Intermittent vertical deflection issue

[tedward]

Searched around and I see a lot of posts about this issue so it seems fairly common but the cause can sometimes be difficult to track down as there are a number of reasons it could happen. I'm hoping to get some insight before I start digging in to help minimize the amount of guesswork I have to do.

Basically, the machine boots and seems fine and after being booted up for a while, the screen will flicker and appear as shown in the image below:

When this happens, I notice a distinct high pitched whine starts emanating from the system, like you'd get from a cheap switching power supply that's failing. If I turn the machine off and leave it off for a few minutes and turn it back on, the issue goes away for a while and then comes back after a few minutes of being powered on. If I turn the machine off and back on immediately, the issue persists. This behavior makes me think that it's either capacitor related or a MOSFET/transistor issue (something getting hot or a cap only showing issues after being charged for a certain amount of time).

I have visually inspected both the logic board and analogue board and all the caps look healthy and I don't see any leakage anywhere, either current of old, but I know that doesn't necessarily mean a cap hasn't started to go bad. As far as I can tell, all of the onboard components are original to the machine (or whoever replaced them was good with a soldering iron).

If someone could help me narrow down what I should be looking for I would greatly appreciate it. I recently moved and all my diagnostic equipment is still packed away until I'm able to properly setup my work bench so I'm currently limited to an old pocket multimeter for troubleshooting.

 

[thellmer]

Look at the 4 pins in the yoke plug (the big coil of wires which sites around the next of the picture tube) where they connect to the analog board - pull the white plug and look for evidence of burned or overheated contacts and clean up as appropriate, then remove the back side plastic cover off the analog board and look at the 4 pins of that same plug receptacle where they are soldered to the board and see if any of the 4 pins have cracked solder around them that needs re-flowed. On "most" of these those 4 pins are directly underneath one of those white double-stick adhesive pads that hold the side plastic cover to the board, so you have to scrape it off to see the 4 pins and their solder connection. Then you can either not add it back or cut one of your own double sided foam squares to replace it, or just a dab of non-conductive glue. These plugs almost always have cracked solder joint resulting in your symptom or if on the horizontal side a thin vertical line from top to bottom of the screen. If the computer is left running over time, the cracks cause the pins inside the plug to overheat which turns the plastic brown and also causes the pins/receptacle to corrode and turn brown or black. If checking for cracks there and/or reflowing the solder (add a dab of new as well as flux if you have it) turns out not to fix the issue, then follow all those traces around in that area and look for evidence of possible other cracked joints around caps, resistors, etc.

 

[tedward]

OK, well, I pulled the analogue board out and checked it as you described. The socket on the board looks clean, no browning or scorch marks. The solder joints on the back look perfectly fine, no cracks and no corrosion. The socket feels firmly in place and attempts to wiggle it produced no movement on the underside of the board. I also gave the cable side connector a look. The board plug side looks fine. I also tried to check the wires going in to the side of the tube itself. I had a hard time seeing them under the cap, and I didn't want to move it too forcefully, but I didn't see anything obviously out of place in there either.
As you suggested, from there I started following traces out on the underside of the board from the four-pin socket and that's when I did notice something interesting. One of the legs on Q1 seems to have no solder on it (see image below). Now, I assume there has to be some kind of electrical connection there or it wouldn't work at all, but maybe after it warms up a bit it moves just enough due to thermal expansion that it becomes a poor connection. Thoughts?

 

[thellmer]

Ah yeah that definitely needs re-soldered and could be your issue. I believe that is one of the vertical output transistors