The XT Dilema

AFAIK it does neither. It just doesn't work, but it shouldn't cause any damage.

One odd feature of the original IBM mono monitor (5151) is that there's no line (horizontal) osciallator. The Hsync pulses from the MDA card drive the base of the line driver transistor (TR22), which is transformer coupled to the line output transistor (HOT for US types), TR23. Getting inside is the first problem. It's not at all obvious how to open up the 5151 until you've done one. Here's how I do it. Unplug the monitor from the mains before you start, and only do this if you know about TV/monitor repair. There's a 13.2kV PSU in that case.

Mind you, the mains PSU is a linear device with a nice 50Hz transformer, so at least there's no capacitors charged to full mains voltage. Prise (Pry) the 2 little blanking caps out at the front of the top grille. Remove the screws found under them. Place the monitor face down, bottom surface towards you. Free the 2 cables from the rear cover. Now undo the 2 screws at the front/bottom corners that hold the cover on. There are 6 more screws in a 3*2 rectangle. Undo the 4 _rearmost_ ones only. These screws hold the mains transformer/heatsink chassis to the case. If you remove all 6 it will fall onto the CRT, and may break it with the chance of a nasty implosion.

Now lift off the cabinet. I have a circuit diagram (from the O&A TechRef) in front of me. The circuitry is conventional : Low voltage AC from the mains transformer is rectified and smoothed to provide 24V across C603. This is then regulated down to 15V by the 7815 IC601 and the pass transistor TR24 (MJ2955, on the heatsink). You can check foir 15V across C605 (or between the case of TR24 and the 0V line) with the monitor turned on. The video circuit consists of the Hex O/C inverter IC201 (7406) and the asociated componets (including the video input transistor TR18). This circuit combines the V and I lines to produce analogue video on R207 (22 Ohm). This is fed to the video output stage (TR19 as a common emitter amplifer feeding TR20 as a common base amplifier). This is then coupled to the cathode of the CRT. The vertical deflection system is a conventional design based on a TDA1170 (IC401). There's not much I can say about this. The horizontal deflection system consists of the line driver transistor TR22, the bas of which is driven by the Hsync signal. TR21 shorts the base to the 0V rail in the event of the 15V rail overvoltaging, I guess for X-ray protection. The collector load of TR22 is the driver transformer T501, which drives the base of the line output transistor TR23. The line output stage is conventional with the yoke driven from the collector of TR23 via C505,and L502-504 and the collector load of TR232 being the line output transformer T502. This produces the 55V supply for the video stage (check on C508), the -170V supply for the CRT grid bias (check on C507) and the 450V supply for the gun electrodes (check on C509). The EHT (13.2kV) is also produced by the line output transformer, of course. I hope that's of some use to you.

-tony