LC575 no screen crash chime

[terryfi]

Hello everyone,

I have a LC575 logic board that plays startup chime but shows black screen and immediately plays crash chime.

I have washed the board twice, there is no visible trace damages. I also desoldered MEMCjr and cleaned underneath it and soldered it back. Re-flowed all other chips pins.

Checked continuity between CLUT and MEMCjr with no issues.

To rule out DRAM, I removed all DRAM from board and used a RAM stick same issue.

To rule out Cuda/Egret issues, I pressed command+power on keyboard and it plays the crash chime again.

I am thinking of checking underneath the DFAC-II or its pins, also checking if ROMs are ok; I might have TL866 adapters for SOJ ROM chips.

I appreciate any hints or advice for next steps. I wish there was some diagnostic software or ROM that could tell us at what stage of boot up the computer crashes, maybe it helps narrowing down the culprit.

 

[terryfi]

Just an update.

I checked most of DFAC-II pins including all ones that go to Cuda and PrimeTime all showing continuity.
The ROM chips are not easy to desolder and if they are bad there is no easy way to replace them. So I am thinking of trying to replace Cuda/Egret chip.

To any Egret experts out there, can Egret failure cause my boot problem? How can be sure Egret is the culprit?

 

[Jagmn]

Have you checked the data and address lines to the SIMM? I’ve had 2 boards where some of these were corroded out. The signals are shared with the on board RAM so just removing that won’t necessarily help.

The 475 I just ‘fixed’ had some traces broken on the address lines both between the SIMM and the on board memory. At one point it just bonged without a SIMM but played death chimes with a SIMM. When I removed the on-board RAM I found several traces between the SIMM and MEMCjr went through the this area first and that was where the corrosion had hit.

 

[terryfi]

Have you checked the data and address lines to the SIMM? I’ve had 2 boards where some of these were corroded out. The signals are shared with the on board RAM so just removing that won’t necessarily help.

The 475 I just ‘fixed’ had some traces broken on the address lines both between the SIMM and the on board memory. At one point it just bonged without a SIMM but played death chimes with a SIMM. When I removed the on-board RAM I found several traces between the SIMM and MEMCjr went through the this area first and that was where the corrosion had hit.

Jagmn thanks for pointing me to the right direction.

After checking SIMM pins, I found out DCAS(3) line to MEMCjr was broken. After budging SIMM pin to the resistor pack and turning LC575; walla LC575 showed the flashing disk.

Remaining issue, even with SIMM budge-wire, I can't see continuity to onboard DRAM DCAS(3), budging that is more tricky because resistor pack and SMD DRAM are on different sides of board.

 

[Jagmn]

Congratulations! Yes; the routing can be a pain. I have occasionally used needle probes to find a closer VIA or other piece of track that is more convenient (with the usual faff of trying to solder it).

 

[terryfi]

Congratulations! Yes; the routing can be a pain. I have occasionally used needle probes to find a closer VIA or other piece of track that is more convenient (with the usual faff of trying to solder it).

Thanks. I was also thinking of getting some sharp probes to make budge work easier.
I tried connecting bootable zip drive and it starts booting but got hit by "bus error". I guess I have to check all other CAS RAS pins, not sure if no on-board ram can cause "bus error". The journey continues...

 

[Jagmn]

My 575 boots fine without the on-board RAM, though I've only booted it from a BlueSCSI so far.

 

[terryfi]

My 575 boots fine without the on-board RAM, though I've only booted it from a BlueSCSI so far.

I tried the logic board on another CC chassis and it worked. Guess the other CC is not 640/Mystic modded.

BTW while studying the SIMM area, I could not find any nearby vias or traces that are connected to SIMM header; I wonder if LC575 logic-board is multi layer. If anyone has info about DCAS(3) vias or traces please share.