Topics

Backlash/Play in the Delta Time Position Control on a 2236


 

I have an otherwise pristine 2236 that has a lot of play/backlash in the delta time knob. I had expected that the extension shaft was loose, but when I opened it up to have a look it turns out that the play is in the pot itself. So I ordered a replacement from QService, and it just arrived. The pot from QService is a perfect match for the one in my 2236, and appears to be NOS, never used (the pins show no sign of ever having known the touch of solder), but when I turn the shaft there is some play in it (about 45 degrees).

I've also noticed that the play in the shaft still results in some change of the pot value, you get a small amount of adjustment over that 45 degrees of motion before you it the higher "stiffness" rotation that results in a much greater amount of adjustment. I've checked the operators manual and there is no mention of the find-vs-coarse feature in the (very short) discussion of the delta time position control. Is this how the delta time knob on the 2236 is supposed to work?

-- Jeff Dutky


 

On Wed, Dec 23, 2020 at 08:36 PM, Jeff Dutky wrote:


I've also noticed that the play in the shaft still results in some change of
the pot value, you get a small amount of adjustment over that 45 degrees of
motion before you it the higher "stiffness" rotation that results in a much
greater amount of adjustment. I've checked the operators manual and there is
no mention of the find-vs-coarse feature in the (very short) discussion of the
delta time position control. Is this how the delta time knob on the 2236 is
supposed to work?
Tek (and others) used pots with that behavior in some instruments, though usually over more than 45 degrees, I'd say about 90. I'm not sure if they did in some 2236's. Not in my 2236 at least. Looking at the pot's body, you may be able to distinguish to mechanisms in tandem.

Raymond


Ed Breya
 

I think the coarse/fine action is normal, and built into the pot.This kind of action is usually provided by two pots ganged together, with a slip mechanism between, to disengage the coarse pot. The schematic may show them explicitly, or not, depending on how the pot is built and pinned out.

You can see if it seems to work and is beneficial by using it in a setup. Rotate it to overshoot the desired spot a little, then reverse direction, and it should be much easier to set.When you reach the other limit, the coarse pot will kick in again.

Ed


Roger Evans
 

In the short wave radio receivers of my youth there was a common two speed tuning mechanism that used an epicyclic ball drive for maybe 6:1 reduction but there was also a peg and slot arrangement that forced the inner and outer to lock together after about 3/4 turn. The locked setting had a lot more friction.

I daresay there are other ingenious mechanisms for the same effect.

Roger


 

On Wed, Dec 23, 2020 at 09:19 PM, Ed Breya wrote:


I think the coarse/fine action is normal, and built into the pot.
My 2236 (S/N 200731) doesn't have such a pot.

Raymond


 

Raymond,

Okay, I broke out the multimeter and I can see what's happening, but I'm not sure I understand how to use this:

The forward block (10K) changes value during both the "loose" (45 degree) and the "stiff" (remaining 300 degree) spans of motion. The stiff range can take the pot through its entire 10K range, but the loose range only covers about 2K change in resistance.

The rearward block (2K) only changes value during the "stiff" span (the other 300-odd degrees).

So this was someone's idea of how to give both a coarse and fine adjustment with only a single knob? And they didn't think to mention this in the operators manual?

I'd be interested to know the part number for the delta time position pot in your 2236, so I could order one that doesn't have this "feature". My pot is part #311-1638-01. Is yours a 2236A by any chance?

-- Jeff Dutky


 

Ed,

Yes, I tried using in the delta time mode, and it works as you describe. It takes a little getting used to, and it has no effect in other modes (which use the second block of the POT, I guess). I see how it is useful, but it's a bit distracting. I would have preferred a second knob for fine control, like they did with the stacked intensity knobs.

-- Jeff Dutky


 

On Wed, Dec 23, 2020 at 10:10 PM, Jeff Dutky wrote:


Okay, I broke out the multimeter and I can see what's happening, but I'm not
sure I understand how to use this:

The forward block (10K) changes value during both the "loose" (45 degree) and
the "stiff" (remaining 300 degree) spans of motion. The stiff range can take
the pot through its entire 10K range, but the loose range only covers about 2K
change in resistance.

The rearward block (2K) only changes value during the "stiff" span (the other
300-odd degrees).

So this was someone's idea of how to give both a coarse and fine adjustment
with only a single knob? And they didn't think to mention this in the
operators manual?

I'd be interested to know the part number for the delta time position pot in
your 2236, so I could order one that doesn't have this "feature". My pot is
part #311-1638-01. Is yours a 2236A by any chance?
Jeff,
According to my SM (070-4204-00), the pot is A10R1617.
In S/N B010100 - B010999, it's a 311-2176-00, 2k2
In S/N B011000 - B014069, it's a 311-1625-01, 1 k
In S/N B014070 - B015819, it's a 311-1638-00, 2k front / 10 k rear
In S/N B015820 and up, it's a 311-1638-01, 2k rear, 10 k front

So there you have it!

Raymond


Ed Breya
 

Jeff, if it works as is, I'd recommend leaving it alone - you can get used to it. I don't know which of two controls is in question here - the one at the top-center, or bottom-right. The B-delay in scopes was traditionally a ten-turn pot, with a readout dial. As I recall, it still is in 2200 models, except for a knob instead of the dial, to save cost (the dial can be more expensive than the pot). If you're talking about the upper one, then it's that skinny trimmer-like one, on the middle of the top board, driven by the long extension shaft. I don't know if it's supposed to be a multi-turn type or not, but your description seems to indicate it's a single turn. If you really want to avoid using a lash pot, you may be able to find a ten-turn mini-sized pot of the same style and fit, and have good resolution (but slower to reach the spot). If the pot in Raymond's 2236 is 10-turn (or even 3 or 5), then that may be the way to go. If you can't get something with more resolution, the lash pot is a decent compromise. If you use the function a lot, you may find it frustrating to set the value right, without higher resolution or coarse/fine control.

If the one in question is the B-delay, then it's easy to replace with a standard ten-turn helipot, and you can even add a dial, if needed.

Ed


 

On Wed, Dec 23, 2020 at 11:11 PM, Ed Breya wrote:


Jeff, if it works as is, I'd recommend leaving it alone - you can get used to
it. I don't know which of two controls is in question here - the one at the
top-center, or bottom-right. The B-delay in scopes was traditionally a
ten-turn pot, with a readout dial.
Ed,
Jeff is talking about the delta-time pot, not the B-delayed pot.
It's on dwg 10 "Counter Front End" in SM 070-4204-00, location K2. A single pot may be used but R1618 / R1619 may have to be changed.

Raymond


 

Raymond,

Exactly right about which control I'm talking about, but I'm a bit confused about the service manual. I have the same PN for the service manual that you indicate, but I can't find R1618/1619, and the delta time position pot is labeled R1617, and indicates that it is only a single 2.2K pot. It sounds like we have the service manuals for each other's scopes (mine is a much later SN than yours, B027035). My SM appears to be the first printing from MAR 83.

-- Jeff Dutky


 

On Thu, Dec 24, 2020 at 12:09 AM, Jeff Dutky wrote:


Exactly right about which control I'm talking about, but I'm a bit confused
about the service manual. I have the same PN for the service manual that you
indicate, but I can't find R1618/1619, and the delta time position pot is
labeled R1617, and indicates that it is only a single 2.2K pot. It sounds like
we have the service manuals for each other's scopes (mine is a much later SN
than yours, B027035). My SM appears to be the first printing from MAR 83.
Jeff,
I don't know where you've got your manual from. I looked through my own PDF and 4 PDF's on TekWiki. They all show the double pot in dwg 10 (K2 area is difficult to read). R1618 and R1619 also are in dwg 10 just to the right of R1617 and both are listed in the parts list. Different copies may have been made from manually updated originals.

The setup for the pots is the standard way that's often used: The two pot's ends in parallel and the runners connected to a common point through separate resistors. The common point becomes the virtual "runner".

Your 2336 was made in Beaverton, USA, mine in the UK, as can be seen from the starting "2" in my SN. There is no straight relation between serial numbers in different regions. This has been brought up and complained about many times in this group. If non-USA production didn't start until a few years after (USA) introduction, regional "early" numbers would cross to US high numbers. Not really a way to tell though.

Raymond


Ed Breya
 

OK, so it is the little pot on top. Regardless of which manual and part numbers are right, one thing that I recall is that pot is very flimsy (mechanically) for its application. A heavy handed user could over-torque it and bend or break it off the board, unless it has some kind of friction release over-run protection. With the number of part changes over the product life, I'm guessing there may have been two problems - one with having inadequate setting resolution, and possibly another needing mechanical changes for pot durability.

Ed


 

Raymond,

I just bought one I saw on eBay that wasn't ridiculously priced (though, I suspect, it might have been the only one I saw, as I can't find any listings now). Maybe I should have been more careful, but I doubt that the eBay listing showed the full title page, so I couldn't have known what printing I was getting. TekWiki only seems to have the later printings. I should put a note in my printed copy that it doesn't match the unit.

I've seen the discussions of Tek SNs. It makes my head hurt. It did reveal, however, a half-memory about one of my dad's scopes (the 2213): what I recall is that he bought that scope essentially as soon as they were available, and the SN (B010299) confirms that memory. Not that I'm impressed by low model numbers, but any time I can have a pre-adolescent memory confirmed I'm happy.

-- Jeff Dutky