Machintosh Plus weird screen problem

[MikkoS]

Hi all! I have this somewhat weird screen problemn on my Mac Plus:



So when I turn the machine on it POSTs ok ( I hear the "bing" also) and then it loads my System disk. Everything seems normal but the screen stays from the start like in the pictures. On the left the brightness knob is turned neutral and on the right it's maximum. So it at least is kinda working.

The screen reacts to the loading of diskette. And if I move the mouse pointer (which I don't see) to the upper bar and press mouse button, I can see extra light at the bottom of the screen (from the opened pulldown menu). No pixels are to be seen.. only light. It is as if the screen is somewhere down there.

I have checked the dead scrolls pdf and resoldered the usual suspects on the analog board but no change.

Any help appreciated.

Cheers,
Mikko

 

[GiGaBiTe]

You are correct "the screen is somewhere down there".

The CRT and/or analog board has a problem with the deflection circuitry that's pulling the cathode beam HARD down to the bottom of the tube. If you could see inside the tube, it'd be a blob of light.

On the CRT side, there could be a short in the deflection coil, or one of the magnets fell off the yoke. You can try placing a suitably strong magnet on the top of the tube in front of the yoke and see if you can pull the image off the bottom of the tube. On the analog board side, I couldn't be of much help because I'm not familiar with the all in one analog + power board design. If you have or can find a schematic, I would recommend going over the deflection sections. I would also recommend reflowing ALL solder joints on the analog board, because they're notorious for micro cracks that cause problems with temperature changes.

You can try tapping the bottom of the analog board while the machine is running with an insulated implement, like the handle of a screwdriver to see if anything changes on the screen, or the computer crashes. If something undesirable happens, it means there's a bad solder connection on the analog board, or a faulty component that has a mechanical fault. Resistors are known for internally fracturing from heat stress, but they're really hard to find unless you can poke them directly to induce failure.

There's also the worst case scenario, the cathode gun has somehow broken loose and isn't aiming straight anymore. If this is the case, there's nothing that can be done to fix the tube.

 

[Hugo Holden]

Typically the vertical scan output amplifier is coupled to the Vertical yoke coils by a coupling capacitor, normally a moderate value electrolytic type between 470 to 1000uF or more. The capacitor isolates the amplifier's DC voltage (which can be about 1/2 the power supply voltage) from the yoke coils. If that capacitor was shorted or very leaky, this could put a DC offset on the coils, though normally as these electros fail, if anything they go high ESR. In some vertical circuits, there is direct coupling to the V yoke coils and the other leg of the yoke is connected to a 1/2 rail voltage reference formed from two transistors, if one of the total of 4 transistors in what amounts to this bridge circuit was defective it could also cause this.

Do you have a link to the schematic so we could have a better guess ?

 

[MikkoS]

Sorry for the late reply. One day when working with the Mac the screen suddenly collapsed to straight horizontal line on the middle of the screen. Just like in The Dead Mac Scrolls PDF pages 49. and 50. Apparently something finally gave up. So, earlier the problem was strange and I couldn't find any information on the net, but now I think it is a known problem. I have ordered replacement parts according to The Dead Mac Scrolls PDF information. Hopefully that'll do the trick!

I will post the results as soon as I have replaced the needed parts.