PS503/Repairs

Tektronix PS503 and PS503A Dual Power Supply Repairs by Bob Haas, VintageTEK Museum @ YouTube
Tektronix PS503A Power Supply Restoration by NFM @ YouTube

PS503A Manual

There are errors in the circuit drawing:

All amplifiers are connected to +5 V or -5 V. In fact they are connected to the +6.2 V or -6.2 V supply.
R48 is missing on the grid location table. It is in F1.
Other parts are missing on the grid location table (Q25, ...)

PS503A: Instable Output/Output voltage is decreasing with time

Problem: The output is instable (±100 mV) and is decreasing with time. Both output chanels behave symetrical.
Solution: The faulty part was C34 (4.7 µF/50 V). C34 stabilises the reference voltage. Measuring C34 with an RLC-meter shows no fault: 5.0 µF and a serial resistance of 5 Ω. The faulty behaviour of C34 can only be seen by applying a DC voltage (see picture).

C34 is shorted by the output-on button. So, replacing C34 with a tantalum capacitor is not recommended.

PS503A: Replacing the voltage indicator light bulb with a LED

to preserve the effect that the indicator lights up with higher output voltage, some resistor have to be changed.

R48 (+ output) and R148 (- output) limits the low current that will always flow when the output is ON. This current will flow at 0 V output voltage. (supplied by +33 V / -33 V)
R55 (+ output) and R155 (- output) limits the high current that will be added to the low current according to the output voltage. (supplied by the 0 to 20 V output voltage)
The LED should be pushed as deep as possible into the bulb holder, so it holds the yellow plastic window part in place.
Choose R48/R148 and R55/R155 according the chosen LED (LED current and power dissipation of the resistor)
R48 is missing on the grid location table. It is in F1.

Pictures

DC current thru a faulty C34 (12 V / 10 mA limit)