PS503/Repairs

• Tektronix PS503A Power Supply Restoration by NFM @ YouTube

PS503A: Instable Output/Output voltage is decreasing with time

The output is instable (±100 mV) and is decreasing with time. 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.

Pictures

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