Help to recover a classic mac (LB)

[doctormistral]

Hi all!

I'm from Italy and I'm trying to recover a mac classic logical board that I've previously recovered after a Max bomb in 2012. After a full recap and a lot of cleaning the mac has come back to life but after a while a put it in a box and kept it until 2017. Than i did a check and it was not working anymore but i decided to kept it in the box because no time to dedicate to it.

In these days of lockdown I decided to fix it but it appears much harder than expected!

Latest Symptoms:

1. chessboard video (sometimes messed), regardless (with or without) ram extension board and (with or without) ROM.
   
2. no startup audio,
   
3. until a week ago sometimes connecting the floppy cable and moving it in the connector (like to simulate a bad contact) the mac booted,
   
   
   now only switch the screen from chessboard to standard desktop - but not boots, nor floppy icon with "?".
   

Tests done:

• The problem is likely in the LB because putting it in another Classic perfectly working I get the same problem: no sound + chessboard.
  
• Recapped again the LB
  
• Voltages are OK +-12V / 5V (regulated from trimmer in the AB)
  
• Various washes with various products. After one of which I made the situation worse moving sympthoms from 3 -> 3.1
  
• Rom is OK because boots in the other working Classic
  
• Unsoldered (hot air desolder)  + board cleanup + re-soldered various chips: CPU, RTC, SOUND, VIA, 2x 74F257A -> no change. 
  
• Checked a lot of PCB lines especially DATA and ADDRESSES between ROM/CPU/VIA using the known schematic -> No problems
  
• The reset signal to CPU correctly rise high after voltage is stable.
  
• Seems that the RTC is doing nothing: I'm using a multimeter and can't find any frequency in the crystal pins nor in the pins 2, 12,13 of RTC chip. I checked this in the working mac and even here can't find and frequency in the pins 2, 12,13 of RTC but there is some confuse/variable frequency in the crystal (may be my multimeter is not enough sensitive).
  

I'm running out of ideas and starting to think there is something deeply wrong in this board!!  

I'm new in this group but I've read a lot of posts, suggestions, even discovered the existence of schematics and many more ... but I'm stuck... 

Find attached some pics of the LB.

Thanks a lot for any suggestion...

 

[doctormistral]

I've just replaced 2 x 74LS245, 1 x 74LS174 and 2 x 74LS257 but unfortunately there has been no change..... so sad

Screen with RAM board installed



Screen without RAM board installed

 

[marcelv]

Have you checked the via's underneath the battery holder?

 

[doctormistral]

Hi @marcelv, yes, I removed the battery holder at least twice to check the board, no problems.

Recently I've also checked frequencies of CPU 7.8Mhz and 32.768 on RTC crystal and they are there.....

 

[doctormistral]

Have you checked the via's underneath the battery holder?

Hi marcelv, yes, I removed the battery holder at least twice to check the board, no problems.

Recently I've also checked frequencies of CPU 7.8Mhz and 32.768 on RTC crystal and they are there.....

 

[doctormistral]

Any other suggestions?

Would it be worth buying an inexpensive logical analyzer to check what is going on on the board?

 

[doctormistral]

After some work i got this behaviour when turning on the mac: Quick checkerboard -> Sad Mac with stripes --(after a while)-> Vertical patterns.

Any suggestion about the cause or about what to check?



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