Editting this for the future persons that find the thread because they have the same problem.

The issue: All displays are flickering on my Bally Flash Gordon (MPU -35). A video is at the bottom of the post.

Possible causes to be examined:
* Check that voltage to the displays is good (spec is 190V DC, manual says voltage can be dropped to 150-170 after displays are burned in).
* Cold solders / bad connections at displays (in this case all displays began flickering at once, but this is a likelihood for a single display flickering).
* Ground/short in machine (disconnect coin door connector, remove GI fuse, solenoid fuse under PF, unplug lamp, aux lamp boards).
* flipperwinkel.nl says to reflow solder at "Pin 36 on the display glass, R21, R22, R29.Q17.U1 pin 13. Unclear yet which board.
* The HV Capacitor on the power/solenoid board can be bad. Check for AC ripple.
* Check the blanking signal to displays - pin 10 on display connector, it should pulse.
* With oscilloscope, look for 430 Hz at pin 3 of U1 on the MPU.

Is that not the 11,700 ufd one that I replaced?

I'll do this as a precautionary / maintenance thing, but since the displays all started flickering together, and I've juggled which displays are plugged in, I don't think this is causing the issue at the moment.

Good idea. I popped the GI fuse to remove them from the equation and no change. I'll try with all fuses that aren't 5V or display power later to do the same in case of ground shorts with a coil somewhere.

Are displays fed with some signal from MPU? I wonder if there could be a failure on the MPU somewhere.

Edit, I got my Hantek USB oscilloscope drivers / software installed and working. I'll do some studying tonight to figure out how to read it. I found this, am I looking to see 320 Hz on each of the highlighted pins? And can I compare to ground braid (easier to attach to than pin 1), or must I compare pin to pin within the chip?

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I've tried disconnecting all but one display (and switched to a different display after to make sure I didn't leave the one faulty one on). Steady and seemingly clean 178 V on the displays. I bumped it up to 192V and there was no difference.

Tried removing the coin door connector with no change.

Will try replacing that other cap later today, along with checking frequency on the display interrupt. Thanks!

To measure AC ripple, I just check for / measure AC Voltage at one of the poles of the cap?

At the positive end of the cap, it oscillates beween 0.35V and 0.15V AC. At the negative end, it sits around 0.01V AC.

I'd missed that line.

Is that pin 10 on any one of the display boards' connectors? Or is that a connector on the mpu, or lamp board, or aux lamp board? The manual isn't as detailed as newer machines.

Yes, pulsing, square wave < 200 kHz, if I'm reading my probe instructions correctly.

Sorry, that was logic probe. I don't have an exact frequency. But 430 < 200,000. Will try with the oscilloscope when I figure out the software this evening.

Silkscreened date (original Bally MPU) is 10-79.

If I've stumbled through using the oscilloscope partially correctly, then I'm getting 343 Hz on pin 3 of U12, and 113 Hz on the blanking signal for the displays. So the timer is running slower than expected. Perhaps I should replace U12, R21, R22 and C16.
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It looks pretty bad, the video is accurate. When I first got this thing home last weekend, the displays were fine, there was no flicker. Then they all started flickering intermittently - all displays would flicker or all would be fine. Now they all flicker.

Not yet. I placed a digikey order last night for other stuff, but they had nothing like that capacitor in stock. Even GPE doesn't have a suitable substitute.

I guess I should get on the google and figure out the calculations to combine capacitors to achieve the correct specs, I'm assuming there are formulas, like for combining resistors.

Nothing notable changed. While playing the first few games, they flickered a bit. And the flicker became permanent. I've wiggled all the things lots of times, and reflowed anything that was recommended so far.

It's ok to go below spec on the capacitance? I thought you could substitute with either higher capacitance or higher voltage rating, but not go lower?

I'm a dummy for not thinking of just using a radial.

I'll follow up here on my progress when I've done stuff. Probably next weekend. It won't be til mid or late next week that the parts arrive.

Thank you muchly!

My stuff arrived today. I replaced the solenoid / power driver cap 160 uF / 300 V with 180 uF/350V. No change to the displays.

I replaced U12 timer555 with NE555P which supposedly replaces timer555. I also replaced polyester capacitor C16 .1 uF 100v with a polyester 0.1uF 400V. No change to displays. But my attract mode ilights are moving at super high speed. I'm not sure if it was like that before or if I'm just noticing it now. Lights seem to move at a normal speed during game play (eg when the lights at the saucer point to the upper or lower playfield before the ball kicks out).

Resistor R21 is 24K.

I guess next I start repinning all the connectors?

The last thing I did last night before bed was reflow all the header pins on the board. After that, it does the 'weewoo' siren thing longer than usual, and the game fails to boot, even though I get seven flashes. Displays remain off. It might be that the MPU is just too old and fatigued. I ordered an Alltek replacement. Hopefully that does the trick, I'll check these things once it arrives next week and I'll report back if that still doesn't help.

I'm wrong. With fresher eyes I get 6 flashes. There's no solenoid power at TP3. I think some sloppy wiring by me grounded the new capacitor to the backbox. Ungrounding it doesn't fix the issue, I'll test/troubleshoot the power issue tomorrow night.

My rectifier board was missing a few voltages. I had a failed BR. Replaced it and I'm back to 6 flashes, a siren, and GI lighting up, but it's still not fully there. TP1 on the rectifier board is now 7.8V, which is a little high, and TP5 is 20V instead of 43. The other BR checked as ok but I might pull it and replace anyways, I'll leave that for tomorrow. This is becoming a farce. :p

Edit: connector wiggling got TP5 up to 43V. TP voltages are all perfect on solenoid board. Missing TP2 (12V) on MPU. Trace it back to the rectifier board, pin 8 of J2 reads 0 on the rectifier board (fuses are all good, tested out of sockets). Will look at the board side of the connector tomorrow for cold solder / evil elves, and replace the other BR that tests ok in case it's not really ok and they failed in tandem somehow. There's so little happening on a rectifier board that I'm sure this should be simple.

I did and I didn't. I cleaned up everything else in the machine, had all the right voltages to the MPU, and still had the flicker. I rebuilt the timer circuit with a new 555 chip, and the related polyester resistor and other resistors, and still had the flicker. I replaced with an Alltek MPU and the flicker cleared up.

So maybe a problem with one of the other IC on the MPU further upstream from the timing circuit. At some point I'll read this all again with fresh eyes and see what else I can do to bring the MPU back to 100% as a spare.