On the Bally AS-2518-35 MPU there is a "ZERO CROSSING DETECTOR" circuit based around chip U14. On most Bally games, and more importantly, in the official Bally literature, e..g, Theory of Operation F.O. 601, this circuit is powered by the +43VDC used for the solenoids. The usual story is that if you can't get the 7th test flash on the MPU LED then either U10 or U14 are bad, or fuse F4 on the +43VDC line is blown. Note that the +43VDC is fully rectified but not smoothed/regulated. That is, it's still pulsing up and down in a sine wave at 2 x 50Hz but only on the positive side:
![24-full-wave-rectifier-1024x368[1].gif](https://www.pinballinfo.com/community/data/attachments/12/12895-b573915293e94b48abf657c74c14927a.jpg "24-full-wave-rectifier-1024x368[1].gif")
So far, so good. I'm fine with everything.
Looking at the schematics for both my PARAGON and VECTOR this +43VDC enters the MPU board on Pin 15 of the J4 connector and then passes through R113, a 2k resistor. This drops the voltage to around +21.5VDC which can be found on test point TP3 before heading onto the ZCD. Many have noted that technically zero isn't actually crossed but the ZCD works by detecting when the wave goes down, or near, to zero. It's also not too picky on the Vpeak voltage and seems happy to work down to around 5-6 volts.
However...!
On GOLD BALL Pin 15 on J4 is no longer fed by the +43VDC from the solenoid circuit, but instead uses one of the 10VAC lines from the Switched Illumination. This is a totally un-rectified AC voltage. Checking the MPU schematic for GOLD BALL shows that the 2k resistor R113 has been replaced with CR52, a 1N4148 small signal switching diode. This effectively provides simple half wave rectification:
![23-half-wave-rectifier-1024x368[1].gif](https://www.pinballinfo.com/community/data/attachments/12/12896-e9a0f768eadefc498810f4993d07382f.jpg "23-half-wave-rectifier-1024x368[1].gif")
Again, no actual zero-crossing and the Vpeak voltage is now around 10-12V but the ZCD circuit is still able to do it's thing.
The question I'm finally getting around to is: When did Bally make this change? Which machine did it start on? Is it unique to GOLD BALL and GRAND SLAM due to the kooky HV/LAMP/SOLENOID COMBO board?
It also means that, for GOLD BALL at least, that a standard AS-2518-35 MPU has to be modified by replacing the 2k R113 with a 1N4148 diode to replicate CR52.
So far, so good. I'm fine with everything.
Looking at the schematics for both my PARAGON and VECTOR this +43VDC enters the MPU board on Pin 15 of the J4 connector and then passes through R113, a 2k resistor. This drops the voltage to around +21.5VDC which can be found on test point TP3 before heading onto the ZCD. Many have noted that technically zero isn't actually crossed but the ZCD works by detecting when the wave goes down, or near, to zero. It's also not too picky on the Vpeak voltage and seems happy to work down to around 5-6 volts.
However...!
On GOLD BALL Pin 15 on J4 is no longer fed by the +43VDC from the solenoid circuit, but instead uses one of the 10VAC lines from the Switched Illumination. This is a totally un-rectified AC voltage. Checking the MPU schematic for GOLD BALL shows that the 2k resistor R113 has been replaced with CR52, a 1N4148 small signal switching diode. This effectively provides simple half wave rectification:
Again, no actual zero-crossing and the Vpeak voltage is now around 10-12V but the ZCD circuit is still able to do it's thing.
The question I'm finally getting around to is: When did Bally make this change? Which machine did it start on? Is it unique to GOLD BALL and GRAND SLAM due to the kooky HV/LAMP/SOLENOID COMBO board?
It also means that, for GOLD BALL at least, that a standard AS-2518-35 MPU has to be modified by replacing the 2k R113 with a 1N4148 diode to replicate CR52.
