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Mac 512k Woes - Long Boot Beep

fri0701

Well-known member
Hi everyone,

Well, this stinks. I decided to try out my Dove MacSnap 548 card in my (previously) working 512k, which upgrades the memory to 2MB.

After snapping the card on the logic board, I turned on the computer - the boot beep was very distorted and lasted a very long time, maybe around 10 seconds. After the beep ended, the screen turned on, but it just showed a single horizontal line.

Now my issue is that when I removed the expansion card, the same issue remains! Can anyone offer any advice? I hope I didn’t just cause myself a huge issue...

(my theory is that it’s something timing-related. The beep still occurs, although it’s distorted, and the video still is being generated I think, but it all seems super slow)

 
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mmiedema

New member
Hi @fri0701, did you ever find a solution to this issue? I have a 512K ED which had a MacSnap 524S installed with the same symptoms. Long distorted bong, horizontal line after a while. Both with and without the expansion installed
 

mmiedema

New member
In case anyone else runs into this issue in the future, it turned out to be a faulty LAG IC in my case.

I checked the 68k with a logic analyser and found that it was stuck in this loop for way to long:
2AE 082D 0001 1A00 L25: BTst.B #$1, $1A00(A5) ; check the VBLANK interrupt flag @sc
2B4 56C8 FFF8 DBNE D0, L25 ; wait until the VBLANK interrupt flag is set @sc
This lead me to check the LAG vsync output which was running at 980Khz, rather than the 60hz it is supposed to. Swapping it out with a known good LAG resolved the issue.
 

fri0701

Well-known member
Hey @mmiedema - actually, I also narrowed it down to a faulty LAG! Something I did managed to blow one of the SYNC signals (I want to say VSYNC, but I could be wrong), which I determined by probing each signal with a scope on boot. I haven't been able to source a replacement yet (other than just buying a new board entirely), so that Mac has remained on the eternal fix-it bench. You don't happen to know a source for working LAG chips other than scavenging other boards, do you?
 
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