Old power supply

The story

This PSU was build by my father back in the 80’s. It has served him well and after he was promoted to glory in 1998 I have used it on my workbench. Today I opened up this old power supply. See photos below:

Old PSU
Backside with power transistors

 

 

 

 

 

 

 

 

 

Old PSU, the inside
Old PSU, the center PCB

 

 

 

 

 

 

 

Origin

From a code on the PCB inside (see photo on the right), it appears to be a design that was published in a Dutch magazine called “Elektuur” in december 1982. That figures; my dad was always reading that…

 

The problem

The large over-spike that I was able to measure yesterday could have been caused by an old – worn out – main electrolytic capacitor, that’s the large round yellowish blob on the left on the last picture.

Verification

To be able to establish wether or not this capacitor is the problem, I have to take it out of the circuit, at least at one side. Otherwise the rest of the circuit would influence the measurement results.

Capacitor isolated from circuit
Capacitor ready for measuring

 

 

 

 

 

 

 

First measurement

I first tried the component tester in the analog oscilloscope:

Measurement indicates a short

This component tester indicates that the capacitor is a dead short. Further measurements with a multimeter an a resistor show that it is charging and de-charching in a seemingly proper rate

Final measurement

So I performed another measurement, now with my new scope. The vertical scale is set to 2 Volts per division and the horizontal time scale to 1 second per division.

Final measurement

 

Conclusion

In this measurement you can see that the time to fully uncharge from 5 Volt is a little over 6 seconds, with a resistance of 240 Ohms over the capacitor. We are talking about a 4700 uF, 63 V capacitor, so the test not entirely adequate, but I’m now reasonably satisfied that this large capacitor is still okay.

 

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Old power supply