We all know that a bootleg is a copy of an original board and often a not perfect one since they could be not well engineered.They can suffers from different issues, the one that occurs most often is an unstable video SYNC.Just like this Snow Brows bootleg:

Board played fine but screen was wavy :

Analyzing the SYNC signal with a scope,  amplitude of pulses seemed a little bit over the standard , +4.16V as average while usually they should be within +4V :

This was most likely the reason why my monitor didn’t like it so I thought about a workaround.Usually it’s suggested to use a 5K Ohm potentiometer set as voltage divider : one terminal to the SYNC signal generated by the PCB, the other tied to GROUND and the wiper (the central one) to the  SYNC pin of JAMMA connector.In this way you can adjust the SYNC signal voltage between the original voltage level and GROUND.I wired the potentiometer in this way and installed it on the JAMMA connector of my supergun.Adjusting it, I got a stable screen but I wanted a permanent solution on the PCB.So I used two resistors as a voltage divider:

In my case I used a 270 Ohm resistor as R1 and a 4.7K Ohm as R2 since, doing a bit of calculation, this should have been enough to bring the signal within standard : 3.93V=4.16V*4700/(270+4700).

I installed the voltagev divider on the solderside of PCB (obviously I first cut the trace going to PIN 13) .

This was what I measured:

My monitor was finally happy with it.End of job.

Got this genuine NebulasRay for a repair:

Game played fine but suffered from a color issue, screen was all yellowish, self-test on boot reported a problem on palette RAMs:

A yellowish screen means that problem is in the BLUE color generation so I started to study the hardware and figured out the RGB circuit:

As you can see from the above picture, there are three 8K x 8 bit static RAMs, each one for a each color.These SRAMs are addressed by the custom ‘156’, their data go to the custom ‘116’ which processes them and generates the different color shades.These digital signals are converted to analog and formed into a single color by three 1K Ohm resistor arrays.Lastly each is color is amplified by a PNP transistor and routed on JAMMA edge pins.

With this knowledge for first I checked the 6264 SRAM @5X (which does the BLUE color), it showed normal activity on data/address bus until I probed its pin 27 (/WE) , it was not toggling like in other two RAMs but it was stuck HIGH so RAM was never written:

/WE signal of this RAM (like the other two ones) are generated by the custom ‘116’:

But with my multimeter I found no continuity, at a closer look the pad of pin 27 of this RAM lost contact with the trace.I promptly restored it and this gave me a fully working board again.End of job.