Blank screen on recapped Macintosh Plus

[goerz]

I recently bought a non-working Macintosh Plus and I recapped the analog board (240V version). Initially it was a success: clear and steady image, bootup chime, stable 5V output. Then after a few minutes the screen went blank and the board started making a"flup flup flup" sound. I switched it off, and when I switched it on again I could hear a perfectly fine startup chime, but the video signal was gone. I already replaced the U3 optocoupler (I also tried to put back the original chip, to no avail). What should I look into?
Thanks,
goerz

 

[cheesestraws]

If you are comfortable doing so and have an oscilloscope, my personal approach here would be to trace the video signals through the video amplifier circuit and see where it disappears. The principle of operation is in this PDF: https://vintageapple.org/gamba2/images/plus_analog.PDF - I traced out the equivalent schematics for the 240V AB (and another member corrected them, for which I am grateful!) - they are on the forum somewhere if you search for them.

 

[goerz]

Thank you for your reply. I appreciate your suggestion, but it's way beyond my skills, so I took a more low-tech approach: I resoldered all the connectors on the analog board. I was successful at first, the image came back, and for a few minutes it was perfect. Than it started flickering and eventually it stabilized as in the image below, a clear sign of overvolting. Essentially it is the same problem as in this thread.
The front brightness knob has become unresponsive. By turning the brightness all the way down on the motherboard I can get an acceptable image, but it's out of focus.
Tapping the side of the screen generates flickering and horizontal lines, that disappear when the vibrations stop.

 

[Kouzui]

I recently bought a non-working Macintosh Plus and I recapped the analog board (240V version). Initially it was a success: clear and steady image, bootup chime, stable 5V output. Then after a few minutes the screen went blank and the board started making a"flup flup flup" sound. I switched it off, and when I switched it on again I could hear a perfectly fine startup chime, but the video signal was gone. I already replaced the U3 optocoupler (I also tried to put back the original chip, to no avail). What should I look into?
Thanks,
goerz

If the 240v board has that large 3.9uf, 35v capacitor right before the CRT connection, what did you replace it with? I did some digging around when I recapped my Plus' analog board last week (120v), and I discovered that it needed to be a nonpolar capacitor, and so I replaced it with a 3.9uf, 1000v film capacitor.

 

[chiptripper]

If the 240v board has that large 3.9uf, 35v capacitor right before the CRT connection, what did you replace it with? I did some digging around when I recapped my Plus' analog board last week (120v), and I discovered that it needed to be a nonpolar capacitor, and so I replaced it with a 3.9uf, 1000v film capacitor.

Could explain it. First time I recapped a Plus AB I almost used a polar cap.

 

[goerz]

If the 240v board has that large 3.9uf, 35v capacitor right before the CRT connection, what did you replace it with? I did some digging around when I recapped my Plus' analog board last week (120v), and I discovered that it needed to be a nonpolar capacitor, and so I replaced it with a 3.9uf, 1000v film capacitor.

I replaced C1 (which was leaking, by the way) with a 3.9 uf 450V film capacitor. This replacement has been tested on other boards I had been working on, so I don't think it can be the cause of the overvolt.

 

[MegaImg]

Their is a Brighness controll on the analog board. Use the nob in the front and put it on high, take the case out and adjust the brightness on the analog board until the lines are not visible.

 

[goerz]

Their is a Brighness controll on the analog board. Use the nob in the front and put it on high, take the case out and adjust the brightness on the analog board until the lines are not visible.

It doesn't work. The lines do disappear if the brightness control on the analog board is set to the minimum, but the image out of focus. Plus, the front brightness knob doesn't do anything.

 

[MegaImg]

Do you com

It doesn't work. The lines do disappear if the brightness control on the analog board is set to the minimum, but the image out of focus. Plus, the front brightness knob doesn't do anything.

Do you recap everything on the analog board?

 

[goerz]

Do you com

Do you recap everything on the analog board?

Yes, I recapped the analog board (240V version) completely (originally it generated a garbled screen), and I also replaced the U3 optocoupler. Initially it was a success: clear and steady image, bootup chime, stable 5V output. Then after a few minutes the screen went blank and the board started making a"flup flup flup" sound. I switched it off, and when I switched it on again I could hear a perfectly fine startup chime, but the video signal was gone. So I resoldered all the connectors on the analog board. I was successful at first, the image came back, and for a few minutes it was perfect again. Than it started flickering and eventually it started showing those horizontal lines. The front brightness knob has become unresponsive. By turning the brightness all the way down on the motherboard I can get an acceptable image, but it's out of focus.
Tapping the side of the screen generates flickering and horizontal lines, that disappear when the vibrations stop.

 

[MegaImg]

Yes, I recapped the analog board (240V version) completely (originally it generated a garbled screen), and I also replaced the U3 optocoupler. Initially it was a success: clear and steady image, bootup chime, stable 5V output. Then after a few minutes the screen went blank and the board started making a"flup flup flup" sound. I switched it off, and when I switched it on again I could hear a perfectly fine startup chime, but the video signal was gone. So I resoldered all the connectors on the analog board. I was successful at first, the image came back, and for a few minutes it was perfect again. Than it started flickering and eventually it started showing those horizontal lines. The front brightness knob has become unresponsive. By turning the brightness all the way down on the motherboard I can get an acceptable image, but it's out of focus.
Tapping the side of the screen generates flickering and horizontal lines, that disappear when the vibrations stop.

Sounds like still have a bad soldering point on the analog board. A crack in one of the soldering points. I will reflow all major connectors. You can check with magnification, but I will re-flow all soldering points. I just have one similar. After some checking, found a couple of cracks in the soldering of other components. This is common in all Mac Analog boards. Specially when you take out to recap it, with the movement old soldering points can give up!

 

[MegaImg]

Check/ replace resistor R5 (470K, 1/4 %-watt, 5%). But again, if you still touch the analog board and behavior take place, you have a crack soldering join somewhere.

 

[goerz]

Check/ replace resistor R5 (470K, 1/4 %-watt, 5%). But again, if you still touch the analog board and behavior take place, you have a crack soldering join somewhere.

Thanks, I will resolder everything then. It's going to take a while...

 

[Kouzui]

Thanks, I will resolder everything then. It's going to take a while...

How does the 4-pin CRT connector on the AB look? if it's fried like mine was, replacing it might be the key if you haven't done that already.

 

[goerz]

How does the 4-pin CRT connector on the AB look? if it's fried like mine was, replacing it might be the key if you haven't done that already.

The top pin of the CRT connector, the one connected to C1 (that likely had failed) has signs of overheating, since it is darkened. Does a modern replacement exist?

 

[Kouzui]

The top pin of the CRT connector, the one connected to C1 (that likely failed) has signs of overheating, since it is darkened. Does a modern replacement exist?

it does, let me find the link

https://www.mouser.com/ProductDetail/Molex/15-31-1046?qs=ldwB34exYZUkWPFk7Nh53A%3D%3D

You can search the part # 15311046 to find it on other sites. Don't let the pic fool you, it says 4P HEADER (and I bought one recently, so I can confirm).

 

[MegaImg]

The top pin of the CRT connector, the one connected to C1 (that likely had failed) has signs of overheating, since it is darkened. Does a modern replacement exist?

CHeck for crack on the soldering point on that connector!

 

[goerz]

it does, let me find the link

https://www.mouser.com/ProductDetail/Molex/15-31-1046?qs=ldwB34exYZUkWPFk7Nh53A%3D%3D

You can search the part # 15311046 to find it on other sites. Don't let the pic fool you, it says 4P HEADER (and I bought one recently, so I can confirm).

Wonderful, thank you! Let's hope this does the trick.

 

[goerz]

Wonderful, thank you! Let's hope this does the trick.

Update: I have another working board, so I started swapping components. So far I have determined that it is not the CRT connector, resistor R5, nor the brightness control (variable resistor R56). Also, the good board works fine on this Plus, meaning that the problem does not lie in the CRT itself or in its neck. Could it be a bad flyback? It seems to be corroded, as you can see from the attached picture.

 

[goerz]

Ok so i tried with a spare flyback I had lying around and that didn't solve the problem. Then I swapped Q12 from the good board to this one and the excessive brightness went away! Also the front brightness knob came back to life. The problem now (besides finding a replacement for Q12 - this one must go back to the good board) is that the screen is flickering cyclically. I am attaching a video (I hope you enjoy the background music). Any hints?