Aug 262017

This is the eleventh repair log for Rainbow Islands, almost a record for a board…

PCB was in fair conditions :

But the TC0070RGB DAC module was missing and its solderside was brutally burnt during removal :

Hence, for first I soldered back a spare module and patched a burnt trace:

On power up I was greeted by a blank static screen, /HALT and /RESET lines of 68000 were continuously going high/low sign that the watchdog circuit was active:

Usually this means that main code is not properly executed due some problems in CPU/RAM/ROM circuitry.For first I focused on WORK RAMs, two Sony CXK6864 (6264 pin to pin compatible).When I piggyback the one @IC26 I got this screen:

The watchdog shut down too, this lead me to remove the chip, it failed when tested out of circuit :

Now I got always that garbage screen.Probing the two 62256 RAMs @IC-IC4 in tilemap generation circuit revealed unhealthy signals on some data lines (good signal on the left for comparison) 

Piggkybacking both RAMs  at same time allowed the game to boot although with scrambled backgrounds:

I desoldered the RAMs having confirm they were both bad:

Graphics was perfect now but sound was faint and scratchy:

I used my audio probe to troubleshoot the sound circuit.This revealed that sound came out from PIN12 (which is the analog output to the TL074 OP-AMP) of  the YM3012 DAC already corrupted.Indeed I could measure on this pin a weak voltage :

But a bad output means also a bad input and PIN11 (the analog input) was not healthy too:


This came from pin 8 of the near TL074 OP-AMP (through a 560 Ohm resistor) 

I pulled it and it resulted faulty when tested :

Fitted a good OP-AMP restored the sound and fixed completely the board.

 Posted by at 7:01 pm

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