by EWBrown » Sun Aug 14, 2011 2:03 pm
First of all, check all of your electrolytic caps in the PSU, they are probably about 45 years old and can be dried out, or otherwise in a failing condition. Look for any bulging or leaking of electrolyte, and carefully, after running the PSU for a while, and then letting the voltage drop bac to zero, very carefully feel if any of teh caps feels unusallywarm or even hot, which would indicate a bad one.
When a gas regulator tube goes bad, it usually has drifted well out of its specs, so the voltage across it will be too high or perhaps too low. They don't generally fail in a short or open mode, though if there is no glow from inside the tube, then the gas has leaked out and it is essentially "open".
For example, an 0A2 or VR150 should measure pretty close to 150VDC across it, but 140-160 VDC is still acceptable, outside of that range means it's gotten weak or otherwise going bad. Also check the series dropping resistor(s) feeding the 0A2(s) if it has drifted out of tolerance, then replace it with a new one of the correct resistance value.
I have two of the Heath Sp17s, one is 100% functional, and has been my tusty lab bench supply for years. The other one, which was $5 at a hamfest, doesn't achieve the full 400VDC, and the DC voltage slowly drifts downwards, after a few minutes' use. I suspect either a flakey 0A2 or more likely, a bad electrolytic cap.
HTH
/ed B
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