Haunted House System 80 MPU will not boot. Displays a "t" in the credit window.
MPU would boot reliably until I installed a 5v super cap for memory. MPU booted fine first time with super cap. On subsequent boots it would hang on boot, no sounds, and display the "t" in the credit window. Removed super cap but problem persisted. If I left the power off for a while it would then boot most of the times. Issue was 50/50 until I did the abetment work and installed new components now its permanent.
1. I abated previous alkaline damage. Removed all clock components (Z2,Z3, crystal, and associated resistors). I also performed the reset mod using the DS1811, verified pins on DS1811 are in correct location. Installed all new components for clock and reset. Reflowed some traces. Tested continuity. Exact same problem persists. Used the following components for new clock area per PinWiki
Crystal - https://www.digikey.com/en/products/detail/ecs-inc/ECS-35-17-4XDN/827601
Z2 - 74LS74 - https://www.marcospecialties.com/pinball-parts/74LS74
Z3 - 74LS04 - https://www.marcospecialties.com/pinball-parts/74LS04
2. Did diode test on U1-U6 and all seemed consistent except U1-5, U1-34, and U1-35 showed OL. Not sure if that is expected. Diode test on Z2 and Z3 seemed fine.
3. Some traces were broken and required jumpers. No shorts that I can find.
4. Removed EPROM but problem persists. I had previously installed the original EPROM backwards and nuked it but flashed an new one and it worked fine for quite a while before this issue appeared so I dont think its an EPROM issue. If I remove the EPROM it has the same behavior and does not flash like it did previously with no EPROM installed.
5. Attempted to ground Pin 40 on U1 but that did nothing.
6. Performed slam switch mod.
7. Added 5V LED. Board is getting good 5v power. Checked at board.
8. Installed PI-80X4 and unit works flawless
Please ignore ugly solder work. Not a solder pro... yet. Backside of clock area looks ugly but everything is connected.
Could the 100uF 10v cap cause this if it was bad? I tested it and when connected to the board is has a resistance of .760 and does not continue to climb. I removed it and rested it and it went to OL, so I am not sure its bad but if this might cause issues I would replace it anyway being cheap.
Would a Logic Probe show anything that the diode test did not on the ICs?
Would going to a TTL oscillator be a better option? https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Replacing_the_quartz_crystal_with_a_TTL_Oscillator
Since this problem started before doing all the rework I am thinking that it is not the rework or anything that I replaced is the root cause but who knows.
Thanks!HH_T_Credit_Window (resized).jpegMPU_Clock_Post_Mod (resized).jpegMPU_Rear_Post_Mods2 (resized).jpegMPU_Reset_Post_Mod (resized).jpegMPU_Slam_Mod (resized).jpeg