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Mac 512k single horizontal line - not usual problem

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ngtwolf

Veteran Member
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Apr 6, 2018
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Los Angeles, CA
Hi All,

a long time back I posted that I had a mac 512k with a single dot, everyone was very helpful in identifying the burnt connector and i ordered a new one and replaced.. Shortly thereafter (worked for about 20 mins or so), I ended up with a long horizontal line.. researching the issue, i did all the things to the analog board (swapping caps, swapping Q2/Q4 transistors, reflowing all the connectors, etc). In the end, nothing worked. My friend ended up dumping a dead spider and dirt infested mac plus on me, so to make a long story short, after everything, it turned out that it was the mac 512k motherboard that was bad, not the analog board, crt or anything else (sigh!). A quick look into things, it appears it must be the PAL chip on the mac 512k, that's all i can guess. So, that leads me to a few questions. One, is that likely what the issue is? And yes, i did reflow the connectors on there as well. It does do the bootup sound, but the floppy on that system is bad so didn't do any blind person floppy tests. Second, what are my chances of ever finding a replacement chip? (I haven't yet done much research on that chip).

In the meantime, I guess I'll be cleaning up that Mac plus... :)

IMG_20190226_004138.jpg

Thanks!
 
The CRT has lost vertical deflection, so you have an analog board problem. Don't leave it turned on that way too long or else it'll burn a line into the screen.
 
vwestlife said:
The CRT has lost vertical deflection, so you have an analog board problem. Don't leave it turned on that way too long or else it'll burn a line into the screen.
Click to expand...

I think you missed what I wrote above, the analog boards are fine. I've swapped the Mac+ motherboard only (no cables or anything else) between the two systems and it works fine, it's specifically with the Mac 512k motherboard itself that causes that issue. I actually spent a long time 'fixing' the analog board only to find out later it wasn't that.
 
If the motherboard is causing a working analog board to experience horizontal collapse, I wouldn't leave that board plugged in for long, it could end up damaging the analog board.

I would try probing the video connector on the motherboard to see if there are any shorts, and probe the PAL chip if you think it might be faulty. If you think there's a fault, try removing it from the board and see if the crt still experiences horizontal collapse.

You can probably find a replacement PAL chip, the problem is though you'll need to have it programmed with whatever was on the original PAL chip. Your best bet would be to get another 512k board that was dead from something else and swap the PAL chip from it to your board.
 
Okay, did some troubleshooting with the scope and a friend (thanks Adrian!), comparing the PAL (HAL, LAG, whatever term of the day they use) and it does turn out that it's not producing the vsync (or barely a vsync) compared to the mac plus. Also clipped pin 40 on the 6522 to confirm it wasn't holding the signal low. So I guess I need to find another one. Or, you mentioned finding a replacement PAL chip and getting it programmed? how would I go about doing that? I'm assuming a PAL from something like the plus would be different (I know the apple part number is different, but is it actually different?).

BTW, just to ease everyone's worries, I never leave it on or even plugged in for more then the minute it takes to probe stuff (both for the sake of the CRT or killing myself). :)
 
You'll need to figure out what kind of chip it is (PAL, GAL, etc.) and see if it can be read by a programmer. If it can, you can try to read it and see if it's readable and not damaged or has protections to prevent dumping.
 
GiGaBiTe said:
You'll need to figure out what kind of chip it is (PAL, GAL, etc.) and see if it can be read by a programmer. If it can, you can try to read it and see if it's readable and not damaged or has protections to prevent dumping.
Click to expand...

Ahh, okay, probably out of the question then. It was basically already bad, and probably made worse by removing it (so i could put a socket in and confirm it was indeed what was causing the vsync issue, which it was). I guess I'll just get the Mac plus working and put the mac 512k aside somewhere until a motherboard or chip turns up... or use it for parts. :) Having one working mac is one more mac than i ever intended on owning.
 
Okay, closing out this thread as solved/completed. Got my Mac+ and my Mac 512K both working. Just a quick write-up on the complete process for future thread viewers with similar issues (since we all hate when threads are never updated after fixed).

The Mac 512K originally had the Horizontal line issue which was caused by a burned CRT molex connector on the Analog board and was rectified by replacing with parts from Mouser (15-31-1046 / 19-09-1046) , but then shortly there-after everything went south and i got the horizontal line again. Turns out it wasn't the usual Analog board suspect this time but rather, the LAG chip went bad. Likely this happened during one of the discharge processes (I was probably too quick on the draw about discharging and didn't use a resistor). Since the LAG chip is a custom chip, replacing that alone wasn't a realistic option but I found another motherboard on ebay for about $40 that was missing Rom Chips so I swapped mine in. The system was now turning on but couldn't read any disks to boot up (Ejecting Disks and getting 0F0064 Error). Double checked all the voltages and adjusted but that didn't help. A few internet threads said something about the A roms not working with the drives i have (OA-D34V-22) but that didn't seem likely because that's what was in the system already, and in the end I can say that those roms do work with that drive model number. It turns out, the problem reading the disks wasn't the drive or the voltage on the analog board (like most things say) but rather a bad drive cable (sigh!). Swapped that out and was good to go.

Luckily in the process I was given a non-working Mac+ which really just needed a good cleaning and reflowed solder joints to get working so I was able to swap the analog board between the two machines and when I discovered the Mac+ worked with either analog board and the 512K didn't, that the problem was the logic board (and eventually the Lag chip on that board). Without a second machine to swap parts with, i'd probably still be screwing with the Analog board. :) I guess the good news is that at least one of those Analog boards is in tip-top shape. Hah! I did replace the RIFA's on both, however. In the end, the fixes turned out to be 1) Bad Yoke Connector, 2) Bad Disk Cable & 3) Bad LAG chip.

The only problem left is I have 1 mouse between the two and only one Keyboard which is bad (bad custom chip on that so no way to fix.. story of my life...), so all I can really do is boot them up and run paint. :)

IMG_20190319_015007.jpg

Thanks to DaveJustDave for the bad Mac's he didn't want to deal with (I think he just likes to keep me busy), Adrian Black for help diagnosing the Lag Chip, and NeXT/Al Kossow for the info on the Bad Yoke Connector.
 
ngtwolf said:
Okay, closing out this thread as solved/completed. Got my Mac+ and my Mac 512K both working.
Thanks to DaveJustDave for the bad Mac's he didn't want to deal with (I think he just likes to keep me busy), Adrian Black for help diagnosing the Lag Chip, and NeXT/Al Kossow for the info on the Bad Yoke Connector.
Click to expand...

Wow that's really cool -- good troubleshooting skills there. There just doesn't seem to be a lot of information out there on fixing these old Macs other than the same older PDF documents from years back. Faulty lag chip, bad floppy cable causing those sad mac errors .. and information that didn't apply when it comes to A ROMs with -22 drives.

Cool! I must say I'm a little jealous of the working Mac 512k even though it's kind of useless over the Plus. :)
 
misterblack said:
Wow that's really cool -- good troubleshooting skills there. There just doesn't seem to be a lot of information out there on fixing these old Macs other than the same older PDF documents from years back. Faulty lag chip, bad floppy cable causing those sad mac errors .. and information that didn't apply when it comes to A ROMs with -22 drives.

Cool! I must say I'm a little jealous of the working Mac 512k even though it's kind of useless over the Plus. :)
Click to expand...

Honestly they're both kinda useless so yeah, i prefer having the Mac 512k if i'm just going to stare at it on a shelf...

BTW, thanks for the 400k floppy disks as well to test with. :p Got anything more exciting than Paint though? Something that doesn't need a keyboard, of course. :)
 
I had a keyboard/font DA on my old Macs. It had a clickable on-screen keyboard image that could be used with a mouse to generate a bit of text for copying and pasting to name a file or folder or create a line of text. I used it once when one of the keys quit working, to grab that particular letter and paste it in to whatever I was trying to type. Got me by till I snagged a fully-working keyboard.
-Ed
 
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