Received some days ago a box of faulty boards for repair.There was a clean Sunset Riders PCB:

Game played fine except for sound since samples were only noise (music was fine instead):

Sound samples are stored in a 8Mbit MASK ROM @1D:

MASK ROMs check reported it as bad:

I was sure device was really bad (Konami ones are well known to be prone to failure) so I removed it :

But it turned out to be good instead since reading it ended up in a good dump and also replacing it with a programmed 27C800 EPROM had no effect .

Sound samples circuit is very simple, the ASIC ‘053260’ (a custom PCM/ADPCM sound chip) addresses the MASK ROM @1D and plays its data:

No other component is direclty involved so most likely the ASIC was the culprit.I removed it :

Took the spare one from a scrap Asterix and soldered it back on my board:

Success!MASK ROMs check reported now a good device @1D hence board 100% working and fixed :

Got this Michael Jackson’s MoonWalker PCB (Sega System 18) for repair:

Board booted but sound was missing at all and game behaved strangely, you could no longer move your character whenever you pressed the buttons or being hit by enemies :

At first glance I thought the FD1094 custom CPU module was slightly suicided so I swapped it with a good one along with the i8751 MCU but with no luck.I began to think of a correlation between the lack of sound and freeze.Probing the Z80 sound CPU revealed /HALT line was asserted while the /INT was floating.The latter comes from one of the two YM3438 FM sound generator @IC88 :

I removed the chip and tested it on a good motherboard, it was bad.But replacing it made no difference, the shared data bus of the two YM3438 was still inactive.So I pulled the other one @IC87:

It was bad too.Replacing it fixed the board.

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.