Mac 512k Woes - Long Boot Beep

[fri0701]

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)

 

[mmiedema]

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]

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]

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?

 

[marcelv]

the LAG is a PAL 16R8

if you can find the code for the PAL (or reverse engineer the source code) you can create a new LAG ic

maybe you can solve the puzzle in the following link:

http://archive.retro.co.za/mirrors/homepage.ntlworld.com/kryten_droid/AppleMac/unitron1.png

Wouter's page : Mac Plus PALs

http://www.retro.co.za/ccc/mac/Schematics/EarlyMacSchematics.pdf