Macintosh Plus Flupping Logic Board

[Aaron Newcomb]

Hello! I am trying to repair a stubborn Mac Plus with an issue on the logic board that seems to be causing the flup, flup, flup. At first, I thought it was the usual problems on the AB, but swapping just the logic board brought the system back to life.

So far I have:
1. Verified there are no dead shorts on the voltage rails with the power off
2. Swapped the 7905 regulator with the one on the knows good board
3. Swapped the TSM chip with the one on the known good board
4. Checked all the passives on the bad board and they all seem to be within spec
5. Checked for cracked solder joints
6. Swapped out the 4M RAM with the known good 1M of RAM from the good board

It is very difficult to troubleshoot with the crowbar triggering. Any suggestions for narrowing down the culprit? Could I safely disconnect the power rails one at a time to see which one is triggering the crowbar?

 

[Aaron Newcomb]

Another clue perhaps. With the 1M RAM in there and the resistor set correctly, I am getting the crowbar not to trip, but the voltage levels on all the rails are fluctuating about 1 - 2% back and forth instead of being nice and steady.

In this state, there is no high voltage and the clock on the CPU is measuring about 7.8 Mhz.

 

[bibilit]

One of the usual suspects is the Optoisolator at U3, cheap enough to have a try, also are you sure all the joints are spotless ? (most are not and some are not easy to spot)

 

[Aaron Newcomb]

One of the usual suspects is the Optoisolator at U3, cheap enough to have a try, also are you sure all the joints are spotless ? (most are not and some are not easy to spot)

Yeah. That's one of the first things I tried when I suspected the AB. I also tested all the caps and all the diodes and transistors. However, then I discovered that the AB works fine with a known good logic board.

 

[Aaron Newcomb]

I think I might try swapping the LAG and the SND chips, but that is more of a guess based on other things I have read rather than any direct evidence for.

 

[Tonust]

just a doubt:
Did you check the fuse in the cable connecting the logic card to the analog card?

 

[Aaron Newcomb]

just a doubt:
Did you check the fuse in the cable connecting the logic card to the analog card?

Yes. I am using the same cable with the known good logic board and it works fine.

 

[Tonust]

ok

 

[Aaron Newcomb]

SOLVED! It was the LAG chip after all. Upon swapping the LAG chip between the known good LB and the bad one, the problem followed the LAG.
Interestingly before I figured this out, swapping the 4M RAM out for 1M RAM and leaving the Mac powered on for a minute or so, the high voltage would just try to start with some squiggly lines and then collapse. So, I guess the lower draw from the 1M RAM and the state of the LAG was just enough almost to start.

Anyway, now I just need to find a donor board. Hope this helps someone else.