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CRT on Mac Plus -- rows randomly offset by 1-2 pixels


Hi, first post, glad to be here.

I have a new-to-me Mac Plus. It appears to be an earlier Mac that was upgraded. It has the Samsung CRT and the "early" flyback transformer without yellow(brown) glue on it. It has a 4MB RAM upgrade.

The problem with the CRT is that some horizontal rows will randomly be offset by 1-2 pixels to the right or left. The attached image shows a single frame of the symptom. In operation, different rows exhibit this problem per frame randomly, and the number of affected rows randomly increases and decreases. It gives kind of a "sizzling" visual effect.

Inside the unit, something is making a faint sizzling/arcing/popcorn sound but I can't pinpoint what it is. Seems more like the CRT neck than the flyback but can't really tell.

I recapped the analog board yesterday, and re-re-touched up all the solder joints, and that didn't make things better or worse.

I also did the diode power mod to the SCSI port on the mainboard, which had no impact on this issue before or after (which it shouldn't have, but mentioning it just in case).

Other than this CRT issue, the computer seems 100%; it was well-cared for in its lifetime, not especially dirty/dusty, hardly yellowed. Works fine with a BlueSCSI to run various programs.




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Clarification: My cap kit didn't include C1 so I didn't replace it. Mine is electrolytic. I've seen it described as the "horizontal deflection capacitor" and highly likely to fail, so that is very suspicious and I will find a replacement for it. Other ideas still very welcome!


Well-known member
Are the shifitings related to floppydrive activity?

Whining noises are usally BU406 or the flyback.
Wiggles on display can be caused by a bad BU406.
Try replacing BU406 and confirm ground is connected to the chassis.


The problem starts as soon as I turn on the Mac. I did try it without the floppy attached and no change.

I'll look into replacing that transistor.

I attached a video of the problem here to better represent it. (note: the image doesn't sway back-and-forth -- that's me being unsteady holding the camera)


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Thank you for responding.

The main rails are 5.04V and 12.1V and steady. If you are asking about voltages in the CRT drive circuitry, I am not sure what to measure.


Here are a few things I just tried. The problem persists -- no better, no worse.

1) Replaced C1 again. My original replacement was a NOS electrolytic and I thought maybe it was bad too, so I put in a ECW-FD2W395K 3.9uf film capacitor. No change.
2) Replaced the horizontal output BU406 at Q3. Had to drill out the rivet holding the old one to the heat sink and put in a screw for the new one. No change.
3) Replaced the 4N35 at U3. It seems like I am just throwing parts at the board now, and I am, but I saw another thread where this chip was a problem and it was cheap to add to my Digi-key order. No change.

So... at least I didn't make things worse? I guess?

I measured voltages on the lines going to the CRT socket. To make things simple (like me), refer to the attached picture, and I will list voltages by color. All voltage readings were stable on my cheapo multimeter.

Red: 304V
Blue: 149V
Green: -28V
Black with line: ground (goes to CRT frame)
Brown: 0V
Black: 0V
Yellow: 16V


Any further ideas welcome. What is the likelihood that it's the flyback?


Active member
Did you do anything to rule out the problem on the Logic board? Could you upload a hi-res photo around the video circuitry area? (UA8~UG8)


I haven't done anything with the logic board. I attached an image of what I think is the video logic...? Appreciate the help.


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Thanks everyone!

I think my next step is to buy a parts unit with a newer (and working) flyback. I wouldn't mind replacing my CRT as well since it has burn-in.


For closure: it was the flyback.

I picked up another grungy Mac Plus with a working display. I did some parts swapping and tried every combo of analog board, CRT, and logic board. The problem followed the old analog board. Then I swapped flybacks between the analog boards and the problem followed the bad flyback.