I have an Allied Leisure game with an early PCB50210125E MPU. I've removed the resistors for the warming circuit and added diodes on all the switch input lines.

When incandescent bulbs are installed, the feature lamps work fine.

With LED bulbs installed, whenever I activate a switch or whenever a coil fires (kickers, pop bumpers, chime coils), lamps that are not supposed to be lit flicker in concert with each activation.

With an LED feature lamp that is supposed to be lit, it stays lit and does not experience flicker or strobing.

I noticed that on the schematics for newer versions of the MPU, there were .1uf capacitors added to each switch line and tied to ground. However, since the LEDs are flickering when both switches and coils activate, I'm not sure that this would be the solution.

What might be the best approach to address the flicker? Add a .1uf (or .22uf or some other value?) cap across each lamp socket to act as a surge suppressor?

Well, I tried a .1uf cap across the lamp socket, and then a 1uf cap, but there was no change with either cap. Then I tried a 470 ohm resistor, followed by a 1.5k resistor, and again, there was no change with either one.

This seems a bit odd....on playfield feature lamps, I'm getting about 4.45 VDC and 1.5v VAC. On GI lamps, I'm getting 6.5 VAC.

Maybe a bad bridge on the 7.5v circuit leaking current? I'll have to check that tomorrow.

Yep, I already removed the resistors.

Across the positive & negative bridge lugs, or between the positive bridge lug and the lamps? 12,000uF or 15,000uF?

Ok, so looking at the game again, there is a 2200uf 16v cap going across the bridge. I'm getting about 5.4v on the cap's leads.

I temporarily added a 15,000uF 25v cap onto the original cap's leads with alligator clips, but no change in voltage at the lamp (still about 4.45v) and they still flicker on each switch/coil activation.

From John Roberts:

The CPU resets all ports when it gets an IRQ is my guess. So it clears all displays, coils, lamps and then resets them to the last status when anything happens like a switch read or a solenoid action.

I don't know what to suggest other than adding a larger electrolytic cap on the lamps to keep them lit for the fraction of a second they are turned off while the CPU resets everything.

I have not attempted to install LEDs in these games so have no experience solving this...

So, I had an idea. Since I only installed a few LEDs in a specific area, I tried installing more LEDs in a different area of the playfield.

Interestingly enough, those LEDs did not flicker on switch/coil activiation.

The group of LEDs that flicker share some common circuity (a pair of 75492 chips, inverter of some sort) and a common PIA. The other group that works fine is on different 75492 pair and a different PIA.

First I tried replacing the PIA in the malfunctioning circuit, but that had no effect. So, I swapped the old one back in.

So, I removed one of the suspect 75492's, and the little logic tester I have didn't identify it correctly. It thought it was a PC817 or TLP5521-1. As it turns out, it's not on the list of chips it can test anyway, so the only way to know is either to swap a known good chip on the board, or install a replacement. Unfortunately, that's not a chip I have on-hand, so looks like I will have to order it.

I swapped 75492 chips, and the problem stayed with the lamp socket.

So, it doesn't seem to be a bad 75492 chip. I'm not sure what could be left

I swapped the rest of the bulbs to LEDs to see if another pattern emerged, but I'm not really seeing it so far.
pasted_image (resized).png

pasted_image (resized).png

pasted_image (resized).png

No luck. A 10k pull up or a 47k pull down had no effect on one of the PIA PB outputs. The lamp attached to the PIA PB output still flickered.

Sorry, I don't have a scope.

I just have a simpler multimeter and logic probe.

I don't think so. Whenever I've traced something out, there hasn't been any surprises.

Newer board versions/schematics don't add anything other than .1uf caps from the switch lines to ground.

[edit]: I forgot about the factory modification on the back. That's not on the schematics (at least, this version), and I'm not exactly sure what's going on there since I haven't traced it out.

I was starting to think along those lines too--that there was something backfeeding to the lamps or 75492's somehow.

It's a little tricky to try to add components to the board in a clean way.

20220731175727612 (resized).jpg20220731175815850 (resized).jpg

Unfortunately, I don't think I'll have time to investigate this further at the moment. Everything else is fully working on the game, so I'm wrapping it up for the Saratoga show happening in about a week. Then I'll probably have to circle back to it after the show is done.

Thanks