Help with dead Mac Plus

[adric22]

I have had this Mac Plus for years with intent to repair it. When I acquired it, it would boot and work but the screen would get very blurry. Tapping the side of the computer would sometimes correct it for a few seconds. I'm good at soldering but never really worked on high voltage stuff, so I've been hesitant to mess with it. I put it in the attic for the past 5 years. I got it out today with intention to fix it. I figured by now surely the CRT and capacitors would have discharged. I decided to just reflow every solder connection on the analog board. Everything looks good, so I turned it on. Now I get nothing. No chime from the logic board and no video on the screen. If I turn the brightness all of the way up there is a faint vertical squiggly line that is moving randomly. I suspect either the logic board has developed a problem or the analog board is not supplying correct power to it.

Any suggestions where to go next?

 

[Macdrone]

Start replacing capacitors on the motherboard then the C1 on analog board and see where that gets you.

 

[djneo]

there is a way to test if the logic board is getting power, if you have a volt meter

tis is the service guide, i didn't find the right pin out. but it might help

 

[adric22]

I'd like to add a little something to this. I've been playing around with it more and I removed the memory modules and ROM chips and put them back in. Now the symptoms have changed a bit. Now the screen stays totally blank, no matter where I set the brightness control on the front. However, I do hear the chime from the logic board, but only through the headphone jack and not through the main speaker. So now I'm thinking the logic board is okay.

 

[RickNel]

Your latest post reinforces the earlier advice to start replacing capacitors. If that does not solve all problems, it will at least eliminate many possible causes and help you to isolate remaining problems.

Rick

 

[adric22]

OK. I've gotten past my fear of CRTs and I've discharged and removed the analog board. I've also used a wire to verify all capacitors are discharged. So now I'm ready to begin replacing capacitors. I am about go to Radio Shack and/or Fry's to hunt down some caps. Whichever ones I can't find I'll order from Mouser. I thought I'd through a couple of photos up here that way if anyone sees anything I should pay attention to, they can let me know. Also the transformer has a lot of "gunk" on it. I don't know if this is normal or if that means it has burned up. So I took a closeup of that also:

 

[adric22]

OK. I managed to replace 10 capacitors. I would have done them all, but couldn't find several at any local stores. One odd thing is that most of the capacitors I used were actually smaller than the originals, even though they boasted the same specs, and some of them even higher. Quite a few I used higher voltage rated caps than the originals simply because I couldn't find the exact matches. Two of the caps were considerably smaller, to the point they didn't fit very well on the board. But I triple-checked that the specs matched.. I'm wondering if there is some other value I haven't taken into consideration such as ESR or something like that.

ANYWAY... I decided to replace the 10 I had and boot it up. There is a definite improvement. I now get a screen showing the disk with a question mark, but it is very faint, and off center. Below is a picture I took.

 

[onlyonemac]

The "gunk" on the "transformer" (which I believe is actually called a flyback coil) is there to stop noise. Otherwise, the alternating electromagnetic fields around the transformer would set up vibrations across the Mac causing it to buzz loudly.

 

[tomlee59]

The Mac Plus does not suffer from the capacitor rot problem that afflicts later models. You've got a loose connection somewhere, so that's what you should be focused on. Did you re-solder the yoke connector? That's the most common cause of the "it gets better temporarily when I whack the Mac" symptom.

See http://68kmla.org/files/classicmac2.pdf

 

[James1095]

I would definitely go over the deflection yoke connector carefully, and check the pins too, they frequently heat up and corrode, which makes them heat up even more.

If that looks ok, there's a non-polar electrolytic capacitor in the horizontal deflection circuit that can fail. Since the HV generation is part of the horizontal deflection, a fault will often cause deflection and brightness/focus problems at the same time.