I'm bench testing a Bally MPU and can't seem to get the 7th flash without the cheat (jumper from U12-3 to U14-14). I am providing the board 5, 12, and 21.5 VDC at the correct test points, but no flash without the cheat, which doesn't let me test U14. PIA input from ZCD circuit is constantly low, making me thing U14 may be bad (Clay's guide).

The whole "zero crossing detector" nomenclature makes no sense to me. The incoming voltage when installed in a game is 43VDC, voltage divided down to 21.5VDC at the test point. There is no zero crossing, it's a positive waveform after rectification on the rectifier board.

In short, is it possible to boot the board up with 21.5VDC on the bench? Or is a bench supply too clean of a signal to allow the ZCD circuit to function properly?

Good idea, can do. Was just hoping to check U14 on the bench. I suppose it could feed it logic on the inputs and verify the ouputs as a basic test.

Also trying to get a better idea of what the ZCD actually does, or how it actually works.

The 43V solenoid voltage is a positive signal, it doesn't cross zero (go negative), the bridge rectifier circuit on the rectifier board prevents that. So how does the ZCD circuit detect zero crossings when there are none? If it watches for changes in the voltage signal, that would make sense. But the solenoid voltage never swings negative, so there are no zero "crossings" to detect. It will be 0V 120 times per second, put never negative. "Zero crossings" are defined as a voltage change from positive to negative. If it was called "zero touching detector" I'd buy it.

Thanks dewdude82, that is what I was after, very helpful!

barakandl, FYI US wall voltage swings between +/- 160V-170V. 115-120VAC is the RMS value.

Also, are you sure the CPU board uses the A/C voltage to time the lamps and coils? I always assumed it was software timed.

vid - yes, that would work. I have a variac that could provide the correct AC voltage, that I could rectify. However, I tested another approach that didn't require fusing another A/C source (a transformer hooked up to mains could source a lot of current) and it worked. Here's what I did:

- Signal generator set to 120Hz, sine wave, output max'd
- 5V to TP5
- 12V to TP2
- Rectified signal from sig gen applied to TP3 (resulted in 11.6Vpk signal, half wave rectified, frequency of 120 Hz)

7th flash no prob.

So this confirms (for me) that the ZCD circuit is not looking for zero crossings at all (see soapbox above and lack of zero crossings in the scope waveform), it's responding to the rectified 43VAC input and doesn't seem to be sensitive to the actual input voltage, a range of input voltages seem to work (i.e. 11.6Vpk/5.6VRMS half-wave-rectified signal at TP3 worked just fine).

Attached are photos of the setup and results, in case they are helpful to anyone in the future. Scope probes were connected as follows:
CH1 - TP3
CH2 - U10 pin 18

photo 1.JPG
photo 2.JPG

My goal is not to get the board to function installed in a game with a certain bench test setup, simply to be able to test the ZCD detection circuit and U10 PIA on the bench and get all 7 flashes.

If the ZCD circuit isn't picky about what waveform or voltage it receives, what makes any sort of bench test methodology invalid if it gets the 7th flash to show the ZCD circuit is feeding the PIA properly and the PIA is responding to the alternating signal and thus allowing the 7th flash?

I don't see how this would result in a PASS bench test and a FAIL game test if everything else in the game is functioning properly.

Zitt, have you gotten that approach to work?

Based on this thread and my bench experience, I think we've established that a constant DC voltage at TP3 won't get the 7th flash, it needs to be a signal that changes over time with a frequency of 120Hz, output of an ATX power supply is regulated and therefore a constant DC voltage.