Ok, so obviously the answer is neither, but I’m just about at my wit’s end
Macintosh Classic. The analog board had the usual leaked caps, leaky diodes, faulty optocoupler, etc. so the wobbling checkerboard was fixed by getting the voltages right - a happy bong, and all good.
But that was a logic board borrowed from a working machine. The other logic board, paired with the repaired analog board, produces a checkerboard pattern. This time it’s stable, not wobbling, because voltages are spot-on.
I recapped the logic board and cleaned it, tried swapping the ROM - no difference.
The (blue) lithium battery had not leaked - but the capacitors had, as you would expect.
Because the checkerboard appeared without the expansion RAM board fitted, it seemed the soldered RAM was faulty. I found a 1MB SIMM, checked the data sheet for the chips (seemed correct), got out the hot-air tool, and swapped the RAM chips onto the logic board.
That got me a screen with multiple, partial Sad Mac icons. Pressing on the RAM chips filled in the missing parts!
I took all the RAM off, bought some solder paste, and carefully soldered the chips on again.
This time, I got the correct Sad Mac display (photo).
It was at this point that I realised the chips I’d used from a 72-pin SIMM have three address lines, not four.
So I removed the chips from the Classic’s RAM expansion board (these being the only other ‘correct’ chips that I have). Soldered them on to get another multiple-Sad Mac partial display, and yes, pressing on the chips caused various lines to flash.
Reflowing all the RAM chips again and tidying up produced - a checkerboard
So I’m right back where I started, but the board looks lovely and clean now that I’ve cleaned it so many times, both with circuit board cleaning solvent and with hot soapy water.
The fourth photo shows the logic board as it stands now, with correct RAM (I think) - this produces the checkerboard display.
Was replacing the RAM a red herring? Or was I further ahead with the Sad Mac? Is it time to get out the oscilloscope?
Cheers for any words of inspiration
-Alex
Macintosh Classic. The analog board had the usual leaked caps, leaky diodes, faulty optocoupler, etc. so the wobbling checkerboard was fixed by getting the voltages right - a happy bong, and all good.
But that was a logic board borrowed from a working machine. The other logic board, paired with the repaired analog board, produces a checkerboard pattern. This time it’s stable, not wobbling, because voltages are spot-on.
I recapped the logic board and cleaned it, tried swapping the ROM - no difference.
The (blue) lithium battery had not leaked - but the capacitors had, as you would expect.
Because the checkerboard appeared without the expansion RAM board fitted, it seemed the soldered RAM was faulty. I found a 1MB SIMM, checked the data sheet for the chips (seemed correct), got out the hot-air tool, and swapped the RAM chips onto the logic board.
That got me a screen with multiple, partial Sad Mac icons. Pressing on the RAM chips filled in the missing parts!
I took all the RAM off, bought some solder paste, and carefully soldered the chips on again.
This time, I got the correct Sad Mac display (photo).
It was at this point that I realised the chips I’d used from a 72-pin SIMM have three address lines, not four.
So I removed the chips from the Classic’s RAM expansion board (these being the only other ‘correct’ chips that I have). Soldered them on to get another multiple-Sad Mac partial display, and yes, pressing on the chips caused various lines to flash.
Reflowing all the RAM chips again and tidying up produced - a checkerboard
So I’m right back where I started, but the board looks lovely and clean now that I’ve cleaned it so many times, both with circuit board cleaning solvent and with hot soapy water.
The fourth photo shows the logic board as it stands now, with correct RAM (I think) - this produces the checkerboard display.
Was replacing the RAM a red herring? Or was I further ahead with the Sad Mac? Is it time to get out the oscilloscope?
Cheers for any words of inspiration
-Alex