Diagonal lines on Macintosh Plus (Solved)

[wottle]

I was running a longer duration test of my Mac Plus (with 030 accelerator) and came back into the room to find the picture was gone and replaced with diagonal lines and the image was stretched beyond the edges of the CRT. I tested my voltages and they are stable and within spec on 12.0v and 5.1v. When powering on the machine, I get a boot chime and then this image. I can see it booting from FloppyEmu, but no change of the image on the screen. I found one reference on reddit for a similar issue a few years ago and they suggested adjusting the G2 voltage on the flyback, the cut off voltage (R53), or possibly the U2 chip. I'll see if I can find the G2 voltage adjustment, I tried adjusting R53 to no avail, and I'll see about ordering a replacement for U2.

But looking for advice if anyone else has seen this problem. The analog board was recently recapped and was working great initially. It's the cleanest analog board with the nicest flyback of my analog boards and I'd really like to salvage it.

Is it possible the extra load from my 030 accelerator caused damage to one of the components on the board?

 

[wottle]

I'm kind of shocked on doing research into this one. I've found several instances of people reporting this issue on various forums (reddit, 68kmla, etc.) and so far none have fixed the problem. Some just stopped messaging, others said they put the machine up for auction / parts, but I haven't found anyone with a solution that worked for the OP. Waiting on an order that has some U2 chips to see if swapping that chip makes a difference.

Sadly, going to name drop some people that may have seen this given the sheer volume of compact Macs they've worked on. @Branchus / @Mac84 / @JDW , ever seen this before? It also seems like @nottomhanks is having a similar issue with an SE/30 with a Micron Greyscale board. Hoping we can find a solution. This analog board is the nicest one I've got with the cleanest flyback - I'd love to keep it functional! Thanks all!

 

[AndyDiags]

This could be a cold soldier joint. They can be very hard to spot sometimes without checking under a microscope. Quickest way to check it to take a plastic wooden stick and poke at it in various places (especially around J connectors) to see if you get any flicker. Also try pull on the corner of the board while running to warp it a bit to check for flicker.
I think I saw the same screen when I was diagnosing this Mac:

https://tinkerdifferent.com/threads/mac-plus-display-too-wide-issue-solved.842/#post-5603

I noticed you said that the 5v rail is at 5.1v. Are you able to adjust it to be 5v?

 

[wottle]

I actually re-soldered all 3 J connectors, as this board came from a machine that would power on with no image, which was fixed when I reflowed a couple of noticeably cracked solder joints. So to be safe, I removed the solder from the connectors and re-applied fresh solder to all the pins. I also replaced U3 (just because I happened to have some spares lying around).

I did adjust the voltage down and was able to get a steady 5v. The 12v dropped a bit, but well within the usable range. I adjusted all the CRT adjustments and they all work great (width, height, focus, cutoff). The voltage adjustment works as well. Flexing the board / poking things with my plastic CRT adjustment tools has not made a difference.

I'm waiting on the U2 ICs to get in and will try swapping that out, hoping that the chip is failing and causing this. If not, I may start replacing caps one at a time to see if one of my new caps I installed is bad.

 

[AndyDiags]

If you turn it on, and wait 10 seconds or so, are you able to see the disk image behind the fuzz? Or if you boot to OS, can you make anything out or does it stay exactly like what you have shown in the picture the whole time?

 

[wottle]

No, no disk icon or desktop. However, it appears to be loading from disk (according to the lights on my FloppyEMU) so it feels as though it is booting.

 

[AndyDiags]

Do you have a spare board to compare multimeter readings?

 

[wottle]

I do. If I swap in a working board from another machine the image is fine.

Although I'm a bit nervous doing stuff around the high voltage stuff on the analog board. Reading the voltages for the logic board is something I've done. What specifically should I be looking for?

 

[AndyDiags]

Compare measurements on the two boards.
Unplug the analog board, pull it out, and put the two boards side by side.
Then using a multimeter compare different points against ground to see if you get the same reading on both boards. You should focus on J1 and J2, and see if any of pins get a different reading between the two boards. If you don't see anything, you can also put the meter in meter in diode checking mode.
If you do see something you think is out of ordinary, I can also compare it on my spare board to get a 3rd reading.

 

[wottle]

So, quick update. I actually received my Mouser shipment today and decided to swap out U2 as that was a recommendation I had seen for similar issues. Put in a new IC from mouser, fired up the Plus, and my picture is back! The geometry is all messed up from my adjustments, but I now get the disk icon and no diagonal lines. I'll maybe poke around with the multimeter on the bad IC vs a good one and see if I can see which pin(s) show a difference. Not an electrical engineer, so I don't really know what I'd be looking for, but it may help others explain why that's a failure scenario for the U2 chip.

The good news is it's solved! And maybe others who are in this state can use this to help fix their issues. Thanks @AndyDiags for all the ideas!

Summary: the U2 chip was bad and replacing it with new stock from Mouser fixed the issue. Ordered part SN74LS38N at https://www.mouser.com/ProductDetail/595-SN74LS38N