PS503A/Repairs

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

Manual errata

There is an inconsistency in the schematics in that the opamps are shown connected to +5 V or –5 V, but this refers to the ±6.2 V supplies – presumably, because before S/N B022065, zeners VR17 and VR117 were 5.1 V parts.
R48 is missing on the grid location table. It is in F1.
Other parts are missing on the grid location table (Q25, ...)

Common issues

Due to age and/or the high supply voltages, the 741 opamps in the PS503 are often problematic (see Bob Haas video for a suggested procedure to test them at the actual operating voltage).
To isolate problems, the current limit circuit can be disabled by removing the respective opamp (U70 / U170).
Some PS503A versions use the infamous, unreliable Texas Instruments IC sockets. They should be replaced proactively, which is easy to do (pull off plastic, de-solder pins individually).
If the unit in question has 4.7 μF/50 V tantalum capacitors, they can be replaced by modern 10 μF/50 V aluminum electrolytic parts if necessary. Tek did that in later production runs of the PS503A (see e.g. Component News #289 (1981), p.7).
Check C10/C20 – with fully loaded outputs, ripple should be ≤2 V_p-p at twice the line frequency (120 or 100 Hz). 60 or 50 Hz ripple indicates a defective rectifier.

PS503A: Instable Output/Output voltage is decreasing with time

Problem: The output is instable (±100 mV) and is decreasing with time. Both output channels behave symmetrically.

Solution: The faulty part was C34 (4.7 µF/50 V). C34 stabilizes the reference voltage. Measuring C34 with an RLC-meter shows no fault: 5.0 µF and a series 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.

Replacing the voltage indicator light bulbs with orange LEDs

To preserve the effect that the indicator lights up with higher output voltage, resistors need to be changed.

R48 (+ output) and R148 (– output) limit the low current that will always flow when the output is ON, even at 0 V output voltage (supplied by +33 V / –33 V).
R55 (+ output) and R155 (– output) limit 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 orange plastic window part in place. A 3 mm LED will fit directly into the original bulb holder.
Choose R48/R148 and R55/R155 according the LED used. Starting points are 15 kΩ for R48/R148, 5.6 kΩ for R55/R155.

Restoration

A carcass of an early PS503A – broken dual potentiometers, missing binding posts, missing knobs – was restored:

Broken concentric pots for voltage replaced with slim 10-turn pots, C10/C110 removed to make space
Tantalum caps replaced, new binding posts, orange LEDs
New opamp sockets, 3 × 2200 μF/50 V radial caps each replacing C10/C110
Final, board (fuse holders replaced with 5×20 mm on component side)
Final, front (plastic frame and latch tbd)