- Game does not work
- No video sync
- Tested watchdog, it is constantly resetting
- Tested CPU clock - OK
- Grounded watchdog, the video syncs and I get just a blue screen
- Did a Fluke 9010a RAM test for range 0x0000 - 0x3FFF - BTS FF @0x0000, which means the RAM is not working all bits reporting bad, at the first address tried. This is highly unlikely that ALL 8 RAMs went bad. So going to testsupporting hardware, especially the address decoding circuitry and the RAM write enable circuitry as that's often problematic
- First though I like to check all sync signals as they are needed throughout and often go bad
- ALl H outputs in SYNC including E7 !HBLANK - OK
- ALL V outputs in sync, chips A4, Br and #10 on !16V - OK
- So the SYNC circuits look good, check the address decoding circuits
- Address decoder @ N2 reseponds OK for a looping ram read @ 0x0000
- OK Address decoder for the RAM range looks OK, let's look at the RAM input / output circuitry
- did a looping write @ 0x0000, checked pin #3 (!write enable) on rams, they are getting the signal.
- did a looping write @ 0x0000, I do seem to get data bits on pin #2 of all 2114 chips.
- p5 (Ram buffer, getting enable signal OK during looping read), all inputs are low (inputs are pins 2,4,6,8, 11,13,15,17,19)
- wrote 0x55 to 0x000 and did looping read, all inputs bits still low
- Validated inputs to RAMs they are getting the correct signals on the write..., but outputs of read are always high impediance/floating
- piggybacked a 2114 since all ram outputs seemed floating, 1 bit got better.
- Pulled that one RAM chip and replaced, now that bit is OK
- Pulled another RAM chip and replaced, that bit is OK
- replaced all RAMS, memory tests run OK.. odd that they ALL were bad!
- Board is now booting but trackball doesn't move left correctly AND the soudns is all messed up, staticy
- replaced the LM324 because that usually is the static problem - no change
- Tested with a known good POKEY, as that's the only thing left in the sound circuit - NO change. Put back original POKEY
- MC14584 @ C9 had already been socketed and replaced, tried a known good MC14584, no change
- Pulled 7474 @ J8 - no change, put a socket in and replaced original
- Issue must be on data bus, tried to track down issue, ended up replacing 2 74LS244s before I noticed that under the POKEY socket there seemed to be some damage
- Pulled pokey socket, saw some smashed traces, found one trace was cut. Repaired, replaced socket and re-inserted POKEY
- Sounds are OK and trackball moves correctly
- Game works!