The advice I gave was based on the messages the update diag rom generates, which can be found here

GitHub - jwestfall69/neogeo-diag-bios: Disassembly of smkdan's Neo Geo Diagnostics BIOS with added features

Disassembly of smkdan's Neo Geo Diagnostics BIOS with added features - jwestfall69/neogeo-diag-bios

github.com

The message in your picture without having a diag cart is an indication the z80 is working fine. If the z80 was dead you would have gotten

Z80 SLOT SWITCH IGNORED (SM1)
SM1 OTHERWISE LOOKS UNRESPONSIVE

IF MV-1B/1C: SOFT RESET & HOLD B
PRESS START TO CONTINUE

Prior to switching to the diag cart, the diag bios notifies the z80 of the pending slot switch and z80 must reply with the correct response to indicate its ready for the switch. If it doesn't give the correct response (for whatever reason, which could be something other then the z80 being dead) the diag bios will give the above error. If it does give the correct response it will do the slot switch, which would enable the diag cart. In your case you don't have the diag cart so failing the test where the diag bios/cart try to talk to each other.

 

[lithy]

Some progress...

Had already removed one cap before I remembered to take a before pic.



Depopulated showing the white stuff hiding under a bunch of the caps, must just be corrosion? A few of the caps came separated from their legs pretty easily.



Cleaned up



New caps installed



And from the underside

 

[lithy]

So with new caps, I now have some sound. Unfortunately there is a constant loud noise, the game audio is playing but it's low and can barely be heard underneath the loud constant noise. The sliders seem to have no effect, I don't seem to get anything through the headphone ports but I want to try again tomorrow to make sure.

Done messing with it for tonight, happy to have some progress though and the recap went smoothly enough and happy with the work I did. I'll see if I can grab a video of the sound tomorrow to see if that can help narrow anything down, I do need to put some of the hardware back on and ground it properly, also the sliders could be swapped over from the better looking watchdog board.

 

[lithy]

Ok testing continues, didn't realize that I guess those jacks on the board aren't headphone outputs? So I hooked the headphone/mem card board back up.

The headphone slider is definitely touchy but I can get it to a spot where it is working and I have sound on the left channel through stereo headphones but not the right. Also get sound on the center channel. Same results on both headphones jacks.

I took the top board off and the loud noise is reduced but still there, if I hover my hand near some of the caps around the audio amp chip itself the noise/interference gets louder.

 

[channelmaniac]

Check grounds on those caps that are grounded on one side and see if you pulled out any plated thru holes leaving a floating cap.

Also, if you need sliders, I can sell you a couple out of my boneyard.

 

[lithy]

Check grounds on those caps that are grounded on one side and see if you pulled out any plated thru holes leaving a floating cap.

Also, if you need sliders, I can sell you a couple out of my boneyard.

Using one of the line-in pins as a common ground, I have continuity to the ground plane on all of the red dotted caps. Admittedly I'm not exactly sure if that's what you mean.



I also checked for continuity at all the caps to wherever the trace went to and got good continuity from every pin.

PXL_20250303_231510407.TS

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Short video showing where the interference noise is coming from (at least as far as when I lightly press on them, not saying they're the source of the issue). The same happens with the two caps by the headphone board connector.

Thanks for the offer on sliders, I might take you up on that at some point, I can grab them off the other board I have, those are in fine shape, but for the moment I still have a dream that it might be fixable too.

Feels like maybe I should get a logic tester so I can start working my way through the circuit, but not sure. Still doing some reading.

Thanks as always for continued suggestions and help.

 

[lithy]

I should note also that just to be sure, I have run a MV1A one slot with JNX Big Red adapter on the cab and sound through the speakers is good. I know it's mono versus stereo but I hear sound out of both speakers so I figure the speakers themselves aren't dead.

If I get motivated I may pull my other 4 slot and run the stereo sound test. Unfortunately I don't have an easy bench testing setup so testing is only possible in the cab.

 

[lithy]

Ok decided to pull the sliders and they basically crumbled.

Headphone slider on the left at least had one good side.



Board after removal



Board after cleanup with IPA and an eraser. Better but not great, anything else I could use safely that might work better?



So, understanding that the condition of the sliders obviously wasn't good, I still get the same interference noise with the board, probably need to understand the audio amp circuit a bit better to see where to look next.

 

[lithy]

Besides the sound, the board has a small graphical issue. You can see it here as vertical lines in the SNK logos on boot up, on the title screen, and on the life bars and power meters. It affects similar items on any cart and changing slots made no difference so I figured it was not an issue on the top board.



So, wanting to take a break from the audio stuff for a bit I was scouring the board for any damage I had missed when I noticed just a little bit of a ding near a couple traces off the PRO-C0 chip. I looked at the pinout and even though I barely know what any of that means I was intrigued that the pins from 42-49 said FIX.

PRO-C0 - NeoGeo Development Wiki

wiki.neogeodev.org

Checking continuity to the SFIX chip on my other board, I worked out which pin from C0 went to where on SFIX and after some checking and rechecking (I really need some magnification, my eyes hurt). I figured out I wasn't getting anything from C0 pin 44.



I was preparing to attempt to bodge a wire and not sure what kind of luck I was going to have with my crap equipment on those tiny ass pins, but I put some flux down, a tiny bit of solder on the iron and just rubbed down along a handful of the pins. A few zoomed in phone pics and a couple redos to undo a couple bridged pins and I got to here and miraculously when rechecking continuity I had restored the connection between pin 44 and the matching SFIX pin.



So not wanting to push my luck further, I just cleaned up the flux with some IPA and took the board back to the cabinet to see if I had actually accomplished anything.



Woohoo! Will the fix last without a wire? I have absolutely no idea, but I'll cross that bridged pin when I come to it.

Now if I can figure out the sound the thread's mission will be accomplished.

 

[lithy]

I ended up checking a bunch of traces on the other bad board tonight after finding some more documentation I could understand. However, after finding 14 different lines without continuity to their destination after just checking the 4 top-bottom board connectors and all of the lines from the bios, I patched the bios to 68k lines and tried both with and without J2 jumpered and I'm still getting nothing.

I may continue to pick at it, but for now, I felt ok pulling the volume sliders from the board and moving them over to the other one.

They're a bit tricky to remove, had to get some heat to them from above and below to finally free them.

After soldering them into place, moment of truth, powered it up with the top board and samsho1 and after setting the unibios mode/region (will have to put a battery in) I heard a nice clear jingle. Checked through the headphones, all good too. Checked in the Sound Test, clean stereo split. Everything seems good.

Tired tonight, but I'll finish up by installing a battery holder and moving all of the good board standoffs, feet, and whatever other miscellaneous screws over from the other board tomorrow.

Now I have a 100% working 4 slot, Mission Complete! Thanks everyone that contributed help and suggestions.