Commodore 1084s-D1 High-pitch whine FIXED!

[Castellen]

No idea which is the correct schematic variant for your monitor, and I'm no CRT monitor expert, though it shouldn't be too hard to work out if the 37 Ohms is an issue or not.  The 125V supply is possibly a fairly low impedance while turned off, in which case you'd be possibly measuring through the transformer winding (20-ish Ohms at a guess) and through R469 (12 Ohms) back to the 125V supply in a low impedance state.  Lift one end of R469 (see attached image) and then measure DC resistance between the collector and emitter pads of the horizontal output transistor, with the transistor removed.  I'd expect the 37 Ohms wouldn't be there then, else there's likely a problem. 

Remove the HOT, tested it in my component tester which says it's a diode between B-E but also says it has a 298pF capacitance. Tested it by meter, E-C Diode affect. B-E 37ohm. B-C Diode affect.

The transistor should not look like a diode between the emitter and collector.  It's just a NPN transistor, so with the red lead of the diode tester on the base terminal, you should see about 0.6V on both the collector and emitter.  Nothing with the reverse polarity.  Nothing with either polarity between collector and emitter, the same way you'd test any other NPN transistor.

So if you're not getting that, the horizontal output transistor is probably damaged, which often happens when the flyback transformer dies.  Have repaired a few 1084S myself with the same symptoms as you describe and have always replaced the transformer and transistor together for this same reason.

I'd suggest replacing the transistor to begin with, and failing that you'll need to start looking at power supply voltages, etc.  They're not particularly fun to work on.

[Castellen]

I'd agree that the schematic doesn't appear to match your monitor variant, which doesn't help the confusion.  For a start, you confirmed that your horizontal output transistor is 2SD1555, which doesn't match the schematic.  The 1555 has the internal base resistor and C-E shunt diode as you say, but the suspect schematic shows these components separately.  The design wouldn't include those if they're already part of the transistor.

Closest schematic I can find from my webserver is sheet 1084S-D2.GIF in http://amiga.serveftp.net/Schematics/1084S_schematics/1084S_schematics-GIF.LHA

The quality is mostly terrible and it's still not the same transistor (though it is similar/equivalent), but it might be a better match to your hardware.

Something doesn't look right with that 37 Ohms you're seeing, needs more understanding.  You should be able to remove the solder from only pin 1 of the transformer (without removing the transformer) and move the pin so it's not touching the PCB pads to further isolate things.

Does the PCB substrate in that area have any obvious contamination, burning or discolouration?  That could be another possible reason for the unexpected low resistance.

Out of interest, what's the DC resistance of the transformer's primary winding?  i.e. Between pins 1 and 7.  I'd be guessing something in the region of 20 - 100 Ohms.

[Castellen]

the LED comes on for a second and then goes off (Is that a bad sign?).

The schematic shows that the LED is run from the regulated +12V supply, and that's derived from a 7812 linear regulator, and that's powered from a 15V unregulated supply that comes from the main power transformer.  So you've probably got some kind of a power supply issue.

Specifics around the power supply vary a lot between the many variants of the monitor, so I'd suggest narrowing down the correct schematic to begin with.  I've copied the other documentation on variants that you pointed out to my webserver so everything is in one place:
http://amiga.serveftp.net/schematics.html#1084

The obvious next step would be checking if the power transformer (not the line output transformer) primary is being switched or not, as if that's not happening then none of the various power supplies will be working.  Since it sounds like it starts working and then stops, the switching supply that drives the transformer primary might be going into some kind of shut-down protection state.  I'm guessing from here as the details of that circuit is completely different between model variants.

[Castellen]

I wouldn't get too carried away with the 105V vs 109V vs 110V, it's hardly going to be exact.

If you did see smoke from the transformer, it could well have been damaged due to the other issue you've since found and fixed.  When you've got a lot of DC current going through a transformer like that, it usually doesn't end well for for the transformer.

I'd suggest doing a bit more research to be absolutely sure that the replacement line output transformer was compatible.  I've previously seen some helpful discussions on various forums around model numbers and compatibility.  Then try again with another transformer.