Mac Plus screen issue

[gkmaia]

I am struggling a bit to identify this fault. The computer beeps once and reads the system disk apparently fine but the only image I get is the attached one.  See image attached.

Following the manual I found here on the forum seems my voltages seem ok. I did read on a post yesterday (https://68kmla.org/forums/index.php?/topic/23533-plus-with-horizontal-sweep-lines/) that my issue could be caused by a fault on 7438. But it seems to be fine. 

So far seems the power block of the analog board is working fine. But the sweep block is not rendering the image. I could be wrong... 

Anyone had a similar issue? And some advise on how to approach it? 

J4

1 - 0v

2 - x

3 - square wave

4 - 0v

5 - 4.7v

6 - 5v

7 - 0v

8 - -12v

9 - 0v

10 - +12v

11 - 4.6v

U3

4 - 330v - 20ms square wave

LM 

4 - 12v

11 - -12v

 

[Gsbtom]

DMS (from your post on that other thread) is Pina’s Dead Mac Scrolls book. A quick google should find it in PDF form (not sure if allowed to post link here).

Looks like your problem might be described on page 54. 

 54 The Dead Mac Scrolls
Symptoms: There is raster but no video display (no desktop). The raster is very bright. Horizontal scan lines are visible. The computer isn't usable (because there's no desktop).
Probable diagnosis: The problem is on the analog board.
Solution: Replace the SN74LS38N chip (14-pin DIP, quad 2-input NAND) at board reference U2.

Replacements available, see link in this post (the whole thread is probably worth a read):

 

[gkmaia]

Thanks mate! I was trying to figure out what was this DMS thing... thanks for the tip. Amazing book!

It is pretty much it. I did test the 7438 and all gates seem fine. I ordered a new one anyway. Lets see if that is it.

 

[gkmaia]

I just replaced the LS38 logic and nothing. Still the same issue. will try page 169 as may be a transistor.... keep you posted!

 

[gkmaia]

Did some more extensive checks. Replaced the LS38. Checked most of the transistors around the LS38 and nothing. Still the same issue. Also re-soldered the large connector on the logic board just to make sure. Nothing.

Any ideas?! Other things to try?

 

[LaPorta]

Could it be something as simple as a bad solder joint?

 

[gkmaia]

Could be but I have redone all the joints on the analog board. Fluxed and added a bit of new solder. Unlikely it is that. I will check continuity on all cables and see if I find anything.

 

[Gsbtom]

Silly question, but does the brightness knob do anything?  Maybe the brightness cutoff is set to max?

 

[gkmaia]

No silly question. Any comment is welcome.

That is a tricky one. Wish I had schematics with voltages and waveforms to test against.

If the brightness you refer if the one in the front under the screen. It works as intended as brightness control. But messing with it does not remove the horizontal lines.

 

[Gsbtom]

There’s a brightness cutoff pot on the board as well, though that seems unlikely.

Mac Repair Secrets does have some schematics, see https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf

also, check Q3 and see if there’s any discoloring on the heat sink.

Edit: On page 104 of Mac Repair Secrets, it suggests maybe the LAG on the logic board might be bad.

 

[gkmaia]

There’s a brightness cutoff pot on the board as well, though that seems unlikely.

Mac Repair Secrets does have some schematics, see https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf

also, check Q3 and see if there’s any discoloring on the heat sink.

Edit: On page 104 of Mac Repair Secrets, it suggests maybe the LAG on the logic board might be bad.

I know what could be then... I am reading the Mac Plus logic is grounded to chassis and I discharged the CRT to chassis without a resistor. Apparently the logic board is sensible to that.

Makes no sense to be the analog as I replaced the ICs and tested all transistors.

Are you able to show me which one on the board the LAG chip? Or just its marking will be enough.

 

[gkmaia]

Just found it!

 

[Gsbtom]

Remember the Plus logic board layout is different. Also, I’m guessing at this stage what the issue could be.

 

[gkmaia]

Remember the Plus logic board layout is different. Also, I’m guessing at this stage what the issue could be.

Yes me too. I am reluctant to go and buy a logic board without covering all possible issues with the analog board. 

All books mention about replacing U2 but none mention which gate of U2 controls the horizontal signal. If I knew that I could trace all between U2 and CRT and see if I find something.