assuming you have done a check for dry solder etc (i expect your tech did that already), you could do the check yourself by running the monitor and then looking at the little lights inside at the end of the picture tube where the little separate circuit board is attached at the back of the monitor. you need 3 functioning glowing lights (electron guns) in there. if theres only two working, i dont think you can fix it.
ps. depending on what u plan to do with the cab, it might be cheaper to just shove any old tv in the cab instead of an arcade monitor (eg. u might find that tv's are free from your local recycle centre, or websites like freecycle...), or if u dont care about authenticity, u could just use a crt computer monitor. arcade monitors especially second hand ones are so old now that u will proably run into problems again in no time at all. the are literally 30 years old, and often heavily used.....