Tektronix 2215 how free to turn are V/div and sec/div knobs ?

[MathWizard]

I've taken the scope apart for other tune-ups, and I'm wondering about the knobs. Both V/div are pretty hard to turn, harder than any knobs on anything I own. IDK what's normal for these old scopes and knobs, they seem good otherwise besides the large force, and they lock tightly in place.

I don't have any torque wrench or anything easy to measure the force. But I'm considering opening them up and adding some lube. I found a couple of old Tektronix mechanical maintenance and lubrication manuals.

The timebase sec/div knob, it turns easier, but it has some freedom to turn a few degrees only CCW, on all timebase positions, it's not that much, but it just feels loose and not nice, like I'd expect. It won't slip tho, it takes a lot of force to go to the next position. I know there's an inner rod for the 10x timebase function.

Does that sound normal ??

If I open these up, I've already put some orientation markings on the switches housings. I'll look for some more repair/teardowns, but there's not much out there on these. And trying to ask a search engine about knobs on an oscilloscope, is harder than finding actual PCB images, which isn't that easy either.

[Andy Watson]

Has teh scope suffered some trauma in its history? I.e. could some thing have pushed the front panel out of alignment?

[MathWizard]

No I don't think so, it all looks ok. I could try winding some string or wire around a knob, and attach some weight and see what the static friction force would be, I could also make a full course meal too tho.

Well I guess I just want to take them part, I hope I don't regret it. I have a desoldering gun with big tips, so it should be pretty easy to desolder safely.

[Kwakerman]

I've repaired a couple of these scopes in the last couple of months and the knobs / detent all worked nice and smoothly so yours isn't the norm.  If your switches / shafts are straight you may have dried grease in the detent mechanism or the concentric shafts.  Another possibility is the knobs are pushed tight against the facia panel and the knob skirts are rubbing and causing extra friction.  Try moving the knobs out first to see if that helps, if not you are left with cleaning and lubricating.

[MathWizard]

I already pulled the rotary switches out, yeah there was old dried grease in there, the detents and spring clips look ok. Just cleaning it made it turn better. Now I'll add lithium grease.

Now in some of these old Tektronix maintenance manuals, they say to use WD-40 on the front end of the shaft with the bushing. Is that frowned upon these days for moisture reasons ? What about light oil, or does that trap too much dust ?? I don't have anything else besides Lithium and Dielectric grease.

[MathWizard]

Holy crap is that ever better now with some Li-grease, and I used a drop of light oil on the shaft. Just the detent section alone anyways, is now like turning the knobs on my Siglent's.

But I forgot to check what position the dials where on before I took it apart, or put them all the way over. I did put marker lines on the assembly.

I'm guessing the RC parts, are free to turn forever, so I might hook them up to a signal gen and see if I can tell what position they are in.

[Kwakerman]

Yep, its quite surprising the amount of drag 40-50 year old grease can cause!

[MathWizard]

Ok so there's no RC parts in this assembly, it's just switches, I have them re-aligned correctly. Now what about lubricating them with something like MG chemicals Nutrol "control cleaner" from a spray can? I read it's ~41% mineral oil, and that it really is meant for pots and switches.

When I bought some Nutrol ages ago, I thought it was just contact cleaner, and it gunked up the one switch I used it on. Maybe I used way too much.

But these 'switches', which I guess are a piece of rotating PCB that makes contact with the pins, don't turn all that smoothly, it pretty scratchy. One of them is a lot worse than the other 3, maybe it's dusty or something. I'm sure I'll break the plastic lock trying to open these black, 260-2025-00 switches, but I could glue something back in their place.

The contact resistances are around 70mR, so that's pretty good, I'll worried I'll mess it up if I use the Nutrol. I can try it on some other stuff first, and see how that goes.

I'm in this deep, should I risk Nutrol? Is that really ok for this ?

[MathWizard]

I missed another 2 carbon composite resistors, they are 100Mega ohm's each. They drive the gate of a F2942 JFET, from a CA3060 transconductance amplifier. I can't find any datasheet for the JFET, I guess it's all input buffer on the attenuator PCB.

I haven't done anything with an actual TC-amp, and IDK the leakage gate current of the JFET or the diode there. I have a Keithley 616 Electrometer, but no probes yet, but I'll check the resistors with a DMM.

How much should I worry about these 100M resistors? I replaced a 22Meg that had drifted to 27Meg Just checking ebay, I see someone selling 2 100M's for near $100US, I'm not spending that much. On Digikey, I could get 10 for ~$12+t/s, I'll do that.

[MathWizard]

I went ahead and took apart the timebase rotary switches as well. That was a lot harder to get apart, they didn't use little metal twist-lock tabs, they just chisel into the metal tabs, to make them split and be wider than the holes. So I mostly tapped them back, but had to use a file too, and crack up a little bit of the plastic case....I can seal it tight with superglue tho.

I think I found why the timebase knob feels like it slips before turning, the little plastic bushing that the inner metal rod fits into, it's cracked in a straight line, all the way through like the letter C, not a ring. I read the plastic can shrink over time and crack on the metal, but it has other forces too from turning.

But it still fits pretty tight, so I'll try what this guy did with epoxy glue. I'm not sure what he means when he said he drilled to more holes in the plastic, what do you folks think, maybe a hole all the way through, so that more expoxy on the inside, is connected to the outside  ??

Also there wasn't any amount of grease on these inner parts that fit together sometimes, to engage the 10x timebase knob and time zoom pot. Should I put a touch? You can see these 3 little white pieces in the pic's and the video, ~19min in is when he starts on the timebase switch. Surely those surfaces should get a dab of grease. But then are expoxy and Li-grease safe near each other ??

[MathWizard]

I didn't bother with epoxy, I just went ahead and used superglue on the cracked piece, the crack didn't extend the full length of the piece, it was just in the skinnier half. And I added some grease to those parts too. I also added a washer, to hold the inner spring in place, I don't think I lost one, and the spring easily extends where it's not supposed to, all the way through the big outer "gear".

But see the actual timebase knob, it bolts to a metal press-fit bushing (I guess that's the name), and the bushing was tight on the hollow shaft (until I had to remove it, since the plastic piece is now glued to the other end of the shaft).

The very end of the shaft was knurled, and just past that, the shaft gets skinnier for a few mm or 1/4", but the excess metal was squeezed out into little fins, parallel to the shaft. This bushing, which is basically a pipe with threads on the outside of 1 end, has 4 slits cut cut on the other end, parallel to it's length, and those fins on the shaft fit into the slits.

But using pilers and a hammer/anvil, I can't get the bushing to hold tight on the shaft. It can slide about 1mm before the knurling stops it falling off.

I really don't want to be hitting this hard, since it's all back together, and superglue is on the other end of the shaft, already a bad choose.

OH NO, now I've cracked that bushing just squeezing it with pilers on the shaft, 1 of the end quarters split off. I'll have to goto a local machine shop and get them to make me one.

I'm wondering if the bushing was meant to be squeezed way more over the knurled end, and only part of it would extend to the fin section.

But I had pushed it all the way down to the shoulder of the shaft, and just tried to squeeze the bushing tight.