First off, A little SAFETY message since this console has high voltage present in it.
Working on equipment with CRT high voltages can be FATAL. YES, FATAL. Be sure to know where the danger area is located on your equipment and take precautions to avoid electrocution and/or shorting out the high voltage supply. Avoid wearing anything conductive (watches, rings, bracelets) and be sure to use insulated tools to protect against unintentional electrical shock. I take no responsibility for injury resulting from trying to troubleshoot your equipment based on my recommendations in the response. YOU HAVE BEEN WARNED.
Tools needed for this repair:
- A copy of the schematics (already provided by the requester )
- An oscilloscope (strongly recommended)
- A non-conductive screwdriver for the X Size deflection potentiometer
- A voltmeter
Troubleshooting your Vectrex issue:
The quickest thing to check is to see if you are getting horizontal deflection signal from the game board before it reaches the monitor. This is on PIN 1 of IC302. If you don't have signal, there is DAC problem (not covered by this response). If you do have signal, the steps are to use a oscilloscope probe while tracing the signal to its eventual output at the deflection yoke.
Take a look at the posted schematics. (specifically pages 33 and 35). Based on you information, the following components are in the signal chain until the output at the X Deflection Coil.
IC305 (pins 11 input, 10 output) -> IC303 (pin 11 input, pin 14 output) -> x-size potentiometer -> Q402 JFET (pin S in, Pin D out) -> IC401 (Pins 8,9,10)
Possible bad news
If you have signal all the way to the output, the worst possible scenario would be if the deflection yoke was burned open. If this is what happened, you are going to need to part out another dead Vectrex to source a new monitor yoke - or just do a monitor swap from a donor machine with a dead logic board. Getting small X-Y black and white monitor yokes is going to be impossible unless you find someone to wind them up for you. (EXPENSIVE)
There is hope
On the bright side of things, it could be something as stupid as cleaning the X-Size potentiometer. These can die just from dirt/dust. Cleaning them with potentiometer spray (WITH THE UNIT OFF) and a small bristle brush could instantly fix the issue. Do NOT use WD40 - use something that is engineered specifically for potentiometers. A way to quickly investigate if this is a problem area is to tap on the potentiometer with something non-conductive and see if the deflection jumps around. If it does, then it's an EASY fix solved with a simple cleaning job.
1) UNPLUG you game and let the power supply fully drain for 30 minutes - 1 hour.
2) Mark the set position of the potentiometer with a sharpie so you can return to the factory setting after you clean it.
3) Spray and brush as much junk off the potentiometer as possible. 4) Take a small screwdriver and rotate the knob through its full rotation in both directions 5 - 10 times.
5) Return the potentiometer back to its setting you marked with the sharpie marker,
6) wipe off any excess cleaner spray with a paper towel, and let any excess liquid thoroughly dry out.
7) power on the game, and see if the problem is fixed.
If the potentiometer was the issue, and you have decent desoldering/soldering skills, i would strongly suggest replacing it with a modern equivalent. They are cheap and will be a significant improvement over the original part.
Good luck to you and hopefully it's not a dead deflection yoke.