30 Jan 2019

Slot PGM - IGS (repair log #2)

I received this faulty PGM slot as part of my Christmas gift from my friend Lorenzo2mars.
Problem was red colour being saturated in-game:

However some screens appeared fine:

Colours bars in the hardware test menu also appeared correct:

So did the RAM check (from my experience colour RAMs U23/U24 correspond to RAM 5 in the test):

To me problem was downstream in the late colour generation stages (DAC). I followed data lines from the colours RAMs leading me to the IGS custom chip U6 (stamped IGS026) then on the other side of this custom were custom resistor arrays (stamped RSA1) and few other components (transistors for amplification and few resistors to drive them). I quickly found each analog colour signal could be mixed with one coming from the expansion connector (same one used for 3p/4p inputs). Was this functionality ever used (photo booth or similar?). I probed the transistors used for the red colour (Q3/Q4) but they seemed ok. I went back to the custom resistor array noted RN1 on the silkscreen and found connectivity was lost between its legs in the middle. Having a spare IGS motherboard in my part bin I was able to replace it. While pulling the faulty one this happened:

It was probably cracked before but I couldn't notice it. Once replaced red colour was back to normal:

An extremely easy repair this time!
Board fixed.

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