Mac 128k video problem

[Pashley]

Been many years since I did repairs on Macs….let alone a 128k. So I got this 128k today, repaired the floppy drive (lubrication). The monitor is weird - see photo. Horizontal thin lines, seems to bright, the the picture is not “solid”, as if it bleeds out a bit from the center. Tried adjusting with the side pots on the main board, not much affect. Do I need to “discharge” the monitor? If not, why do you discharge the monitor? Just for repair safety? THANKS!

 

[bibilit]

First step is to reflow all usual weak spots on the Analog board.

Then, i will use a wood stick to poke around, trying to improve or change the behavior, that will help to locate the problem.

 

[Pashley]

Actually, it was just the forgotten about brightness control. Duh!

 

[cheesestraws]

If not, why do you discharge the monitor? Just for repair safety? THANKS!

Exactly for repair safety. You discharge the CRT when your hands are going to be anywhere near any of the bits that are normally at high voltage and might remain so for a while after the thing is turned off.

It isn't anything to do with the ongoing performance of the monitor, and it's everything to do with the continued performance (and indeed existence) of the person doing the repair .

Don't confuse discharging the CRT, which you do before a repair, with degaussing, which is about dissipating long-lasting but low magnitude disturbances. The need for degaussing also wouldn't look like this.

 

[joshc]

If the brightness knob is allowing it to look like that at maximum brightness, then it means the cutoff control on the analog board is set too high.