Pinball Repair - Flickering LEDs 

Good questions, and I'm probably not the guy to answer them! But I'll try: Once an SCR has power to the gate, it allows power to flow from the anode to the cathode, but the power to both gate and anode are needed to keep the circuit open and current flowing. I assume then that low voltage to the anode can then cause the circuit to fail, or at least cause it to fluctuate. This appears to be a problem only for a small number of (40-year-old) SCRs, so it may be more of an issue of wear. As to why SCRs rather than FETs, I couldn't tell you. I can say, though, that the lamp driver board is the most dependable item in the backbox. MPU, regulator, sound and rectifier boards need lots of maintenance, and are more and more frequently replaced, but most machines still have the original lamp driver boards working away. So, using SCRs seems to have been a reasonable choice. Only the unpredictable switch from incandescent to LED has caused any issue.

@@davesthinktank The SCR is turning on/off the power supply voltage to the lamps/lights on the pinball playfield. The SCR is in "parallel" with the Lamp bulb or LED. When the LED is in parallel with the SCR its causing the SCR to self oscillate or fluctuate the SCR gate voltage or anode voltage causing intermittent on/off which I'm not sure why. The datasheets to SCR's doesn't say anything about the SCR's Load impedance.

​​@@waynegram8907you have to consider the difference between electronic components in a schematic, and in the real world. In a schematic, everything works perfectly. In the real world, things deteriorate and sometimes fail. What may have worked perfectly with an LED 40 years ago may not be up to the task any more.