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.