[time-nuts] HP 5065 Rb update

[ed breya]

Paul S. wrote

"The pots on the digital counter and its actually quite small. The 3 
leads all fit into an area about the size of a pencil eraser."

Yeah, I know what you mean. I had assumed the pot in question was one of 
the more modern, fairly compact ten-turn helipots, that are about an 
inch diameter and length (body size), as opposed to the really big 
old-school ones (about 2" D and L). There is at least one style even 
smaller, about 1/2" D x 1" L. These are true helipots, built with a 
helical mandrel carrying many turns of fine resistance wire. Below this 
size range, I don't know if a helical pot structure can be built 
practically, so it's possible that these small clock face style ones may 
be more similar to the small multi-turn trimmer pots, with a short, 
straight mandrel, and the wiper driven linearly by a worm-screw 
mechanism. The difference in construction has a huge effect on available 
resolution. A helipot will have one loop of mandrel for each shaft turn, 
so its effective length is quite large, compared to a short, straight 
element, with gear reduction to provide multiple turns.

Regarding the 5065A, I suppose the clock style pot was for saving space, 
with the pot actually inside the knob and counting assembly, and hardly 
anything behind the panel - pretty slick. I think I still have somewhere 
the actual pot from the 5065A carcass I got many years ago (used to 
build Z3801A into), and I just found a few of probably the same kind, 
that I had collected over the years. The whole front is a knob about 
3/4" D x 1" L, with a clock face (except 10 vs 12 points) readout in the 
tip. The back looks like a standard 3/8" D mounting shank, with only the 
pot terminals sticking out.

So, it's possible that they managed to squeeze a helical pot mechanism 
and readout in there, but it could be something else. I've never had any 
of these apart (which probably requires destruction), so don't know 
what's in there. One way I could picture it working, is that the knob 
itself is the wiper carrier, shaped internally with the right screw 
pitch. The mandrel could then be wrapped around an inner structure, and 
the readout mechanism in the center and tip. Another option is that it 
could be helical, but not necessarily ten full turns-worth - maybe five 
turns, and 2:1 mechanical reduction, and so on.

I think these kinds likely are still made, so data sheets may describe 
or show what's inside.

Ed