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Tektronix 545B oscilloscope

This Tektronix scope was a real piece of high-tech in its time. It is solidly built, the American way. It contains an impressive number of valves and delivers really good performance. There is a little problem, though.

This scope has been built around 1965. The vertical bandwidth is 33 MHz, which is an impressive performance for the time it was built. It has a delayed timebase with several trigger modes.

6AU5 alight

The glow from the 6AU5 tube reflected from the CRT shield.

This fine piece of equipment weighs about 35 kg. The 545B contains more than 50 tubes and it consumes about 500W. The cooling fan makes a noise like an aircraft. Light, heat, size, noise, this is professional technology from the early sixties.

Unfortunately, my 545B has a problem. When it is switched on for about 40 minutes, the HV power converter collapses. I have delved into this and after much testing, the only part that I have left to blame is the HV transformer. You can see it at the lower right corner of the photograph of the HV subassembly. My diagnosis was confirmed by people on the Tek forum on Yahoo Groups.

	   HV rectifier cascade and HV transformer.

HV rectifier cascade and HV transformer.

This phenomenon occurs in a number of Tek scope types of the same age. It is loosely called the "Lossy Potting Problem". It is caused by the potting compound, a waxy material used to seal the windings an keep moist out. But as he potting deteriorates with time, it starts to cause electric energy loss. These increase up to the point where the 6AU5 valve cannot put enough energy into the transformer to keep the HV stable. The losses increas with temperature, explaining the typical warming up period before the HV starts to collapse. It seems there are people who rewind these transformers, but they are as hard to find as white ravens.

I have used this scope sparingly for a number of years. I always had to keep the sessions short because of the HV problem. Its noise and heat production helped to keep sessions short. I recently acquired a better and more compact scope, so now my 545B is just a piece of antique. But as it is so large and my hobby shack is so small, I'd gladly leave it to a responsible foster fasther. In 2012 I sold it to a person who collected Tek scopes.

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