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Zaccaria/Associated Leisure MVGS Neptune Maintenance 2022

02/06/2022 - Northwest Pinball and Arcade Show (2022)

Zaccaria MVGS with Arian Mission
Zaccaria MVGS with Arian Mission

The game ran OK through most of Thursday but eventually stopped booting. A spare game PCB was swapped in that continued running OK for the remainder of the show.

04/07/2022 - Game PCB #PA20200226 repair

Arian Mission with Arduino ICT Arian Mission on the bench

Jumping straight in the with the Arduino ICT found:

Further RAM testing on both CPU A and B sides found that no RAM was reliably accessible. The "RAM Check" and "RAM Write All AD" both produced a consistent display on screen that suggested the RAM write operation was generally reliable and the RAM read operation was unreliable. During initial fault finding the character display became briefly flashing intermittent and then was lost entirely, leaving a blank screen. "RAM Write All" had no effect on the display and RAM read had changed from unreliable to returning no valid data at all :( The new failure syndrome was confirmed with the Z80 CPUs reinstalled.

There were no known schematics for this PCB set with the closest being a poor scan of the older game Marvins Maze that provided a rough guide to how the two CPUs are connected up to the shared memory space. Each fault-finding step ended up as two parts - tracing the relevant portion to build the schematic and then using the scope with Arduino ICT to inspect the signals.

The buffers & latches on the CPU PCB along with Marvins Maze schematics and measuring with the scope during Arduino ICT RAM operations identified the common address bus leaving the video PCB as a 12-bit bus present at unpopulated resistor arrays RA1 & RA2. The common data bus was present at RA3.

After a "RAM Write All AD" to write data == address the RAM on the video PCB was inspected with the scope:

In all cases the address bus was active. Active data and ~WE Write Enable as also seen during a RAM writes. Various individual RAM tests revealed that the ~WE input of all the RAMs were common, so chip select alone was gating access to specific RAMs. Checking RAM write state on IC H2 found: Checking IC H2 (6116):

Pin 1 - A7idle lo |Pin 24 - Vcc idle hi
Pin 2 - A6idle lo |Pin 23 - A8 idle lo
Pin 3 - A5active |Pin 22 - A9 idle lo
Pin 4 - A4active |Pin 21 - ~WE idle hi
Pin 5 - A3active |Pin 20 - ~OE idle lo
Pin 6 - A2active |Pin 19 - A10 idle lo
Pin 7 - A1active |Pin 18 - ~CE active

It appeared as if only 6 address bits were active for video display. Check IC C6 pin 8 (~MREQ) measured a 1.25uS pulse. Using ~WE as a trigger and dual scope channels found that ~CE is not always active with ~WE. To continue working this path would have required tracing the schematic for the RAM ~CE circuit.

At this point I realized that the new failure syndrome may be easier to trace - why was there no video display? The power on state of the RAM did provide at some active data bus bits on IC H2 to work with. Checking IC H8 (27256):

The upper address bits being idle on the graphics EPROMs suggested the video RAM data output wasn't connecting through to select characters in the graphics ROMS. I began to work through tracing the schematic for IC H2 data bus. Active data was present on IC G3 (LS273) pin 7 input but corresponding output pin 6 was idle high. Checking the latch input on IC G3 pin 11 found it idle high - data was not being latched through to the output. Working back to IC C2 (LS367) pin 5 output was high as was input pin 4. The enable pin 1 was idle low and thus enabled. Working back again to IC B15 (PAL16R8) pin 16, whilst checking IC B15, touching pin 8 with the scope lead caused the video display to re-appear and the scope revealed that pin 8 was floating with noise. I was really hoping that this was an input to the PAL rather than the PAL being bad :( Tracing the signal schematic found pin 8 connected to output pin 13 of IC A15 (LS194). The rest of A15 outputs all looked OK on the scope. Replacing IC A15 (LS194) fixed the RAM test, and the game booted and played OK. Not having schematics made this repair a slog taking several hours.

Arian Mission schematic notes page 1 Arian Mission schematic notes page 2 Arian Mission schematic notes page 3 Arian Mission schematic notes page 4

The schematic notes were captured for future use.

28/03/2023 - Monitor chassis #015 repair

Hantarex MTC-900E on the bench

Pre-show testing found a intermittently vertical rolling picture and the vertical sync pot was adjusted all the way to one side as the closest lock. The chassis was removed and setup on the bench where it also had vertical roll. Checking TR1 (BC237) output on a scope didn't reveal any issue (a common failure point). The vertical sync pot RV15 (220K) track measured 860K in circuit, suggesting it was bad. Removing RV15 and measuring out of circuit found the resistance between all pins around 1M to 2M confirming it was bad. After replacing it the vertical lock point was approximately pot centre and the chassis was declared fixed.

30/03/2023 - Game PCB PA20200226 repair

SNK Arian Mission self-test failure

Pre-show testing found the game PCB not booting with a "CPU B ROM 2 ERROR" self-test failure. The game PCB was removed for repair.

SNK Arian Mission game PCB on the bench Arian Mission self-test failure

Which ROM was ROM2? Removing IC D1 (27128, P6) didn't change the error. Removing IC D2 (27128, P5) resulted in a ROM 1 error. After reinstalling the all the ROMs back into the PCB, the PCB reported a ROM 0 error and crashed. Checking the pins found IC D1 and D2 pins corroded and after a cleaning the game ran reliably.

Arian Mission game power armor screen Arian Mission slight graphics corruption

Whilst the game PCB now booted and played OK, there was still a minor sprite graphics corruption issue that was left as is.

Arian Mission game title screen Arian Mission game power armor screen

The repaired monitor chassis and game PCB were fitted back into the cabinet and tested OK.

MVGS Neptune Maintenance 2023

prswan@gmail.com