Keeping old equipment alive
A few weeks ago I fired up an old HP 204D audio oscillator and found that it wasn’t working anymore. Instead of a clean sine wave, I was getting sinusoidal bursts with a 60 Hz repetition rate. Oh, power supply failed, I thought to myself. I opened the unit up and couldn’t see anything obvious. A search of the Internet yielded a copy of the manual. I was surprised to learn that the linear supply was on all the time. It made troubleshooting easy as I could operate the supply with it disconnected from the unit. The oscillator dates from 1978. The power supply was a simple linear supply with a discrete transistor regulator built on a single layer PC board (Figure 1).
Figure 1 The linear power supply for the HP 204D oscillator is a simple discrete transistor regulator on a single layer PC board.
A quick check with the scope showed what I had expected; the electrolytic filter capacitor in the positive supply was open (Figure 2). Probes on the rectifier outputs for the positive and negative supplies showed that the positive rail wasn’t being filtered.
Figure 2 Looking at the rectifier outputs of the positive and negative supplies, it was easy to see that the positive rail (top trace) wasn’t being filtered.
De-soldering the 100 µF filter cap and checking it with an ohmmeter told the whole story; it wasn’t a capacitor anymore. Then I thought, “Oh heck, I don’t have any parts, I recycled them in New York.” I could have looked online and ordered a replacement but I wanted to get this repair finished that day if possible.
I spoke to my older son, an optical physicist, who has lived in Orlando for some time. He suggested I try Skycraft Surplus, a nearby surplus house. He was right. An old time electrical/electronic surplus store offered bins of components, old test equipment, motors, and materials all of types. I hadn’t seen a store like this since they levelled ‘radio row’ in New York. Fifty cents bought me a replacement capacitor; but, I spent an hour in the store, more reminiscing than shopping. Then I was on my way marveling at how things go in circles.
With the capacitor replaced, the power supply worked like a charm and I had my oscillator back. I also had a new place to shop with my grandson for science project components.
Arthur Pini is a technical support specialist and electrical engineer with over 50 years experience in the electronics test and measurement industry.
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