Tektronix 2246 Time Base Linearity Issue - Page 1
Forum section: electronics / repair

author: W3KW
author_rank: Contributor
timestamp: October 16, 2022, 12:46:07 pm
content: I bought a gorgeous 2246 and she arrived extremely professionally packed. What a relief when that happens. I bought a Sencore PR570 that is now a parallelogram….not so professionally packed. Anyway, I fired up the 2246 and every switch and light performed perfectly. The face looks brand new. The trace is bright and all the measurement functions seem spot on. Here is the weird part. The horizontal axis doesn’t display in a linear fashion. As the internal calibration signal (square wave) travels across the screen, the display is slightly compressed more and more with each cycle. The first cycle is spot on the reticle. By the end of the screen, they are 2 pips off. I used the built-in time measurement and each full cycle measures exactly spot on and each the same. I can use the VAR pot on the time/div selector to show more cycles on the screen, and when turning that off, it seems I just need a bit more rotation of that knob to get the display to match the divisions perfectly.I have the service manual, which is obviously excellent, but it doesn’t show pictures of the problem you are calibrating. Is this the correct calibration for my issue? (See attached)Thanks for the help.

author: Testtech
author_rank: Regular Contributor
timestamp: October 16, 2022, 10:26:22 pm
content: Can you post a picture of the screen?

author: W3KW
author_rank: Contributor
timestamp: October 17, 2022, 02:40:17 am
content: Yes. I’m traveling. I will asap.

author: W3KW
author_rank: Contributor
timestamp: October 17, 2022, 02:52:29 am
content: The attached photo is from the 2246 service manual but shows what I’m seeing. The test signal lines up perfectly on the left side of the screen, but as the signal moves across the screen, the signal appears to be compressed and leads each succeeding reticle. My 2465b does not do this. Nor does my 465 or 2235. Using the internal calibration signal, the other three scopes present an identical relationship on the time axis all the way across the screen. Again, the scope is new to me so I may not fully understand what I’m looking at. I’m anxious to get home to do more discovery and diving into the manual. The 2246 presents a heck of a lot of features for the money. Wow. Ironically, a new Siglent of the same price likely has 50 more functions, but there’s something so special about Tektronix! Gotta love them.

author: bdunham7
author_rank: Super Contributor
timestamp: October 17, 2022, 03:29:20 am
content: There's nothing terribly complicated going on there if that picture is accurate. Either your a calibrator frequency is slightly off (high) or your horizontal sweep rate is a bit slow. Typical CRO stuff, needs adjustment or calibration. Post an actual photo of your scope in action so we can confirm this. Do you have a reasonably accurate AWG and frequency counter?Edit: Just curious--if you already have a 2465B, why buy a 2246? Don't be shy, admitting you have a problem is the first step...

author: TimFox
author_rank: Super Contributor
timestamp: October 17, 2022, 03:37:00 am
content: When the signal leads the reticule more and more across the screen, that isn't linearity but a calibration error between the signal frequency and the time base rate (as mentioned above).It is possible for an analog CRO to have a linearity problem, where the time/division changes across the horizontal axis, which is a different effect.Do you have another generator or oscillator to compare against?

author: W3KW
author_rank: Contributor
timestamp: October 18, 2022, 05:14:19 pm
content: I’m suffering from G.A.S. I’m full of it. The real answer is I watched some videos of the 2246 in use and I lived the functionality. Plus, I acquire gear during the summer for winter restoration projects. 4 Tek scopes. That’s a crap load of Caps. Plus batteries.I’ll post pics of my screen shortly. I do have an Agilent SG and good counter also.Thanks for the help.Wes

author: David Hess
author_rank: Super Contributor
timestamp: October 18, 2022, 06:51:17 pm
content: I agree with the earlier comments; that is just a calibration issue. It is easily corrected if there is a problem.