Single-frequency monitors are found on every other computer, e.g. Sun workstation or Mac. They can only accept a narrow range of scan frequencies. For instance the 20" monitor you usually get with your SPARCstation will display between 63 and 70 kHz line and 60-72 Hz frame rate if you tweek the internal pots.
There are also monitors intended for older computers which cannot accept ANY standard PC resolution. You might have one of these, in which case sell it to someone with an Atari or Amiga. If you have young healthy ears you can recognise it by the loud whistling noise it makes when operating. Don't get rid of it yet though; there are other reasons for monitors to whistle (for instance when they are feeling cheery)
Worst of all are the 'CGA', 'EGA' and 'MDA' monitors perpetrated by PC manufacturers during the Eighties. These only accept 'TTL' digital video, and thus can display a very limited set of colours (in the case of CGA, four!) What's more they have weird scan rates. If you have a CGA or EGA monitor you are best to use it for testing your new twelve-bore.
MDA (aka Hercules) monitors are slightly more useful, as they can just about be fragged up to display normal composite video, but it's not really worth the hassle.
You can recognise any of these monitors by its small size and 9-pin D video connector. (Don't confuse with a Sun/Hitachi mono monitor; it has a 9-pin video socket but is much bigger and more useful.)
While you have the lid whipped off, identify the input connections, the PCB usually offers some clues (like it might have 'R', 'G', 'B' etc printed on it) There are schematics elsewhere on the Web for many popular monitors.
Once you have figured out the connections it is time to fire up. Get the RGB output from your computer wired to the monitor so you will see a picture (of sorts!) Also have a normal VGA monitor on hand. Now acquaint yourself with the setup program supplied with your video card; pay particular attention to the method of testing different resolutions, and most importantly, escaping back to normal VGA.
Wire the sync up so you have normal and inverted sync waveforms on hand (use a 74LS04 inverter chip or similar.) If you only have one sync input on your monitor, you will need to build a sync combiner, but don't worry about this yet. You want to make sure it works first...
With the normal VGA monitor plugged in, prepare your video card to switch to the resolution you want to try. (1152 x 864 at 70Hz is good for most Sun-style monitors.)
If you are unsure, then turn the monitor on with no signal applied, and hold the probe of a frequency counter near (not touching) the yoke terminals. The frequency you get is approximately the monitor's intended line scan rate.
Now, unplug the VGA monitor and hook in your jerry-rig. If you only have one sync socket on the monitor, connect the line sync to it. Now (using the appropriate hot key or a click on a previously-positioned mouse) switch to the new resolution. You might see the line scan lock on; if not try the other polarity of line sync. If neither works fiddle with the H-hold preset. You should end up with a picture that is legible but rolling vertically.
If the picture is too wide for the screen, or appears wrapped round, you are running at too slow a line rate.
If it is too small, the line rate is too fast.
If you have a multiscan it might lock on at whatever resolution you try!
If you cannot get the picture to appear, then press the appropriate hot-key to return to VGA mode, plug the VGA monitor back in, choose a new resolution, and repeat.
Once it looks like the monitor might work, note the polarity of sync you used. If you have two sync inputs repeat the process for the frame sync. Otherwise it's time to build that combiner circuit.
T.B.C.