Apple Macintosh Classic II rap - why does it do this?

[solidpro]

I've recapped both the analogue board and the logic board. The logic board tests good in other machines. Does anyone know why my machine does this constant reboot thing?

Classic II on YouTube

 

[powerlot]

Your power supply or some other component on the logic analogue board have issues.... the wavy lines and lack of vertical deflection suggest a fault in the CRT circuit or its supply voltages.

Did you recap the whole analogue board, if yes which guide did you follow and which components did you use?
Have you checked your work (correct caps, orientation, no leftover electrolyte or other shorts)?
Do you see arcing / sparks with the cover removed in a darkened room?
What are the voltages like?
Does something smell weird?

 

[Hugo Holden]

There is a defect in the power supply with both the height and width being too low. There is ripple on the power supply output voltage too, you can see what amounts to a recording of it down the sides of the raster scan with the horizontal scan width being modulated by the ripple. When there are defects like this in both the H & V scan, it is certain to be a power supply issue. Check for overheating in the regulator too.

It should be easy to sort out with the scope, looking at the input and output voltages around the voltage regulator.

Also did you replace the capacitor, that looks like an electrolytic capacitor (but isn't exactly) in series with the yoke's H deflection coils ? If so, what part did you use ?

 

[GiGaBiTe]

I would also recommend reflowing ALL solder joints on the analog board with new leaded solder. Analog boards are notorious for cracked/dry joints from the extreme heat inside the unit.

Clean the potentiometers, all of them. Hose them down with deoxit gold or other contact cleaner spray and wipe them back and forth several times until all of the oxidized carbon works its way out. Make sure you mark and reset the potentiometers to their original position.

Neither of these are likely your main issue, but they will contribute to erratic behavior if not fixed.

Also, did you replace the horizontal deflection capacitor? I think it's a 3.9 or 4.5uF bipolar HF capacitor. Nobody makes these anymore, so you have to sub them for something like a polypropylene microwave capacitor. DO NOT USE A NORMAL CAPACITOR, bad things will happen. If the original capacitor tests fine in a capacitor tester, I would say use it rather than replace it.

 

[Hugo Holden]

Also, did you replace the horizontal deflection capacitor? I think it's a 3.9 or 4.5uF bipolar HF capacitor. Nobody makes these anymore, so you have to sub them for something like a polypropylene microwave capacitor. DO NOT USE A NORMAL CAPACITOR, bad things will happen. If the original capacitor tests fine in a capacitor tester, I would say use it rather than replace it.

Exactly what I was wondering in post #3.

There is no electrolytic cap available from any supplier that can replace this part. Generally the originals had an ESR of only about 0.1 Ohms or less. They can be replaced with a film capacitor through it is better to use a high voltage rated one, typically 250V rated. Or better as you say, don't replace the original, they seldom ever give any trouble. All they do is fool people who have been coached to do global re-caps on VDU's (maybe by internet videos) and replace them along with many other good original electrolytic caps, simply because these S correction caps have the appearance of a bipolar electrolytic capacitor.