Hello Pinball World!
I own a Bally Flash Gordon (1980, with AS 2518 35 MPU) and I have a problem which causes headache… I try to describe it here.
Hitting a certain round target causes TILT. In test mode, the target usually causes a "07" display (TILT) instead of "15" (the correct code), but sometimes it is displayed correctly one time, then wrong again:
- the target is linked to the MPU together with all playfield switches on connector J2, pins ST1 and I6
- the TILT assembly which is involved in the problem is on connector J3, pins ST0 and I6.
At the beginning, it happened only when the pinball was "warm", now it happens all the time.
The capacitors and the diodes on both switches as well as the 6820 are OK (I replaced them). Using freeze spray on all the components on the MPU board as well as C and CR of the switches does not help (but sometimes causes a "reset" of the pinball).
I have tried many many things, to make it shorter I describe the "minimal" configuration under which the problem happens:
- all microswichtches are OFF
- I have disconnected everything on J2 and J3, except:
- on J2, I put a diode directly between ST1 and I6, to simulate the target without being influenced by the playfield switch matrix
- on J3, I put two wires on ST0 and I6 and link the with the tilt assembly (the big one on the left side with the hanging knob and the ball in the cage, which I disconnect from the "original" wires (but using the normal wires makes no difference).
Then, in this configuration:
- the target is still detected as "tilt" but
- if I replace the tilt assembly with the capacitor and the diode alone, the target is detected correctly
- in configuration 2, if a pin of the capacitor or of the diode touches a "big" metal mass, like the cage around the hanging knob, the target is detected again as "tilt" even if the metal mass is not fixed to the cabine.
Look how ST1, ST0 and I6 (here named "PB6" and measured separately on J2 and J3) look like:
When the problem occurs, I6does not go down to 0V after ST0 goes low (and ST1 high), but stays at about 1-1.5V instead. When the problem does not occur, I6 goes down to 0V. Playing with the value of the capacitor on the tilt assembly has an influence on the level at which PB6 goes low.
I notice that the ST0 and ST1 signales (PA output of the 6820) are a total mess and look totally chaotic. I would expect all STx to come cyclically, one after another but I observe that sometimes, they are high together. I don't know it this is "by design" (which I don't understand), or if it is the symptom of a problem.
I don't understand what happens. The problem must be on the MPU board, but there are so many diodes, capacitors and resistances that I have no idea where to search. Replacing all on the MPU board will only cause more problems because of all the soldering works required. I wouldn't like to replace my MPU board with a modern universal MPU because I like the idea that my pinball is "original".
Thanks and regards