MVS MV-6 Repair Logs

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Repairer: channelmaniac

Troubleshooting vertical lines on the MV6 6-slot system

Do you have vertical lines on the screen either on the moving foreground objects or background objects on your 6 slot board?

On the 6 slot board there are a bunch of 74LS251 8 to 1 data selector ICs. The ICs nearest slot 6 are for the foreground objects and the ICs nearest slot 1 are for the background objects.

The easiest way to check these chips for the bad one is to put a test cart in slot 5 and flip DIP switch 8 to pause the screen where you can see the problem images on screen.

Next take the logic probe and check pin 15 of each 74LS251. Look for the IC with no signals on that pin. If you find one without a signal then check the trace between the slot and that pin.

Not fixed? Next check pin 4 of each 74LS251 for data output. If one is missing then replace the IC.

Still not fixed? Next take the logic probe and put it on pin 15 of each 74LS251. Slide the tip over to where it momentarily shorts between pin 15 and 16. Look at the screen for lines. See lines pop up? That's not the chip you are looking for. Finally find one that doesn't pop up any extra lines when you short it across? OK. That's the data line that is the problem. Follow pin 4 of that chip and find where it connects to go to the bottom board.

Can't find where it goes to the connector? Check a neighboring 74LS251 IC and see where it goes. If you can find it then check for a bad trace between the suspect data line on pin 4 and the connector to the bottom board and patch it.

Still not it? Check the connection between the top and bottom board. If there is no signal then desolder the connector, disassemble it, pull out the offending connector piece, clean it, reassemble, and resolder it back in place. Test it.

Still no? Follow that trace to the chip it goes to and if the trace is good then replace the chip.

If that doesn't do it then call a priest and have the board exorcised.

Symptom: Stuck in Watchdog (a.k.a. Click of Death)

6 slot board would constantly reset... Logic probe on pin 18 of the CPU confirmed that the reset line kept bouncing. This was the watchdog circuit trying to kickstart the CPU into booting.

Checked the data lines and enable lines on the work RAM - checked fine.

Checked the enable lines on the backup RAM and discovered that the WE* line on one of the RAMs was missing its signal and the OE* line on both RAM chips was missing its signal.

Repaired the bad traces to fix the bottom board.

It appears that something corroded the plate-thrus on the board. Seems to be a common occurance on all models of the MVS boards.

Symptom: Graphics corruption

Found and repaired 4 gouged traces on the top board. Traces were damaged due to insufficient padding when item was shipped. The top cover was bent and the traces were gouged by it.

Diagnosing bad RAM ICs

Here's the info on the 6 slot board:

Upper Backup RAM: E9 Lower Backup RAM: F9

Upper Video RAM: L9 Lower Video RAM: K9

Upper Work RAM: J9 Lower Work RAM: H9

Just keep in mind that if you have an error at the bottom boundary then you need to check traces.

For example a video error of:

Video RAM Error:

Address    Write  Read
00000000   5555   5455

Shows up on the zero boundary. You need to check to see if the traces (wiring that is on the circuit board) are all good. A bad trace or a bad chip can each cause this type of error.

Symptom: Vertical lines in graphics

Some background objects had vertical lines in them - in this case the "shock Troopers, 2nd Squad" floating logo had lines. It's the logo that is on the screen when it is playing in demo mode. I traced it to connector CN8 and started tracing it on the bottom board.

UGH. It goes straight to NEO-CT0

As one final step before replacing that surface mount chip I checked the continuity between the top board and bottom. Bingo! No continuity. Next step - pull the boards apart and look at the connectors. They had something sticky on 2 of the pins and up in the socket.

Desoldered and disassembled the socket then cleaned the pins on both connectors. Reassembled, resoldered into place, and tested. Board is fixed.

Moral: Don't discount those board connections! Look at them closely to see if there is a foreign substance in there causing the problem.

Symptom: Bad Video RAM and graphics corruption

Battery leaked and caused corrosion on the board and it had a Video RAM error at $8000 - Written 5555 Read 5FFF.

Replaced and socketed both CXK5814 ICs to fix the Video RAM error. Removed the battery and cleaned up the corrosion some. The board would work but then had a funky video problem.

This wasn't a normal vertical line through the images type of problem. It looked like horizontal lines through all images. It started with overlay graphics, moved to foreground moving objects and then on to backgrounds. It also got worse as the board heated up.

Resoldered the NEO-B0, PRO-C0, and LSPC surface mount ICs. This helped some but not much.

Running a finger across the board would cause the problem to get worse if you touched the area around the 74HC32 IC at C6. Replaced that IC but it had no effect.

Broke out the component cooler and sprayed the PC board under each chip around the 74HC32 and found that the clock driver IC, a 74LS368 was bad. Spraying it with component cooler immediately fixed the graphics problems until the chip heated up again.

Replaced the chip and played a game of Shock Troopers 2nd Squad.

Symptom: Missing Audio on slots 4 through 6

Jumpered a bad trace between pin 1 on IC B13 and pin of B17. Tested Game.

Symptom: Slots 4 and 5 dead

Slots 4 and 5 would show crosshatch when a known good cartridge was installed. Cleaned the slots aggressively with very fine emery cloth as the slots had bad corrosion problems due to liquid (possibly rodent) damage.

Symptom: Missing video on odd numbered slots

Games would play fine in the even numbered slots. The odd numbered slots had audio, but missing video - foreground, background, and text were all missing. Repaired bad trace between D3 pin 11 and C29 pin 15. Tested.

Symptom: Slot 1 not working

Repaired 3 burned traces on the top board and tested. It appears the board was powered up wet since the traces that were burned were once that are grounded and next to the power pin on 3 of the 74LS244 ICs.

Symptom: No Audio

Board had very faint audio when running hardware test. Recapped board and tested. Board had strong audio in test mode but no audio for games. Reset backup RAM and tested. Corrupt backup RAM was causing the audio to not work.

Symptom: No audio

Could not put board on bench without first contacting customer. The NiCd battery on the board was replaced by a NON-rechargeable lithium battery and the charging circuit was NOT disabled. This was dangerous.

Replaced battery with a NiMH rechargeable type, installed Audio Cap Kit, and tested. Replaced one incorrect standoff and reassembled system then retested.