Mac Plus video issue / question

[Cam]

Hello.

I recently got my hands on a dead Mac Plus (no sound, no video, no nothin') and managed to get it to the point it chimes, and gives a really dim, distorted video output (see attached). I think the LB is OK since I can see the ?disk icon so I've been concentrating on the AB when the contrast (brightness?) dial is adjusted to the max setting. I've replaced all the caps, except C1 which I'll be adding to my next Digikey order.

Any advice as to what could be wrong? Is something else dead on the AB likely to be the cause?

 

[dv-]

Yeah, unless your CRT is absolutely screwed that’s gotta be analog board.

If you haven’t yet, find a PDF copy of the Dead Mac Scrolls; it includes a list of symptoms, and the troubleshooting steps for each. Archive.org will have it when they’re back up, there are other places in the meantime.

Your symptom looks kinda like the fly back transformer failure on page 16. Those are kinda hard to find; you have to replace with a like for like part and there’s nobody making modern equivalents afaik.

 

[bibilit]

Hi, not easy to help, but the very fisrt thing to have a look at is at cold solder joints or broken ones, next is to have a look at voltage outputs (at the floppy port)

You have adjustments on the side of the analog board, your image is stretched out and blurry, try to adjust it also first.

 

[LaPorta]

I’d agree with the above, but definitely look at the solder joint/component side of things first. Unless a monkey went in and turned all the adjustment pots widely out of spec while you weren’t looking, it shouldn’t be THAT bad. It looks like all the geometry is off and the cutoff also is not working properly. Not saying it couldn’t be adjustments, but I don’t think it would all go out of spec at once without a part failure. I’d leave the pots where they are and if the failed component is replaced with a good one, it should come back reasonably where it should…then you can fine tune it.

 

[Cam]

Thanks for the advice on this, it's the first time recapping a board hasn't fixed it for me.
The Dead Mac Scrolls mentions that the BU406 might be bad, so I'm going to track a replacement down before trying to find a fly back transformer (another flavour of unobtanium).