Macintosh 512K Scrambled Screen

tylamb19

macrumors member
Original poster
Dec 27, 2014
72
4
Hi all,

I picked up a Macintosh 512K and a bunch of accessories for $62 online last week. It was advertised as untested and when I turned it on, I got a pretty weird looking pattern on the screen. I think the computer is trying to show a sad Mac as there is no startup chime, but somehow the screen gets horribly scrambled when it’s trying. I would think this is RAM, correct?

Also, does anyone know anything about the Digi-Graphics Simmer modification board? It looks like it adds external SCSI and 4MB of RAM. This computer has one and I think it might possibly be part of the problem, but it also should make RAM a lot easier to change if it is bad. Thanks for any help you can provide!
 

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Opry99er

macrumors member
Mar 9, 2018
31
9
Hi! Nice computer you have there. :)

I have seen this kind of issues on compact Macs before, and it typically relates to capacitor issues on the logic board. Might be worth pulling out the ol' T-15 and seeing if you have any bulging or busted caps.

Hope you post your progress here... always love seeing old computers come back to life!!
 

tylamb19

macrumors member
Original poster
Dec 27, 2014
72
4
Hi! Nice computer you have there. :)

I have seen this kind of issues on compact Macs before, and it typically relates to capacitor issues on the logic board. Might be worth pulling out the ol' T-15 and seeing if you have any bulging or busted caps.

Hope you post your progress here... always love seeing old computers come back to life!!
Thanks for the advice! It doesn’t appear to have any obviously bulged caps on the logic board though, but I probably should re-cap it just to be safe. It is a good computer.

The case is in really good shape aside from being a bit dirty, but that’s easy to fix. It came with the aforementioned upgrade board installed, keyboard and mouse (in their original packing) and an 800K external floppy, also in the original box and packing. I think it was a deal and a half at $62.

I’ll keep the thread updated with progress.
 

Opry99er

macrumors member
Mar 9, 2018
31
9
Excellent! I'd have bought 5 of them at that price! You did indeed get a great deal on a computer in such nice physical shape.

Looking forward to the progress updates!
 
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tylamb19

macrumors member
Original poster
Dec 27, 2014
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Ok, I have a small bit of progress. Moving the RAM sticks around in their slots gives me different scramblings on the screen. I also tried booting with a known good ROM from another 512K and it still scrambled. I also tried booting the bad board with the other 512K’s case and analog board but that didn’t solve it either. So the logic board definitely is the issue here. I’ll post pictures of the board with and without the expansion board on it. Let me know if you can see anything weird, I certainly can’t other than the haphazard nature of the install of the expansion board.
 

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MacTech68

macrumors 68020
Mar 16, 2008
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Check that none of the soldered ends of the wires are touching adjacent pins. Check that the resistor soldered to the PAL chip is NOT touching pads on the board proper. Check that the pins on the underside of the upgrade board are not bent. When plugging the upgrade board in, take care that the pins underneath are NOT off-set to the sockets by a pair of pins.

And finally, the ROM-Hi and ROM-Lo are in each OTHER's sockets, ie, ROM-Hi is "342-0341-C" and ROM-Lo is "342-0342-B" (at least I can't see the designer of the board forcing a swap of the ROMs).
 

tylamb19

macrumors member
Original poster
Dec 27, 2014
72
4
Check that none of the soldered ends of the wires are touching adjacent pins. Check that the resistor soldered to the PAL chip is NOT touching pads on the board proper. Check that the pins on the underside of the upgrade board are not bent. When plugging the upgrade board in, take care that the pins underneath are NOT off-set to the sockets by a pair of pins.

And finally, the ROM-Hi and ROM-Lo are in each OTHER's sockets, ie, ROM-Hi is "342-0341-C" and ROM-Lo is "342-0342-B" (at least I can't see the designer of the board forcing a swap of the ROMs).
Alright, I swapped the ROMs around and still no change. I checked all the connections from the main logic board to the daughter board and they’re all solid. I also reflowed the weaker looking solder joints on the wiring connecting the MLB to the expansion board. Still results in the same behavior.
[doublepost=1554174804][/doublepost]I am also getting a hiss/static/buzzing through the built in speaker constantly while the machine is powered on.
 

MacTech68

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Mar 16, 2008
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I've tried to look for the jumper settings to see if that relates to the MacPlus jumpers for 2 & 4MB configurations but can't find any documentation.

I'd suggest trying one memory SIMM in each slot to see what happens. If you find a slot that one SIMM works in, test the other three in that slot.

Unless you have known, working 1MB or 256K 32Pin SIMMs, or at least another machine that you can more easily test them in, you'll have to use the above method.
 

tylamb19

macrumors member
Original poster
Dec 27, 2014
72
4
I've tried to look for the jumper settings to see if that relates to the MacPlus jumpers for 2 & 4MB configurations but can't find any documentation.

I'd suggest trying one memory SIMM in each slot to see what happens. If you find a slot that one SIMM works in, test the other three in that slot.

Unless you have known, working 1MB or 256K 32Pin SIMMs, or at least another machine that you can more easily test them in, you'll have to use the above method.
Alright, so after a little more testing, I’ve found that the bottom two slots are functional while the top two slots are not. If I put a SIMM in both of the lower slots, the computer comes on to the scrambled screen. With one or two SIMMs in the upper slots, the computer doesn’t turn on at all.
 

MacTech68

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Mar 16, 2008
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So the screen is blank with just one SIMM in ANY slot?

Try a pair of (matching) SIMMs in just Slot S1 and S3 ONLY.

Any idea where the 3pins on those jumpers go to? At least one pin of JP'B' goes to ground from what I can see. A pic of the underside of the upgrade board might help.

Also a nice pic of the two types of SIMM would be nice (just to check the spec & size).

Ultimately without known working SIMMs it's a case of experiment - which may lead nowhere. :(
 

tylamb19

macrumors member
Original poster
Dec 27, 2014
72
4
So the screen is blank with just one SIMM in ANY slot?

Try a pair of (matching) SIMMs in just Slot S1 and S3 ONLY.

Any idea where the 3pins on those jumpers go to? At least one pin of JP'B' goes to ground from what I can see. A pic of the underside of the upgrade board might help.

Also a nice pic of the two types of SIMM would be nice (just to check the spec & size).

Ultimately without known working SIMMs it's a case of experiment - which may lead nowhere. :(
The computer powers on to a scrambled screen if there is a single SIMM in S1 or S2 but if a single SIMM is in S3 or S4 there is nothing on the screen at all. When the SIMM is in S1 or S2, the screen looks like a half of the fully scrambled picture. Putting another identical SIMM in slot S2 makes the picture “whole” but still scrambled. Pictures of everything (the scrambled screen with 1 SIMM in S1, the screen with matching SIMMs in S1 and S2, the bottom of the expansion board, and each pair of the SIMMs) are on this post.

On to the pins. The right side (relative to the direction of the text) pin of both jumpers goes to ground. The middle pin of JPB goes to a pin on U4 it seems. The middle of JPA goes to a pin on what appears to be the SCSI controller. The left pin of JPA and JPB go to various pins on the RAM, SCSI chip and cable, and interconnect to the board. I assume because of the trace size that this is a power rail of some sort. I’m no circuit designer or analyst so this might not be completely correct, I’m just following what I see.

I also ordered a working (boot tested) Mac Plus board for $15 with what I can tell are the same type of SIMMs to test this system with. Once that gets here, hopefully I’ll be able to test a bit further.
 

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MacTech68

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Mar 16, 2008
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Just a quick note: You're upgrade board is more like a MacSE logic board (ie, same RAM SIMMs and apparent config - but with a different rear port arrangement and power connector). But, it may have a third party board too using SIMMs).

Also, on first glance, there seems to be corrosion on most if not ALL of the RAM SIMMs - maybe that's the ticket. :confused:

Corrosion on the mother board and upgrade card seem non-existent.

LATE EDIT:

Seems like (without checking the actual board without a multimeter) that the jumpers go straight to the PAL chip at U3. The outer pins of each jumper is Ground or +5volt. The center pins go to the PAL chip - This really doesn't reveal much since it depends on how the PAL chip is programmed. However, I'll bet it's just like the MacSE where you can set it to use 256K RAM SIMMs or 1MB RAM SIMMs and the other jumper is for 2 SIMMs or 4 SIMMs populated.
 
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tylamb19

macrumors member
Original poster
Dec 27, 2014
72
4
Just a quick note: You're upgrade board is more like a MacSE logic board (ie, same RAM SIMMs and apparent config - but with a different rear port arrangement and power connector). But, it may have a third party board too using SIMMs).

Also, on first glance, there seems to be corrosion on most if not ALL of the RAM SIMMs - maybe that's the ticket. :confused:

Corrosion on the mother board and upgrade card seem non-existent.

LATE EDIT:

Seems like (without checking the actual board without a multimeter) that the jumpers go straight to the PAL chip at U3. The outer pins of each jumper is Ground or +5volt. The center pins go to the PAL chip - This really doesn't reveal much since it depends on how the PAL chip is programmed. However, I'll bet it's just like the MacSE where you can set it to use 256K RAM SIMMs or 1MB RAM SIMMs and the other jumper is for 2 SIMMs or 4 SIMMs populated.
Alright, the known good board and SIMMs got here today. The Mac plus board with the working SIMMs boots fine with this analog board, floppy, etc. I took the SIMMs out of the plus board and put them into the modded board. Still no boot, but a drastically different picture this time.
 

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MacTech68

macrumors 68020
Mar 16, 2008
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Australia, Perth
Have you tried swapping the ROM chips - at least that's easily removable.

But, I also think we should check the basics, like the 5 volt rail.

Use the metal chassis for ground and (on the multicolored cable that plugs into the motherboard) the blue wire is + 5 volt. What reading do you get with the bad motherboard fully populated and with floppy drive connected ?
 

tylamb19

macrumors member
Original poster
Dec 27, 2014
72
4
Have you tried swapping the ROM chips - at least that's easily removable.

But, I also think we should check the basics, like the 5 volt rail.

Use the metal chassis for ground and (on the multicolored cable that plugs into the motherboard) the blue wire is + 5 volt. What reading do you get with the bad motherboard fully populated and with floppy drive connected ?
Swapping ROMs is no help. Reading at the blue line is 4.99V when populated with the old RAM.
 

MacTech68

macrumors 68020
Mar 16, 2008
2,187
152
Australia, Perth
That voltage looks fine. :)

Well, unless you have a logic probe or better, it's not looking good for that motherboard.

The only thing else I could suggest is using some contact spray in the sockets that the upgrade board plugs into. At a pinch you could use some WD-40 but you might be better off spraying some into a cup and using a q-tip to "dribble" some into the sockets. :confused: o_O