Sir Constantine . . . . .
Looks like your unit circuitry is just a POWER oscillator that drives the transformer , and uses no IC's.
It's probably using that other single small signal bipolar transistor in assisting in clamping down the base drive level to the power output to regulate the overall power output.
I mentioned the use of the series lamp to keep from blowing the output transistor again.
If you have made the supply come up . . . . without blowing up . . .excluding that "
special resistor " . . . you might disregard the precautionary power limiting safety lamp..
Figure that "
THE R18 " resistor function is for voltage dropping for the power to the LED enclosed with in the 817 family of optical isolator.
And . . . . that you have had one burnt up and also the the follow up guesstimate that you installed.
In a no power plugged in state, test the LED inside of the 817 to see if any semblance of a LED is still inside, or it is now being just a fused together blob of semiconductor material, now reading as a dead short.
OR . . . . you might possibly get a diode test result, somewhat akin to 2 series arranged diodes.
If the later suggests that a LED might still dwell within, then lets initially use this procedure, where you cut the line at
RED X . . . . OR . . . just don't have the R18 resistor installed.
Then you do DC voltage monitoring across any of the filters of the 12v DC output, and then power up the unit
JUST LONG ENOUGH to see what level of now unregulated volts shows on the meter . . . .15---18----20 ?
To keep this low voltage / cold supply side, still all isolated and safe, get a 9V transistor type battery, and use 2 flying wire leads, to connect battery + to a 470 ohm 1/2 W resistor and a lead to the other - terminal of the battery.
Your free end of the 470 resistor is going to connect to the pad of old R18 that went over to the 817 optical isolator.
Your batteries free flying lead will be held in hand to be temporarily touched to the negative power output terminal of the power supply.
If all is well in the 817 and the HOT side of the power supplies regulatory circuitry that that 817 phototransistor feeds, there will be a
decrease in power supply output voltage, that you had read before, while holding that connection, just after powering up, for a short time needed to monitor the output voltage again.
This time it could be down to 9? 7? volts, but rise just as you lift the wire for the battery supplied LED power.
If this passes regulation control in this manner. . .then I will tell you how to evaluate the sets 431 existing feed back circuitry, to see if it is working..
If it doesn't, start looking for a common 817 OI as most computer power power supplies use, and u might have 1 in Le JUNQUE BOXE.
SCHEMATIC REFERENCING . . . . ..
73's de Edd
.....