Help needed with dead Mac 512K board

[TimR]

Hi, I've attached a picture of my 512K Mac and the pattern on the screen at start-up.

Until yesterday it worked more or less perfectly but would hang sometimes and exhibited issues associated with under-volting. I'd fitted a Rominator
to it and it ran fine from that. Following Larry Pina's PDF guide I changed out CR15, CR17 and CR18 on the International Analog board for 1N5234B diodes, all inserted the right way around and soldered correctly. When I restarted the Mac I got the checkerboard screen and my multimeter would only show a maximum of 3.9v from the blue 5v wire to the mainboard when trying to adjust. The original diodes were part number 1N48 or something like that. I decided the diodes could be the issue and put the originals back in the analog board. I can now get 5V when adjusting the voltage on the analog board again, but have the checkerboard on the 512k board still.

To troubleshoot I tried the motherboard out of a Plus and that works with the 512k's analog board, so I've done something bad to the Mac 512k mainboard.
I tried removing the Rominator and putting the stock ROMs back in, but still get the checkerboard.

Anyone on here done anything similar and managed to resolve it?

 

[RetroViator]

It could be corrupted RAM or ROMs, but I got a checkboard pattern when my Mac SE had a bad solder connection on the analog board that resulted in the 12V rail only providing ~3V.

 

[Daniel Hansen]

My first thought is that this is an issue related to the reset button, or something on that circuit - that's what that screen looks like to me. Have you tested the board outside of the case (just hanging out the back)?

 

[TimR]

My first thought is that this is an issue related to the reset button, or something on that circuit - that's what that screen looks like to me. Have you tested the board outside of the case (just hanging out the back)?

Hi, yes, just tried it hanging off the 512K chassis and also a Mac Plus chassis, does the same thing with both.

 

[TimR]

It could be corrupted RAM or ROMs, but I got a checkboard pattern when my Mac SE had a bad solder connection on the analog board that resulted in the 12V rail only providing ~3V.

Hi, my analog board is okay now. Tested a Plus board in the 512K chassis successfully. And also tested the 512K board in a Plus chassis and that fails. The Roms are okay too as it does the same thing with the stock ROMs or the ROMinator. It could be a dead RAM chip or other component, but I don't see how too low a voltage could damage the board. I've also tried Deoxit'ing all the connectors, just in case.

 

[Daniel Hansen]

Interesting. I agree that it seems unusual for low voltage to result in an issue on the logic board, and is more likely coincidental. Have you inspected the solder joints and pins of J7 on the logic board? Can you verify the reset button / circuit isn't shorted?

Otherwise, it's time to break out the oscilloscope and check address lines.

As an aside, CR15, CR17, and CR19 should all indeed be 1N 5234B and shouldn't result in low voltage on their own... I'd be curious to know what was going on there. But it's working now.

 

[Kay K.M.Mods]

What the above guys are saying is generally helpful.

If the ROM is bad, the checkerboard will come out more beautifully. It's just my experience.
In my opinion, Unfortunately, one of the possibilities is that the RAM has burned out. Normally, RAM problems appear in Sadmac, but in your case the ROM itself does not work. If you don't have an oscilloscope, remove the RAM one by one, socket it, and install a working RAM to check. D41256C should be used for RAMs.
The other is that there are two CAPs that lead to ROM, which may be bad, but it seems unlikely when looking at the context of the failure.

Of course, my reply is not perfect, so please collect a lot of information and make your own judgment. Good luck!

 

[jajan547]

Bad ROM I actually have a spare set I made.