|Lines running through the truck graphics.|
So it's time to get the schematics out and fix the issue, but first let me step back to the original fault when I turned the cab on after firing it up for the first time... See the image below, the game was not playable, the background was scrolling madly and a constant audio tone was coming out of the speakers.
|Original board fault from September 2013|
I sent the board to Andrew Welburn (P-Man) who fixed the game and sent through the following diagnosis along with the fully working board.
> Your pcb is now fixed... brief fix summary : Address buffer at 1A had a couple of stuck
> bits and was replaced.. the cpu was dead as a doornail and was replaced, the zero page
> ram @ N2 was bad so it was replaced along with a socket install too. There was also a
> loose patch wire on H5 the graphics ram buffer that was rectified.
So where did I start with the current issue; Reading the Operation & Service manual confirmed that the tail cab data is stored in two ROMS at positions J5 & L5. These can be read as 2716 ROMS and in my programmer the checksums they gave matched the checksum I generated looking at the MAME rom file in a Hex Editior so these weren't the issue, and I must say I had a good look for dry solder joints before looking into component failure, my initial assumption was that the road journey to the show had caused some of the 35yr old solder to fail...
|Fire Trucks Tail circuitry|
So testing with my logic probe I confirmed the Address and Data lines were happily pulsing away, the data lines from the two ROMs pass through an LS273 at position M5, again the data was coming out of this pulsing away looking like what I think is OK. I did note that If I shorted and Input pin to Output pin (i.e. 2&3 or 4&5) It would create a similar fault to what I was already there but in a different position on the graphic. Next in line to check was an LS151 at M6 this is where all the ROM data is turned into a single output called TAILVIDEO. The select lines on this chip were pulsing away and none of the data inputs seemed bad so I was a bit stuck...
Hoping for the day that I find a leg on a chip that is dead, stuck low or high, I decided to order an LS273 and an LS151 and try piggybacking them to see if the problem is fixed, first up I piggybacked the LS273 at M5 and it made no change to the fault. BUT I had more luck with the LS151 at M6. Boom upon piggybacking this chip the video fault had gone. YES! I removed the faulty chip, put in a new socket, inserted the chip and the fault was gone. I suspect that one of the Data bits of the LS151 was internally shorted.
It would have been nice to find the exact fault, but I'm happy it only took a few hours in total to get the cab running 100% again. Until the next time...