Does the slingshot work ok if you start a game or does it only lock on in diagnostics?
Did you ever manage to get the displays working?
Do you have a copy of Clay's system 3-7 repair guide? I've been searching it for more info concerning this issue, and found some interesting info. It looks like there may be another PIA chip involved with the special solenoids (typically pop bumpers and slingshots) that only gets involved during diagnostics. I realize this doesn't address a coil locking on in diagnostic mode, but does reference some interesting info regarding special solenoids and potential problems they could have only in diagnostics. Clay's guide goes into detail in section 3e. Here's what he wrote:
--Special Solenoids that work in the Game, but Don't in Diagnostics--
The special solenoids work fine while playing the game, but don't in the diagnostics. Should anyone really care? After all, the only use for the CPU control of the special solenoids is in the diagnostics. But I guess that's up to you to decide whether it should be fixed (after all, if the game plays, who cares?), but I'll try and explain why this could happen.
In diagnostics, the special solenoids are controlled by the CPU via PIA chips. Sounds simple enough, that's how the other 16 CPU controlled coils work. But it's not that simple, because unlike the other 16 CPU controlled coils which are controlled by PIA IC5 on the driver board, the special solenoids use *four* PIAs for their CPU control! Here's a logic chart:
Note one of the special solenoid controlling PIAs is on the CPU board, not the driver board! (Better check that 40 pin interboard connector.) So if the solenoid IC5 PIA was replaced thinking it would take care of the special solenoids in diagnostics, chances are 5 out of 6 that the problem will *not* be fixed! (Usually it's the 7408 chip anyway.) And the special solenoids are not controlled by the PIA's "normal" ports. Instead they use CA2 and CB2, which are two specialized ports on the PIA, at pins 19 and 39. Leon's test chip does test pins 19 and 39 of all the PIAs, but unfortunately the pulses are usually not as clear as testing the "normal" port at pins 2-17 of the PIAs.
So, according to this chart, it appears IC18 comes into play when the left slingshot (aka kicker) is activated in diagnostic mode. IC18 is on the CPU board and is primarily used for the score displays, but apparently also functions for left slingshot (but *only* in diagnostics).
I gotta admit this is a weird problem, i hope this info is of help!