Demons's World repair log #1

Another PCB found in a recent job lot purchase :

Board was dead, no sign of life.Dumping some EPROMs allowed me to identify it as Demon's World, a run and gun arcade video game developed by Toaplan.
Probing the 68000 main CPU revealed both /HALT and /RESET pins were stuck low :

The reset circuitry is a typical one built around the 'PST518' voltage detector IC plus all parts needed by its typical application circuit (resistors and a capacitor)

In particular the role of the capacitor is primary because, when the power supply voltage drops below a certain threshold, it suddenly discharges generating thus the required reset signal.
I removed this 10uF 16V electrolytic capacitor and measured it out-of-circuit:

As you can see from above picture, its capacitance was OK but the ESR was beyond typical values according to this table :

 Fitted a good capacitor and the board booted up into game but the colors were clearly wrong :

Inspecting the board I noticed some burnt resistors just near the JAMMA edge connector :

These resistors are part of the R/2R ladders (one for each RGB color) used to convert palette digital bits into analog signal.I figured out each R/2R is a 5-bit DAC with values of 220-470-1000-2200-4700 Ohm.Measuring them in-circuit gave me good values except for one that showed high resistance whereas it had to be 220 Ohm :

I replaced it but no changes, colors were still wrong.Measuring in-circuit the replaced resistor gave me a value of only 133 Ohm hence there had to be something holding it low.One side of this resistor is tied to the output (pin 12) of a 74LS273 @24M which was almost shorted to GROUND :

I removed the IC :

It failed the out-of-circuit testing exactly in the gate of shorted output :

Installed a good IC :

This restored correct colors :

 Board 100% fixed and a good (weird...) game added to my collection.