The transistors are multiplexed.

When this happens, usually reflowing the solder on the ppb board relay fixes it.

That's the power supply. The PPB is the one below the CPU and the power supply - will have a bunch of ceramic resistors on it, and a relay. Not sure if you reflowed the relay on the PPB as well. That's the one that usually causes this problem.

Burnt connectors are par for the course on the PS and PPB. Order the right parts and repair that, before it becomes a bigger problem. GPE has them.

09-18-5121 on this page goes on the board:
https://greatplainselectronics.com/products.asp?cat=98

03-09-1126 on this page goes on the wires:
https://greatplainselectronics.com/products.asp?cat=88

The pins are on this page:
https://greatplainselectronics.com/products.asp?cat=89

You'll want
02-09-1104 - Female
02-09-2103 - Male

So it's not just the pins. You need to reflow the solder on the 8 pads that connect the left/right relay on the PBB. The flashers are shared with the first 8 coils. See the coil chart where it has 7l/7r etc. These are controlled by q29 on the CPU board. This is a tip122. You can test for continuity from the tab of q29 to the wire in the harness directly below. Its a black wire with red stripe. If you don't have continuity there, reflow that pin on the CPU board. Next, follow the black with red striped wire to the PPB. Reflow that pin. Next reflow all 8 pins on the relay, itself. If this doesn't solve it, replace the relay.

Right, you have to reflow the *relay* pins, not the connector pins.

For the most part the guys have you covered as it seems there's a problem with the L/R relay not energizing during the flasher test which could also account for coils firing randomly during play. However, you should also be aware of this service bulletin in case after repairing the relay you still get random coils firing:

http://www.sternpinball.com/downloads/sb53.pdf

And if you don't have ROM version 5, you probably should get the set from John - at least until Chad finishes his code update.

viperrwk

It is possible the logic that drives the relay is not working. Check the components in front of the relay. It is probably works like this. PIA -> 7408 for blanking -> 2n4401 to drive the relay.

PIA you should see high / low activity when the relay should be activating
7408 for blanking IC you should see high / low activitiy on the output when the relay activating
2n4401 same as above.

You can test the 7408 (or whatever TTL they may be using) and 2n4401 power off with diode test on your DMM.

Its likely the relay has failed. I also gave more instructions and no evidence any of that was considered. I'm not typing it again. Look above for q29. It should tell you how to trace this from the CPU board. Check continuity of the wiring from CPU board to ppb board as state above. You can test the relay also by shorting the tip of q29 to ground and see if the relay fires. If not, and you've reflowed and tested all the continuity above, I would replace the relay. If it does, I would assume issue is in CPU board.