I have one for this thread..about the watch dog circuitry.

My board LED flashes at 5Hz and wont boot. If I bend pin 13 of U2 out, the board boots just fine! I get 6 flashes in this case.

What can cause this? Ive literally tested ALL address and data lines. No shorts or open circuits! I have tested all chips and they are fine.

I have a MPU-2 and have correctly set it up for 2 2716 star trip roms.

Im baffled!

To make matter worse, Im looking at the reset section in the parts pages, and the resistor values are not the values I have in my board! However, my board has the same resistors you have posted on your MPUs.

Ok, the manual is incorrect. I have 3 boards that are the same. The discrepancy is in the resistors that hook the U1 and U2 tantulum caps to ground. The parts manual says 220k on these, but everything I own has 22k in them. The other discrepancy is in the tantulum caps. The manual has 1uF on all 3. However, my working atilla has two 10uF (labelled 106) caps on U1 and the U2 cap is 1uF (labelled 105).

I once heard that there were supposed to be two 1uF and a single 0.1uF. Either way, I put new caps on. They are all 1uF. The board is now flashing correctly, but it is an endless cycle, so it keeps going after 6 flashes. Im assuming this is because I used all 1uF caps. I think I narrowed it down to aging tantulum caps.....but now I need to know the correct values!

Any ideas?

Thanks! I used a 1uf and two 10uf. That was part of my problem. The other problem was that D2 between the ROMs was flaky. If you are in a pinch, electrolytic caps will work They are on mine. I think everything is working on mine but the displays. I need to read about this. The displays have everything lit, so its all 8s on everything. I know the ribbon connector at the cpu is a little worn. I am hoping that is it and not some chips on the MPU. Whats weird it that when I wiggle the ribbon connector plug on the MPU, some of the playfield lights turn off and on. Weird, but at least everything else is working.

Ok.....I had a coil that was locking and some strange light behavior when I powered the star trip on. It was not the driver board. It tested fine. When I unplug the ribbon connector to the driver board, no coils were locking. So, I figured it was the old 74139 chips. Replaced those and the problem persisted.

By chance I noticed this...when I unplug the display ribbon from the CPU, all of my problems are gone! That leaves me with 4 options:

1. Bad display chips on CPU distorting the cpu signals. . Unlikely....I replaced them.
2. Bad chips on the display shorting back to the CPU and distorting CPU signals. I doubt this is the issue as they appeared to work fine before.
3. The card edge connector has damage on 2 pins and they are shorting together. I patched them up. However, I get the same problem and I see no continuity between pins.
4. Star trip is MPU-1 and the CPU I am using is MPU-2, which has 2 display connectors. I assume the MPU can be used on system 1 and 2 displays?