Macintosh Plus display problem

[Stanley]

Hello everyone! This is my first post here, as well as my first classic Mac restoration project, and although I have read a lot on the subject, it turns out that I need your help.

I have recently bought a Mac Plus (240V), which apart from its intermittent display problems seemed to work quite well. When I received it, it turned out that the display was no longer working at all and the computer was tripping home electricity. I have already replaced the box capacitors with X2-class ones and the issue is gone, however I can’t get the display to work.

I have completely disassembled the Mac and checked all analog board plugs (no corrosion whatsoever). I flicked through Dead Mac Scrolls and older discussion threads, and resoldered appriopriate connector pins, as well as the flyback transformer. The computer seems to power up properly & read floppies, however it still displays no picture. Every now and then I smacked the cabinet, but have never seen anything on the display.

I think that the CRT is working, as a quiet „electrical” sound can be heard while switching the computer on and off, similar to an old television set. Does anybody have any idea what could possibly be wrong? My only guess is the nonpolar C1 capacitor may be dead, however it looks perfectly alright from the outside (as a matter of fact, the whole analog board seems to be in a reall good shape).

I will be extremely grateful for any help in bringing my Mac back to life!

 

[marcelv]

have you tried adjusting the brightness on the front?

 

[Stanley]

Yes, I have, it didn’t work. On the other hand, perhaps I should have resoldered the brightness knob too, just in case.

 

[bibilit]

The Plus is known for having weak solder in the Analog Board side.

The flyback transformer can be bad also.

 

[Stanley]

I have already resoldered the transformer, yoke connector, and two other connectors. Are there any other joints on the analog board that could be responsible for the lack of picture?

I’m trying to rule out as many potential reasons as possible, before I move to more radical solutions such as replacing the flyback transformer.

 

[nvdeynde]

It can be a lot of things on both the logic or the analog board. It's hard to troubleshoot remotely.

Do you see the bulb of crt  tube glow when it's powered on ?

Otherwise if it i glows, it can be a video issue that it gets no signal from the logic board. Check the wire harness for continuity. 

[SIZE=11pt]If you turn up the brightness cut-off pot on the analog board, there's a point you go into over brightness and should see arcing lines on the crt. Try this first. [/SIZE]

[SIZE=11pt]If you see the arcing lines then at least the CRT and flyback can be excluded. [/SIZE]

 

[bibilit]

http://www.ccadams.org/se/classicmac2.pdf

 

[Stanley]

Thank you @nvdeynde for that piece of advice. I have turned that little knob, and for a while, a vague white shape flashed on the screen. If I understand correctly, that proves both the CRT and flyback transformer are working (by the way, there was a spark between the knob and screwdriver while increasing cut-off, but no worrying sound or smell followed).

@bibilit I have already read the document, but thanks for the link, I will keep it for future reference. Now I’m going to check the cables as soon as I get my hands on a potentiometer.

 

[BadGoldEagle]

I had a similar problem with my 128k. Reflowed every connection on that board, still no go. Turned out to be the flyback. But it could be something else that has gone wrong on your board... 

I would recommend testing Q3 (international Q2) and the BU-406 power transmitter. The flyback usually goes down along with those parts. Have a look at Larry Pina's Macintosh Repair and Upgrade secrets ©1990, especially around page 100. It'll tell you exactly how to test those parts. 

Edit: BTW, what type of flyback do you have? Some are more prone to failure than others...


Manufacturer


Parts Number


Macintosh Model


Non-Bleeder Type Flyback Transformer


ZFAT


AC-058


128K, Early 512K


Taiwan Totoku Electric Co.


157-0026-B


128K, Early 512K


Tai-Ho/Taiwan R.O.C.


TH-1565


Late 512K, 512Ke, Early Plus


Monitron/Taiwan R.O.C.


157-0026-B


Mid Plus


Bleeder Type Flyback Transformer


Monitron/Taiwan R.O.C.


157-0026-C


Late Plus


Tai-Ho/Taiwan R.O.C.


TH-1565C


Mac SE


Lifon/Taiwan R.O.C.


157-0042-A


Mac SE, Repaired analog board, SE/30


Lifon/Taiwan R.O.C.


157-0042-B


Mac SE, Repaired analog board, SE/30


Lifon/Taiwan R.O.C.


157-0042-C


Mac SE, Repaired analog board, SE/30

Also, you should replace all the electrolytic caps, for peace of mind.

Here's a link to another thread you might find interesting.

 

[Stanley]

The flyback transformer is Lifon, B-type unit. According to your chart it must have been replaced, as my Plus is one of the earlier beige models.

I will surely have a look at the parts you suggested and recap the board in the future. I just wonder whether the „arcing lines” I have seen on the CRT don’t rule out the possibility of a broken flyback.

 

[Stanley]

OK, I have tested the three wire harnesses for continuity, and they are completely fine.

I guess it’s time to have a look at the analog board once again. The capacitors may be in a worse condition than their exterior suggests. Any advice about which ones I should replace first (i.e. which can be responsible for completely no raster)?

 

[Stanley]

Just a few photos of the board in question:

https://www.dropbox.com/s/892xfl02o40ovmw/Analog Board 1.jpeg?dl=0 

https://www.dropbox.com/s/oxua3vzp0ebvqrr/Mac Plus.jpeg?dl=0

...and the logic board:

https://www.dropbox.com/s/tphtkqxdi8wo16h/Logic Board.jpeg?dl=0

 

[nkomarn]

@Stanley Did you ever fix the issue? I have a similar issue with my Classic II. If you got it to work, would you be able to share?