Sir Subi . . . . . .
. . . .my making a schematic . . . .
Is that a functioning board and what is its purpose. ( A solid state version of a current loop supply for a teletype ? )
Since the currently drawn out connections do not seem to make a viable working circuit.
I have initially simplified the drawing somewhat, by dropping the clutter of unused lines that do not terminate into circuitry components, plus numerous "U" wire crossover symbols.
As the circuit layout is now being drawn . . . it is deviating from the norm, with its power input on the right side and the output at the left .
Also, the DC positive voltage is at the bottom versus the top .
UNLESS your concept is biased by a native language that is normally read from bottom to top or from right to left.
I see the 100k pot wired in order, with it using R1 and R2 on either end to establish its effective range with its center wiper feeding its variable output setting to the base of the small signal NPN transisistor.
I see the incoming 76 VAC coming into a Full Wave Bridge configured with D1-2-3-4 and a (1000) pf mylar noise / spike filter shunting them and then a 220ufd @160Vdc E-cap storing up the raw rectified DC supply.
LOOKING FOR THE LOGICAL B+ POWER FLOW PATH . . . . .
I now seem to see the sole viable flow path being from the MAIN B+ coming in at the bottom #1 line from the joined cathodes of D1 and D3, with filtering supplied by " BIG cap C".
The first feed off is being to R2 . . .47k . . . and it then going to the collector of small signal NPN transistor 2SC1815.
So with a 100VDC B+ hitting it,that transistor will then be able to see no worse than 2.25 ma of current with a potential .225 watt of total dissipation.
Whoa . . . .horsie . . . . . . that potential voltage level would exceed the collectors 60V spec below.
ABSOLUTE MAXIMUM RATINGS at Ta=25℃ Collector-Base Voltage Vcbo 60 V Collector-Emitter Voltage
Then the B+ feeds up to R3 . . .560k . . . to the max cw lug of the 100k pot . . . .while its other max CCW lug goes to R1 . . .6.8k . . . and then to ground. No problem is seen there as they are just seleccting out a specific range of adjustment capability .
Then things get wierd, with the B+ then going into the anode of a GREEN LED and exiting out its cathode to go up to the " b " terminal of your O/P connection.
It would then have to pass out thru that " b " terminal and come back to enter as your "a" terminal, which then goes direct to the collector of your 2SC2233 "power" transistor, since it is capable of 40 watts when heat sinked.
BUT that transistor also has a 60V collector spec.
Sooooooooo . . . confirm the units raw DC supply voltage, seems like it should be ~ 100 VDC with using a 76 VAC supply input.
That incoming voltage then goes solely to the 2SC2233 collector, and if its base was being FULLY driven on by the small signal transistor the power would flow out the 2233 emitter to the R4 resistor . . 120 ohm . . . to complete a ground connection.
But all hell would break loose, as 100V at ~ 833 ma would vaporize your earlier series LED with its 20-30 ma rating and your R4 resistor to ground, unless it was a 100 watt unit.
So looks like there must be an error in interpreting the strip board connections wiring.
Seems like I . . . DO ! . . . do see two component side wiring U jumpers being installed near the transistors, and also a yellow wire jumper.( or component lead wire extender ? )
Are they being accounted for in your wiring ?.
Now if this unit is supposed to be a supply andwith its output being at the a and b terminals, the top a connection would go to the emitter of the power transistor and the transistor collector would also be receiving appreciable + supply voltage.
The b connection would be going to ground / negative, and not thru the LED as it is now, in its path to ground..
If the LED is only being a power indicator, it would be wired between ground (cathode) and have a series current limiting resistor from the LEDS anode to the a output connector. ( But the LED brightness would vary with different settings . )
If the smaller transistor was intended to be making a Darlington interface into the power one, there would be an interconnection between the two collectors.
REFERENCING TO THE MARK down BELOW . . . . .
The YELLOW is questioning the output connections.
The PINK is questioning only a sole connection being made to the collector.
The GREEN is questioning the wattage of the R4.
The ORANGE is questioning the collector solely going over to the A output . . . . . or if you have reversed the collector and emitter.
Did I make any error in my chopping out of dead end portions and line crossovers ?
REFERENCE . . . . .
73's de Edd ....
Appearances are not everything; it just looks like they are.