Hi!
This is a "primary-side" regulated power-supply that contains THREE distinct power-supply sections:-
1) A low-power section consisting of U5 (TNY280) and T1 which provides +/-12 V for the front panel voltmeter/ammeter PCB on the front-panel;
2) A second (momentary self-oscillating) "housekeeping" supply - this consists of T2, Q17, Q18 and Q21 and provides by bridge-rectification, the startup and control supply for the main PWM controiler U4, (SG3525) for the first few cycles of oscillation, until PWM drive is produced from pins 11/13 of U4 to maintain the control supply. Transformer T2 is both a self-oscillating housekeeping transformer during the initial start-up phase, and then the main drive transformer for the half-bridge output stage once enough volts has built up across C20/C21 for U4 to become operational;
3) The main variable high-current supply, consisting of U4, half-bridge transistors Q21 & Q22, main controlled transformer B1 and current-sensing transformer B2. The primary return for B1 is the centre-point of the two main HT smoothing electrolytics C23 & C44.
A) Begin by checking the power-supply voltage for the SG3525 U4 across pins 3, (-ve) and 13, 15 (+ve), about 12V DC should be present across these points - if low, make sure C20 & C21 aren't low in value or o/c., whilst if missing test Q17, Q18, D19, also Q19, Q20 and D15, and the soldering to the pins of the housekeeping supply/driver transformer T2.
B) If a satisfactory supply-voltage is present across pins 3 & 15 of U4, transfer your meter-probe red lead to pin 5 of U1 - the voltage here should vary 0 to +5V with adjustment of the front-panel voltage control. If the voltage on the terminals of the Voltage Control is missing, this points to a wiring-fault, an internal failure of U4 or a possible fault in U1 or U2 - pin 16 of U4 is a 5V Reference O/P used by the voltage & current controls, derived from internal circuitry within the PWM controller U4.
C) Next, if you measure a good variation at pin 5 U1 with the front panel control, transfer the red meter lead to pin 1 of U4 - absence of control indicates a fault in U1, Q8, Q9 or Q10, best tested by substitution. Preset controls W2 and W3 adjust the max/min output voltage limits. Feedback to close the control-loop is from the positive O/P terminal via resistors R48, R49 & R50 in series, whilst a.c. feedback to help cancel noise and O/P ripple is via C30 and R44, the high-value electrolytic C29 purely a d.c. blocker.
D) Make sure approximately 7.5V is present across D4, if you need to replace it, use 1 x BZX61C7v5, and make sure C13 is OK as the SG3525 may oscillate if C13 is low in value or O/C, this is an internal reference for the internal error-amplifier in U4.
You should be able to get 2SC1815 & 2SA1015 cheaply in packs of 10 on fleabay, alternatively I've found BC182L (NPN) and BC212L (PNP) are drop-in substitutes but PLEASE make sure you get the "L" type!
Let us know how far you get with all the tests I've given here, and I'll post some more if you're still stuck!
Note:-
In all the above I've assumed the voltage/current displays on the front panel are lit, proving the +/-12V supply from T1 present and correct, or reasonably so!
The circuit drawing for the front-panel PCB presumably was drawn/designed on the basis of being fed from a centre-tapped 50Hz
mains-transformer winding, but the circuit works just the same using the rectified DC supplies from T1!
Chris Williams