Received this original Nichibutsu Terra Cresta PCB for a repair:
Board had severe graphical issues, backgounds were all messed up and moving, sprites absent:
The first thing I noticed after my visual inspection was that both boards were fully populated with Fujitsu TTLs therefore I was pretty sure all the faults were due them.To troubleshoot them I used my HP10529A logic comparator for TTLs up 16 pin and a logic probe for 20 pin ones.The backgrounds data are stored in two 27256 EPROMs @15F and 17F on CPU board so I went to probe around and I found a 74LS273 @18E with stuck outputs:
This was confirmed also by a logic analyzing of the device:
Once desoldered the device failed when tested out-of-circuit:
I got improvements, now backgorunds were almost formed but still scrambled and sprites visible although not perfect:
With my HP10529A I found a 74LS157 with floating outputs @17D on CPU board:
Chip failed the out-of-circuit test:
Backgrounds were now 100% restored but sprites missing lines with some garbage on screen :
At this point I focused on video board since all objects circuitry lies there :
Probing around the sprites EPROMs, I found a 74LS367 @1F with bad outputs:
and a 7LS257 @3C (involved in sprites RAM data bus) with stuck outputs, also this failed its test:
Now sprites were perfect but doing some comparison with MAME I realized that characters were totally missing!
Found a 74LS257 @14G with missing input pin 15 (/OE ) in the area of the character ROM:
I traced it back to an output of a 74LS367@20B on CPU board:
Logic analyzing confirmed its outputs were floating and chip failed once tested out-of-circuit:
Characters were back:
I was archiving this repair when, during my test, I experienced some sound issues, sometimes audio was distorted:
This board uses an YM3526 OPL IC paired with an YM3014 DAC although chips are marked with Nichibutsu part name (‘TC 148509’ and ‘TC 1409’) :
Probing pin 2 (the analog output connected to the OP-AMP) of the ‘TC 1409’ revealed a weak signal:
I replaced it with a YM3014 :
This gave good sound back.Board 100% fixed and evil Fujitsu once again defeated.
