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Mac Plus screen unstable when cold

goerz

New member
I got a broken Mac Plus I repaired. Just connecting the power chord (without even turning it on) used to make the main electricity switch trip. I recapped the analog board, replaced the optocoupler, recapped the logic board, and added a diode in the empty location on the logic board to provide SCSI terminal power. Now it works well, however, the screen shows faint horizontal "waves", especially when the luminosity is set low, that disappear only when the Mac is warmed up (very warmed up... they take about 20 minutes to disappear, and the Mac Plus can reach pretty high temperatures). I tried to capture the effect on video but it's not easy. Could it be a power issue? Should I be worried?
Thanks,
goerz
 

cheesestraws

Well-known member
Does it look anything like this?


For me, this turned out to be corroded solder joints on non-cap components in the PSU, right at the bottom of the board. If you can, measure the ripple on your voltage lines (you can access these through the floppy port at the back). If not, you might be well advised anyway to carefully inspect the solder joints on the AB and redo any that look dodgy.
 

goerz

New member
Does it look anything like this?


For me, this turned out to be corroded solder joints on non-cap components in the PSU, right at the bottom of the board. If you can, measure the ripple on your voltage lines (you can access these through the floppy port at the back). If not, you might be well advised anyway to carefully inspect the solder joints on the AB and redo any that look dodgy.
Thank you for your answer. My problem is not as dramatic as what you can see in your video. It's more of a pulsating effect, that becomes more evident when I decrease the luminosity. I'll try to capture it on video if I can.
I measured the voltage at the floppy port and it is 5.048v and -12.051, stable. However, when I put my ear on the top left vent, I can hear a very feeble "tick-tick-tick" noise coming from the case. Is it the flyback transformer? (EDIT: I just read it could be the 800K floppy drive, I'll try to disconnect it and see whether the ticking noise goes away).
 
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cheesestraws

Well-known member
Mine also emitted a quiet ticking, FWIW, until it had warmed up; I somehow forgot to put that in the post. This feels unlikely to be the flyback to me, but I don't know a huge amount about CRTs, so that's only intuition.

If you can measure stability, are the HSync and VSync good solid frequencies?

I'd still personally be inclined to reflow lots of solder joints first; those ABs are known to cook themselves, and hairline cracks in the solder from repeated heat/cooling cycles are the most common failure mode, which often do resolve as they warm up because the solder expands. The fact that yours improves with heat does suggest a mechanical issue.
 

goerz

New member
Mine also emitted a quiet ticking, FWIW, until it had warmed up; I somehow forgot to put that in the post. This feels unlikely to be the flyback to me, but I don't know a huge amount about CRTs, so that's only intuition.

If you can measure stability, are the HSync and VSync good solid frequencies?

I'd still personally be inclined to reflow lots of solder joints first; those ABs are known to cook themselves, and hairline cracks in the solder from repeated heat/cooling cycles are the most common failure mode, which often do resolve as they warm up because the solder expands. The fact that yours improves with heat does suggest a mechanical issue.
Thank you for your suggestion, I'll reflow the non-capacitor components on my analog board. Unfortunately, I don't know how to measure stability and HSync and Vsync frequencies, if you have any link or pointer it would be great.
 
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