Zaccaria/Seevend Fitter Maintenance 2023
01/06/2023 - Northwest Pinball and Arcade Show (2023)
Fitter is second one from the right and barely visible. It's the only picture I
have of it at the 2023 show.
On show day 1 Fitter developed a graphics fault. The spare game PCB was swapped it
and it ran OK for the remainder of the show.
24/06/2023 - Fitter Game PCB PA20191217 repair
On the bench the PCB had the same fault of bad background graphics & scrolling. I
decided to connect the Arduino ICT first and detected two failures, "E:v07 9800 00 07" and
"E:v06 9800 90 80" indicating problems with one block of video RAM. Checking with a simple
"RAM Write-Read" returned "v07: 08 08 08 08" and "v06: 80 80 80 80" suggesting problems
accessing the RAM. Using a Data == Address pattern with "RAM Write All AD" and inspecting
the two failing RAMs 4-bit data bus nibbles with a scope found:
- IC 6 pin 11 - active
- IC 6 pin 12 - idle hi
- IC 6 pin 13 - idle hi
- IC 6 pin 14 - idle lo
- IC 7 pin 11 - idle hi
- IC 7 pin 12 - idle hi
- IC 7 pin 13 - active
- IC 7 pin 14 - active
Running a "RAM Check" did result in data appearing on all the data pins. Checking the address
pins found:
- IC 7 pin 1 - active
- IC 7 pin 2 - active
- IC 7 pin 3 - idle lo
- IC 7 pin 4 - idle lo
- IC 7 pin 5 - idle lo
- IC 7 pin 6 - idle lo
- IC 7 pin 7 - idle lo
- IC 7 pin 8 - idle lo
- IC 7 pin 17 - active
The idle address inputs for pins 4,5,6,7 originated from IC 16 (74157 dated 7935). Checking
IC 16 inputs found:
- IC 16 pin 3 - active
- IC 16 pin 6 - active
- IC 16 pin 10 - active
- IC 16 pin 13 - active
The control input ~OBJRAM was high and toggled low during the RAM check but there was still
no output from IC 16. Trying a piggyback of IC 16 didn't change the display but the scope
showed 0.7V clamped outputs. I noted that this was the 2nd bad 74157 of 10 on this PCB,
the set being ICs (1,10,20),(15,16,25),49,56,(27,17). Replacing IC 16 with a 74LS157 fixed
the game.
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