Hi.
The Neon reference tube is indeed known to "go funny" after some years.
The isotope contained to lower the striking voltage only has a 10(ish) year half life.? After 20+ years they can become rather fussy about at what voltage they strike at (the resulting transient high voltage, can sometimes trigger the over-voltage protection, and blow fuses.)
After 30 years, there is possibly only some 12.5% of the isotope left, regardless of if the tube was used or not.
Plus, the maintaining voltage can drift over time too, as a result of being used.? There are many who have substituted a string of Zener Diodes to do the job.? But, look at the Zener data sheets, and choose ones with a suitable temperature coefficient to suit etc.
Also See:-
He has "Lots" of other interesting instrument (HP and others) repair's documented too...
I also have recollections about a very high value (85 M Ohms?) resistor in the only 141T I ever crossed swords with during an extended visit (thanks to TWA going bust!) to central California. That had gone very high value, so was replaced with ten of 10M resistors (one series element had two in parallel to get the desired value) the whole lot encased in some insulation tubing.
I don't recall in what part of the PSU that resistor was in, maybe the CRT circuit, but it prevented the thing from powering up correctly, I can't find a good quality image of the schematics just now, plus my notes from back then are long gone.
I do recall it still had the neon tube in the main PSU.? At that time (early 90's) that was not exhibiting? problem!
A nice instrument to use when working, and a superb variable persistence storage tube too.
Regards to All.
Dave B (in the UK)
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