Apr 222019
 

Got for repair from the States this Teenage Mutant Ninja Turtles PCB (actually the US 4 Player ROM set)

Board booted up with noticeable graphical issues, both sprites and backgrounds were affected as they were missing parts.Also colors were wrong, screen was blue tinted (hard to distinguish in the video and pictures though)

The palette circuit is made of two 2k x 8-bit static RAMs, a couple of latches (74LS273), some open collector buffers (74LS07) and finally three ‘052535’ RGB DACs that outputs each color to respective JAMMA edge connector pins :

A blue dominant means the RED color has some troubles so I went to probe around this part of circuit and found the 74LS07 @D23 (from Fujitsu, obviously…) with an output (pin 4) shorted to GROUND :

Chip failed in that pin when tested out-of-circuit:

Replacing the TTL IC restored the correct colors so I moved on the grahical issues.I launched a MASK ROM check which reported two bad devices @K4 and K6:

They are two of the four 4Mbit MASK ROMs that store sprites data:

I removed the first device @K6 :

I dumped it, the resulting buffer of my EPROM programmer was empty so device was really bad :

I launched again a MASK ROM check, the device @K4 was reported as good this time so the bad one @K6 was affecting it (data/address busses are shared)

I replaced the bad MASK ROM with a programmed 4Mbit EPROM (I used a Macronix MX27C4100)

Sprites were restored and  check no more complained :

But backgrounds were still missing parts :

This part of graphics is entirely handled by the ‘052109’ and ‘051962’ custom ASICs:

On a visual inspection I found a lifted pin on the latter:

I reflowed the pin and this fixed board completely.Repair accomplished.

 Posted by at 8:43 pm