Well, it is not exactly my "cup of tea" but it is fairly common to work on a piece of equipment without the diagram.
As the the monitor is still in working condition, we can assume that problem is due to deterioration of component(s), soldering joints or the combination of the above.
There should be a trimpot or two, located on the neck board of the picture tube. One of them should have the effect on the level of screen raster and the other on the focus.
[Note: a proper, isolated trimpot adjusting tool is required to do the adjustments while the monitor is operating. I do apologise for mentioning it here as I am sure you know this. I do it because of the safety, for the benefit of potential other but less experienced readers that may open this post.]
I would also examin the soldering of the whole unit, taking particular note of areas around components that dissipate the heat (like high power resistors, transistors, e.t.c). I would resolder the suspect ones and continue with examination of the PCBs, but this time - on the component side.
There could be obvious signs of deterioration of electrolitic capacitors (the sign is a bulging top of the can but not always). They seem also to deteriorate in two measurable ways, like loss of capacitance (capacitance meter is helpfull) or incresed ESR (effective series resistance). There are ESR meeters available, I have built one around 10 years ago from a kit purchased at a Dick Smith store. It is a very usefull piece of instrument and I still use it today.
As the unit is old, it is probably a good idea to replace not only suspect capacitors but the whole lot of them (electrolitics). It is not a job I particularly like but without the diagram I found it to be an effective way to solve the problem and little electrolitic capacitors are not expensive. The only real expense here is the time it takes to do it
.
I am almost convinced the problem here is the overall detrioration and I sincerely hope that you will fix it.
As for the diagram, I can not be of any help to you.
Regards,
xanadunow