HP 721A Power Supply – Repair

HP’s cute little power supply. Another surplus-store rescue. I was assuming it to be a simple repair/restore. However, it had more plans for me.

 This is how the unit looks, post-cleaning and liberating it from all the labels it had on it

0-30V, linear regulated power supply with germanium PNP transistors. Here is a quick look at the internals.

Problem#1 – Bad capacitors. Yes C3, C4, C5 were bad and they were leaking 950uA@25V . So all three were replaced. The right side pic is of a modern replacement, showing zero leakage current after few seconds of initial charge.

Problem#2 – Output is never zero. The minimum output level is ~5V, even with the voltage control knob at CCW. You can find that in the picture above.

This is caused by the faulty pass transistor 2N375 (Ge). Multimeter test was showing leakage between emitter and collector. My tektronix 575 showed the problem straight away.

For reference, the below trace is from a working PNP/Ge transistor – 2N2137. Yes, the trace is upside down for PNP on the 575.

I was suspecting the tin whisker problem. Anyways cut it opened to check what is going on, and fortunately, there were no whiskers. It was bad/burned out die

Not explaining the schematic here as it is very close to textbook linear power supply.

I was lucky to find a replacement 2N375, installed the new one with a bit of heat sink compound.

Problem#3 – Power cord- was rotten. Replaced it

And here is the power supply after a quick calibration. Calibration procedure is very simple, all you need to adjust is the max voltage and max current.

HP 721A


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