(Topic ID: 234120)

Multiplexed lamp doesn't work - 6821 PIA


By newbieinKC

10 months ago



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  • 16 posts
  • 5 Pinsiders participating
  • Latest reply 10 months ago by newbieinKC
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#1 10 months ago

---Bally 6803 Control Board---

I am down to the final two playfield lamps (wired in parallel on the same circuit) that never light. I have tested the sockets, bulbs, wires, diodes, connectors, etc. without finding any problems. The bulbs are getting voltage, the board never closes the circuit to light it up.

The rectifier for this lamp (MCR106-1) drives two lamp circuits. These two bulbs are on the circuit that uses the "A" phase (J12-4 below). The bulb on the circuit that uses the "B" phase (J11-1) works as it should.

Does this mean my problem might be in one of my 6821 PIA chips? I am thinking about swapping them to see if a different light goes out. No risk in that, right? I don't have a known good 6821 PIA right now.
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#2 10 months ago

If the B side works as it should, then that would imply to me that the PIA is working fine and it pretty much has to be an issue with the voltage at the lamps. You say you're getting voltage but are you sure it's the right AC voltage? Is it possible the diode is bad?

#3 10 months ago

Did you replace the SCR for that lamp circuit? I had one that would test ok with a meter, but was not functioning properly in circuit. My lamp was dim when on. Replacing the SCR solved the issue. That threw me for a loop as I work on electronics for a living. I have dealt with transistors that measure properly, but do not operate properly. Usually in the gain stage for audio or current draw.

I replaced my bad SCR and threw it away.

#4 10 months ago
Quoted from zacaj:

If the B side works as it should, then that would imply to me that the PIA is working fine and it pretty much has to be an issue with the voltage at the lamps. You say you're getting voltage but are you sure it's the right AC voltage? Is it possible the diode is bad?

When I check ACV to ground at the "hot" side of the lamp, the voltage seems to jump around a bit from 8-11V (There are lots of flashy lights and the lamp test function flashes them all at once, so I opted to test the voltage outside of test mode). When I test the other side of the bulb/diode, it it calms down to 9.0-9.5 VAC. The DMM I used today does not have a very sensitive AC setting (200V is the lowest range), so I am not sure how accurate the readings are. Plus, if I understand how multiplexing works, I should only be getting one side of the sine wave at this particular lamp socket. I don't know how that affects a DMM.

The diodes test fine in both of the lamp assemblies that don't light. I tested them with a DMM and 9V battery. Hook it up with the correct polarity, DMM reads ~900 and bulb lights with 9VDC. Reverse polarity, open circuit no light.

You don't think I risk any damage by swapping the two 6821 chips and check to see if a different lamp doesn't light, do you?

#5 10 months ago
Quoted from Skidave:

Did you replace the SCR for that lamp circuit? I had one that would test ok with a meter, but was not functioning properly in circuit. My lamp was dim when on. Replacing the SCR solved the issue. That threw me for a loop as I work on electronics for a living. I have dealt with transistors that measure properly, but do not operate properly. Usually in the gain stage for audio or current draw.
I replaced my bad SCR and threw it away.

I have certainly been considering it, but others have suggested that the SCR would not affect only one phase of a multiplexed circuit. I don't know enough about how they work to know if that is a possibility. I don't have a MCR106-1 to replace it with right now or I would replace it just in case.

The only weird thing that I can find with my DMM is that pin J12-4 (the one that doesn't work) shows some continuity to ground with the machine off and the wiring harness disconnected. None of the other light pins, including its "sister" at J11-1, do that.

#6 10 months ago

I think controlled lamps run on DC voltage. Only the GI lamps run on AC.

#7 10 months ago
Quoted from jj44114:

I think controlled lamps run on DC voltage. Only the GI lamps run on AC.

From what I understand, the multiplex comes from chopping a 20.5VAC sine wave in half and sending the top to the A phase and the bottom to the B phase (or vice versa). It ends up looking a lot like 11VDC voltage to the lamp, except that it flashes 60 times a second and is only on half the time.

From pinrepair.com, speaking of the 6803 Control Board:
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#8 10 months ago
Quoted from newbieinKC:

From what I understand, the multiplex comes from chopping a 20.5VAC sine wave in half and sending the top to the A phase and the bottom to the B phase (or vice versa). It ends up looking a lot like 11VDC voltage to the lamp, except that it flashes 60 times a second and is only on half the time.
From pinrepair.com, speaking of the 6803 Control Board:[quoted image]

#9 10 months ago

Yea, I guess the 6803 games are different than the earlier Bally games.

#10 10 months ago

I swapped the two 6821-PIA's on my board and the same two lamps are still out. Which tells me that I have two good 6821's and the problem is somewhere else.

Back to the MCR106-1 I have read tech sheets on it and still cannot understand what it does. Can anyone explain to me what it might do to let one phase of AC through and not the other? -OR- is it possible that the diodes are installed backward on the lamp assemblies?

I am going to check the other circuit that uses the MCR106-1. Most lamp circuits use the smaller 2n5060 SCR, apparently because there is only one lamp on each phase. The circuit in question apparently uses the beefier MCR106-1 because there are two lamps in parallel on one phase.

I am now seriously suspect of the MCR106-1...

#11 10 months ago

I disconnected J12 and pushed a wire into the harness at position 4 and grounded the wire and the bulb came on. 100% the problem is that the board is not grounding J12-4 when it should. Yay! Progress!

I still don't know what that means. Is there a component on the board or maybe something wrong with my ROM? Running out of possibilities...

I am going to order the MCR106-1, along with some plastic parts that need to be refurbished and try that next.

#12 10 months ago

Most common fail mode for SCR is the lamp never turns on. Probably a bad SCR.

do you have a logic probe? if so you can stick it on the 4514 chip at the pin you have highlighted. If you get pulsing there much like all the other lamps during the test mode the SCR is bad.

#13 10 months ago
Quoted from barakandl:

Most common fail mode for SCR is the lamp never turns on. Probably a bad SCR.
do you have a logic probe? if so you can stick it on the 4514 chip at the pin you have highlighted. If you get pulsing there much like all the other lamps during the test mode the SCR is bad.

I don't have a logic probe.

The goofy part of this whole issue is that the lamps that don't light are probably the most meaningless to game play. I want everything to work, but if I had to pick one thing that didn't, the 10K rollover lamp(s) would be #1 or #2 on the short list.

Like I said, I plan on buying and replacing the MCR106-1. If that doesn't fix it, I am not sure I will take it any farther (said that before).

Thanks for the advice.

#14 10 months ago
Quoted from barakandl:

Most common fail mode for SCR is the lamp never turns on. Probably a bad SCR.
do you have a logic probe? if so you can stick it on the 4514 chip at the pin you have highlighted. If you get pulsing there much like all the other lamps during the test mode the SCR is bad.

I have thought about this. I will for sure get flickering during test mode because the multiplexed lamp on the same pin on the 4514 works fine. I might measure half the voltage because only one phase is getting through...

Can anyone tell me if one pin from the SCR drives the J12-4 pin and a different pin drives the J11-1 pin? If that is the case, one side of the rectifier is cooked.

#15 10 months ago
Quoted from newbieinKC:

I have thought about this. I will for sure get flickering during test mode because the multiplexed lamp on the same pin on the 4514 works fine. I might measure half the voltage because only one phase is getting through...
Can anyone tell me if one pin from the SCR drives the J12-4 pin and a different pin drives the J11-1 pin? If that is the case, one side of the rectifier is cooked.

Should be the same pin. Although I guess the trace going to one could be bad, or the connector or wiring?

#16 10 months ago
Quoted from zacaj:

Should be the same pin. Although I guess the trace going to one could be bad, or the connector or wiring?

I figured it out. I am an idiot. The wiring harness was not making contact at pin J12-4. I (finally) tested it by shoving a stranded copper wire into the wiring harness and connecting it to the J12-4 pin and the lamps worked. I have installed a temporary solution (stranded copper wire "shim") and now everything works. I will get a new wiring harness as a more permanent solution.

Thanks, everyone, for your help. As it turns out, I should have heeded the first line of questioning when calling tech support for an electronic device:

Are you SURE it is plugged in?

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