jmacz
Well-known member
Leaving this here for future searchers looking at debugging a 512K with the flup flup flup noise from the analog board.
(sorry, no pictures on this post)
Corroded 512K Logic Board
I received a 512K logic board that was in poor condition. It was evident the Mac it came from had seen moisture, most prominently in the front lower left corner as that part of the board had more visible signs of corrosion. I pulled out my own working 512K and plugged this logic board in. No bong/chime, no video, and the analog board was making a flup flup flup sound. My analog board is fully working (and recapped) so I knew that wasn't the issue and something on the logic board was drawing too much current or something else was going on.
Usual Reference Guides for Troubleshooting
I consulted the usual two repair guides that have a good deal of information:
But most of the info talks about analog board issues. I did check the few things it mentioned on the logic board including the 7905 negative voltage regulator. I pulled it off the board and tested it, and it seemed to be properly generating -5V so looked ok but I decided to replace it anyway. There wasn't much else in those guides for when the issue is due to the logic board.
Basic Checks
I then tested the usual things... all voltage lines were good, all voltage values were good. Checked both the oscillator and the crystal, both were generating clock signals at the right frequency. Reflowed the important solder joints (the cable coming from the analog board, etc). All good on those fronts.
ROM Check
I had two 27C512 EPROMs available so I found 512K ROM dumps online, figured out how to split the single ROM image into two parts, and then flashed the two ROM chips (HI and LO). No change. I tried testing the stock ROMs and the newly flashed ROMs in my working 512K and both were fine (should have tried this first instead of spending time flashing ROMs but not a big deal).
Schematics
I then searched and found a couple schematics for the 512K but the Apple one was difficult to read. The Beck-Tech one was much more legible but it has lots of errors: a few component IDs were mixed up, some signal lines were mislabeled, two different signals had the same name, but one very obvious one was the 6255 data lines are wrong... the schematic showed D0-D7 when it's actually D8-D15.
Trace Damage
Anyhow, the Beck-Tech one was the best I had so I used it to beep out ALL the connections. Found 6 broken traces (corrosion). 4 of 6 were in the moisture impacted area. 2 were elsewhere. Around this time I also figured out the reset switch was broken. It wouldn't trigger. Doesn't cause the issue but nonetheless it was broken. But none of that helped. Still flupping.
Memory Check
I then found posts talking about how the memory goes bad. Unfortunately, no sad Mac screen, I'm not even getting to that point so no information on potentially which chip is bad. I decided to replace them all. Found new old stock on eBay and got them at a reasonable price. Installed them (using sockets for easier testing later) and still no luck. I should have expected this as I think the flupping is an issue earlier in the startup process.
Started Replacing Chips
I decided to replace some of the more common chips. I knew if I had to replace any of the PAL chips then the board is toast and destined to be a donor since those chips have not been reverse engineered yet. But the common chips are cheap and readily available so I thought why not? Worst case I have some of these chips lying around in case I need them for something else.
I suspected the corner of the board that was hit with moisture so I started there. Removed the four 253 chips in that area, two 257 chips, and the two 393 chips. I put sockets in for each of those positions. I should have gone one by one testing each chip but 1.) I wanted to have a single desoldering session while I was in that work flow and 2.) I decided I wanted that section of the board to be socketed.
I replaced all of those 8 chips with fresh new ones and BONG/CHIME. Got video. Got a flashing disk icon!
Which Chips Failed?
I then went back to see which chip was the problem. Well, it was total carnage. Both 257 chips were dead. Both 393 chips were dead. One of the four 253 chips were dead. I went back and swapped in the original memory chips. All of those were fine.
Lack of Patience = Left Over Parts
It takes a week usually to get parts from Mouser, two weeks+ from eBay, even longer from overseas. This means ordering a larger batch if you don't want to go slowly and lose time waiting for more parts. I probably should have just purchased stuff in the moisture impacted corner but that lack of patience and losing time resulted in the following parts which I didn't end up using:
What's Next?
This 512K board isn't out of the woods yet. I need to replace the reset switch and fully test the machine to ensure everything else is working (serial ports, floppy port, mouse port, etc). I know something's going on with the mouse port... vertical moves aren't working, only horizontal.
Anyhow, wanted to leave this here in case someone searches in the future for causes of the flupping noise from the analog board.
(sorry, no pictures on this post)
Corroded 512K Logic Board
I received a 512K logic board that was in poor condition. It was evident the Mac it came from had seen moisture, most prominently in the front lower left corner as that part of the board had more visible signs of corrosion. I pulled out my own working 512K and plugged this logic board in. No bong/chime, no video, and the analog board was making a flup flup flup sound. My analog board is fully working (and recapped) so I knew that wasn't the issue and something on the logic board was drawing too much current or something else was going on.
Usual Reference Guides for Troubleshooting
I consulted the usual two repair guides that have a good deal of information:
But most of the info talks about analog board issues. I did check the few things it mentioned on the logic board including the 7905 negative voltage regulator. I pulled it off the board and tested it, and it seemed to be properly generating -5V so looked ok but I decided to replace it anyway. There wasn't much else in those guides for when the issue is due to the logic board.
Basic Checks
I then tested the usual things... all voltage lines were good, all voltage values were good. Checked both the oscillator and the crystal, both were generating clock signals at the right frequency. Reflowed the important solder joints (the cable coming from the analog board, etc). All good on those fronts.
ROM Check
I had two 27C512 EPROMs available so I found 512K ROM dumps online, figured out how to split the single ROM image into two parts, and then flashed the two ROM chips (HI and LO). No change. I tried testing the stock ROMs and the newly flashed ROMs in my working 512K and both were fine (should have tried this first instead of spending time flashing ROMs but not a big deal).
Schematics
I then searched and found a couple schematics for the 512K but the Apple one was difficult to read. The Beck-Tech one was much more legible but it has lots of errors: a few component IDs were mixed up, some signal lines were mislabeled, two different signals had the same name, but one very obvious one was the 6255 data lines are wrong... the schematic showed D0-D7 when it's actually D8-D15.
Trace Damage
Anyhow, the Beck-Tech one was the best I had so I used it to beep out ALL the connections. Found 6 broken traces (corrosion). 4 of 6 were in the moisture impacted area. 2 were elsewhere. Around this time I also figured out the reset switch was broken. It wouldn't trigger. Doesn't cause the issue but nonetheless it was broken. But none of that helped. Still flupping.
Memory Check
I then found posts talking about how the memory goes bad. Unfortunately, no sad Mac screen, I'm not even getting to that point so no information on potentially which chip is bad. I decided to replace them all. Found new old stock on eBay and got them at a reasonable price. Installed them (using sockets for easier testing later) and still no luck. I should have expected this as I think the flupping is an issue earlier in the startup process.
Started Replacing Chips
I decided to replace some of the more common chips. I knew if I had to replace any of the PAL chips then the board is toast and destined to be a donor since those chips have not been reverse engineered yet. But the common chips are cheap and readily available so I thought why not? Worst case I have some of these chips lying around in case I need them for something else.
I suspected the corner of the board that was hit with moisture so I started there. Removed the four 253 chips in that area, two 257 chips, and the two 393 chips. I put sockets in for each of those positions. I should have gone one by one testing each chip but 1.) I wanted to have a single desoldering session while I was in that work flow and 2.) I decided I wanted that section of the board to be socketed.
I replaced all of those 8 chips with fresh new ones and BONG/CHIME. Got video. Got a flashing disk icon!
Which Chips Failed?
I then went back to see which chip was the problem. Well, it was total carnage. Both 257 chips were dead. Both 393 chips were dead. One of the four 253 chips were dead. I went back and swapped in the original memory chips. All of those were fine.
Lack of Patience = Left Over Parts
It takes a week usually to get parts from Mouser, two weeks+ from eBay, even longer from overseas. This means ordering a larger batch if you don't want to go slowly and lose time waiting for more parts. I probably should have just purchased stuff in the moisture impacted corner but that lack of patience and losing time resulted in the following parts which I didn't end up using:
- Motorola 68000 processor (from eBay)
- DIP 64 2.54mm pitch socket for the 68000 (from eBay)
- New 74LS161 chips (from mouser)
- New 74LS166 chips (from mouser)
- New 74LS244 chips (from mouser)
- New 26LS32 chip (from mouser)
What's Next?
This 512K board isn't out of the woods yet. I need to replace the reset switch and fully test the machine to ensure everything else is working (serial ports, floppy port, mouse port, etc). I know something's going on with the mouse port... vertical moves aren't working, only horizontal.
Anyhow, wanted to leave this here in case someone searches in the future for causes of the flupping noise from the analog board.
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