Dec 242016
 

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.

 Posted by at 3:53 pm