Check all of the coils associated with the solenoid expander board. They should all have two diodes. I can almost guarantee someone replaced one at some point with a single diode coil.

Medusa_coils (resized).jpg

As you can see in the red box above, the solenoid expander switches between two solenoid busses. A coil on one buss shares the same ground line with a coil on the other buss. The solenoid expander determines which coil will fire (i.e. which of the two coils will have power) when a given driver transistor on the SDB is turned on.

The second diode prevents power from reaching the inactive buss. Looking at the diagram above, if the buss with the outhole coil is powered, as is shown by the solenoid expander relay, power will pass through the second diode on the outhole coil, through the coil and reach the ground line (A3J5-10). Since the outhole and saucer coils share the same ground line, power will also appear on the saucer coil. It will pass through the coil and if there wasn't a second diode to block the power it would reach the inactive solenoid buss, which causes all kinds of strange problems with the wrong coils firing at the wrong times.

Yes they should all be three lug coils with two wires, one open lug and two diodes on the coil.

Driver and pre-driver transistors may be damaged too due to the stress from handling the current of two coils firing at once. After you find and replace the incorrect coil, if any coils are still weak you'll have to replace their associated transistors.

Yes, I concur.
I've made a Tech Chart for Medusa.
Maybe that will help a bit.
The coil for the outhole needs to be an AT-26-1200 (with 2 diodes).
All the other coils are also listed on my Tech Chart.
The file is on my website www.inkochnito.nl, but here is a copy.
The coils are also listed in the manual on page 20.

Peter
www.inkochnito.nl

Bally_Medusa_Tech_Chart.pdf

Thanks for a great informative post.

Some of you techs are simply amazing