Bally -35 MPU Repeating 7 Flashes

[Nedreud]

Having problems with the -35 MPU for my GOLD BALL. Thought I had it all working as got 6 flashes on the test bench, but when it was put back in the the pin it became very erratic. I've checked the +5VDC regulator (I installed a new 15,000uF cap too) which seems fine, so I put it back on the test bench.

GOLD BALL doesn't use the +43VDC to power the ZERO CROSSING DETECTOR, but instead uses the unregulated and unrecitifed 10VAC from the Switched Illumination. As such, R113, a 2k resistor, is replaced with CR52, a 1N4148 diode.

On the test bench I don't have an AC supply for the ZCD so I use the trick of feeding the output of the DISPLAY INTERRUPT GENERATOR into the ZCD by connecting Pin 3 of U12 to Pin 14 of U14.

This gets the 7th flash on the test bench just like when it's hooked up the power in the backbox, but a second later the MPU resets and the 7 flashes repeat again and again in an endless cycle.

Stuff I've read online and in the Bally THEORY OF OPERATION F.O. 601-2 suggest that U7, the NMOS RAM, might be faulty but I've tested with a known-good one from my other pins. That leaves either of the 6821 PIA chips, U10 or U11.

Anyone else seen this on a Bally -35 MPU before?

 

[Nedreud]

Only other unknown is the U2 and U6 EEPROMS (2532 and 2732 respectively) as I've not had them in a working GB. At this stage I have to assume they're OK as the 1st flash test is passed. Mmm... maybe I should re-jumper the board for my PARAGON ROMs and see if keeps cycling round the 7 flashes. That would indicate the problem is not related to the U1-U6 ROMs as the PARAGON ROMs (U1, U2 and U6) work fine in the PARAGON MPU board.

I may have to pull the MPU from VECTOR which has the same alternative jumpers to run EEPROMs in U2 and U6, and pop in the GB ROMs. And I could also pop the VECTOR EEPROMs in the GB MPU. That way I could rule out all ROM-related silliness.

If the VECTOR MPU works with the GOLD BALL ROMs I could then try it in GOLD BALL to see if there's at least a semi-working game...

This will also be a great time to get out my new Hantek dual-channel PC oscilloscope

 

[Nedreud]

I don't want to jump the gun just yet but I think I found the problem...

After pulling all the chips, fitting new U10 and U11 6821 PIAs and reflowing the solder on every single chip pin and through-board hole, I realised the only part I hadn't replaced was the CPU, the Motorola 6800. Looking closely it wasn't even a Motorola 6800 but a AMI S6800P from KOREA. I had assumed that because the board was booting and running tests that it was good.

So, I pulled the authentic Motorola MC6800P from PARAGON and voila! No more round and round rebooting. The final nail in the coffin for the AMI chip was putting it in the PARAGON MPU which then started to exhibit the same rebooting behaviour.

Next test is to get the GB MPU back in the cabinet and start connecting cables... this could be a very jolly New Year indeed!