The Simpsons repair log #3

Another board from the box of the “death”: The Simpsons

It immediately appeared to be a “desperate case”, board was heavily “harvested”, many components were missing:

A smashed TEST switch too:

I tried to power up the board and obviously it didn’t work, it was  in an endless loop:

Summing up, these were the missing/broken components : 

• 6264 RAM @16G
• 3.579545MHz oscillator @4G
• ER5911 serial EEPROM @12C
• MB3722 amplifer plus electrolytic capacitors @C21 (1000uf 16V), C16 (1000uF 16V), C114 (330uF 16V)
• momentary switch @16A  

I reinstalled the RAM, the oscillator and the EEPROM (temporarily omitting components in audio circuit)  and board showed the self-test reporting bad devices @12C (ER5911 serial EEPROM), 6G (sound Z80 code 27C010 EPROM) and 7G (2k x 8-bit SRAM of sound system)

I cleared the first error by re-initializing the EEPROM (holding down the TEST switch while powering up the board) 

As for the other two errors, I dumped the EPROM @6G and code didn’t match any existing MAME set so I reprogrammed the device with correct code.This cleaared the error but the one regarding the device @7G was still present.Probing the device (a Sony CXK5814, a 2k x 8-bit static RAM compatilbe with 6116) revealed unhealthy signals on data lines:

Once removed, the chip failed the out-of-circuit testing:

Finally the self-test no longer complained so board booted into game but colors of some backgrounds were wrong and sprites were mssing parts:

The MASK ROMs check reported a bad device @16L, this is a 8Mbit one containing sprites data:

 

I replaced it with a 27C800 EPROM but with no luck so I focused on the background colors issue.The hardware uses an ASIC marked ‘053251’ which is responsible of palette and priority:

The chip outputs 11 bit of palette index called ‘CO0-CO10’ on schematics:

Probing the outputs with a scope revealed bad signal on pin14 ‘CO4’ (healthy signal on the left for comparison)

I removed che custom and replaced it with a spare take on a donor board (Premiere Soccer)

This fixed the color issue but sprites were always wrong :

and device @16L was still reported as bad:

Schematics don’t cover this part of circuit but I knew the design.The two ASICs marked ‘053246’ and ‘053247’ work in tandem, the first generates the address lines to the sprites MASK ROMs and the latter read and process their data:

I checked connections between the ASICs and MASK ROMs, no problem found.Actually the data lines from MASK ROMs are not directly tied to the ‘053247’ ASIC but there are some 74LS374 in the middle (two for each ROM) which latch data bits.Piggybacking the ones @12K and 10K restored partially the sprites.I desoldered the first and it failed the out-of-circuit test:

 

This improved sprites but not yet perfect :

So I removed also the other 74LS374 @10K, it failed too!

This fixed the sprites issue completely.As for lack of sound it was only matter to fit the missing MB3722 amplifer and electrolytic capacitors  @C114, C21 and C16.

Another board torn from the darkness, long live to it!