Jul 162017
 

Received this original Moon Cresta PCB (board ‘SMC-2FJ’ manufactured by Nichibutsu) for repair :

Board booted up but some sprites were upside down and partially doubled :

Some Fujitsu TTLs were present so I went through them with my logic comparator.I quickly found a suspicious 74LS08 @6H:

It was really bad once tested out of circuit:

Now sprites were correctly placed but I noticed that the bullets were missing:

Looking at schematics there are two separate circuitries which generate yellow and white bullets.Probing them I found that the two active LOW signals /YBPS and /WBPS were stuck HIGH all the time also when you pressed the fire button and generated the bullets:

 

 

The signals are geneated by a 74LS139 @4C:

My logic comparator complained about pin 11 everytime I pressed the fire button and hence activated the selection line B on pin 13:

I pulled the TTL and I had confirm it was bad in that specific gate:

Fitted a good TTL and bullets were displayed again.Board 100% fixed.

 Posted by at 10:49 am

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