Author Topic: Tektronix 2465A CT lessons and Leroy's Mystery  (Read 320 times)

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Offline bnordlund

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Tektronix 2465A CT lessons and Leroy's Mystery
« on: February 27, 2021, 07:42:33 pm »
I just resurrected a 2465A CT.  I first want to express my appreciation for the threads here that were such a great help (alpher's "screwed the cal data" thread comes to mind).  I thought I would summarize my experience and list some important lessons.  As has been noted elsewhere, the 2465A seems to be underrepresented in the boards, and the "CT" version is even less common.

The "A" version, at least for me, is perfect for keeping it working.  The configuration memory uses a separate backup battery, so there is a reasonable chance of retaining the calibration when changing the battery.  It also uses mostly through-hole components.  Possibly most important, it uses  the "pre-Maxim" horizontal hybrid IC, which is said to be far more reliable than the later Maxim version.

Originally, it was not functional.  However, there was noise on the CRT.  I was happy to find that the 2465A uses the stand-alone memory battery, instead of the Dallas nightmares found in the later versions (and so many other models I have dealt with).  The battery voltage measured 3.7V, so I left it alone, knowing it will eventually need to be replaced, but not for now.

Testing the power supply rails on J119, I found that the -5V and -8V were dead, so I dove into the power supply.  I tested several bad electrolytics.  Because of the work involved in extracting the power supply from the case, I decided to replace all electrolytics, the safety capacitors, and the large poly Sprague.  After the reassembly, it powered up and appeared to be fully functional.  However, the CRT seemed to be a bit out of focus.  My rush to clean up the display led to some careless mistakes.

Early 2465A's (before serial number B014330), DO NOT have a CAL 08 (CRT Cal) menu item.  Instead, the CRT is adjusted the old fashioned way, without any help from the control board.  The procedure for those adjustments is at the very end of section 5 of the service manual, nowhere near the CRT Cal section for other variants.

If one (me) blindly hunts around in the menus on the 2465A CT, for the CRT (CAL08) procedure, they might see an entry called "CT CAL 81".  It is important to understand that this is NOT a misprint of "CRT" just missing the "R"!  By going into that item, the scope expects you to calibrate the timer settings for the high resolution "Counter Timer Trigger" option (so, I guess it is "CTT", but missing one "T").

If one finds themselves in a menu they don't want to be in, it is important to understand how to navigate with the front panel keys.  It is even more important to not panic and shut off the power (yep, that was me).  Worse yet, don't go back in to another calibration routine (CAL02, vertical cal) and do the same thing again.

After panicking, there was a "Fail 04 1x" error, and a "Fail CT CAL 81" error.  These are indicators of a bad checksum in the battery-backed RAM configuration data, and also out-of-range configuration values.  In many cases, these are interpreted as being an indication of a weak memory battery, but the battery still tested at 3.7V.  My hope was, if I could successfully complete the two aborted calibration routines, I might get it back on its feet.  My hope was that "Fail 04" could be fixed with a successful CAL02, and that the "Fail CT CAL 81" could be fixed by a successful "CT CAL 81", without having to perform a full calibration.

Lesson #1: The following Trigger buttons are how you navigate in the calibration routines:
    1. "A/B-Menu" is like an "escape", used to exit the diagnostics / calibration routines
    2. "Mode (Up)" and "Mode (Down)" step you through the menu items
    3. "Coupling (Up)" is like an "enter" key, and causes the selected item to execute
    4. "Coupling (Down)" is like a "backspace", used to back up and redo a step (if the step failed)

Lesson #2: The service manual is nice to have, but is missing some details that are important to know.  I found the most help by searching for information here on the EEVblog forum and on the TekScopes group.  Additional help was found on the wb0smx.net web page. The wb0smx page is specific to the 2465A, but not the "CT", so it covers the main calibration routines, but not the "CT CAL 81" process.  His observations and pictures are very helpful for verifying what the screen should look like before you proceed with each step.

Lesson #3: If anyone has a working 2465 family scope, there is a simple way to "back up" the configuration data, found in the battery-backed RAM.  Using diagnostic routine EXER 02, you can step through the 111 calibration entries.  If take a video with your cell phone, you can always step through the video if you need to.  After my experience, I would recommend that this be done on any working scope, before the cover is removed for any work, and to have on hand in case the memory battery suddenly goes dead, or if anything else goes wrong.  There is not a clear-cut way to restore that data, but at least having that data might lead to a way to restore operation if other methods fail.

I captured the RAM contents while it was corrupted, then again after I completed the two calibration routines.  There are three blocks of data that had changed, in addition to the last entry (at 6E: possibly the checksum entry?).

I have read that others have not been able to clear even the Fail 04 1x error without completing ALL calibration routines, in order.  I can say that this was not true in my case.  I only did the two calibration routines, and both of my errors were cleared.  Therefore, a partial calibration can not only be saved, but can clear the errors.  Since Fail 04 errors could possibly reflect a checksum error from any calibration data, the fact that I actually knew what data I had screwed up may have helped.  I guess a point would be that "Fail 04" does not mean that CAL04 is invalid, it apparently means that any data could be corrupted.  If one is dealing with a bad backup battery, causing much or all of the data to be bad, that may explain why a full calibration might be necessary much of the time.

Lesson #4: This scope seems to be more forgiving in the calibration requirements than the later TDS models I have tried to "cowboy calibrate".  Even with all the wrong calibration equipment, I was able to get it to accept the signals.  Some steps did fail, by displaying "Level" instead of proceeding to the next step.  However, this is where pressing "Coupling (Down)" will back you up and let you "retry" the step.  This can be done over and over, until it finally accepts it.  I experienced this on the 10V step for the CAL 02 routine.  To get the 10V level, I had to use a second (analog) signal generator.  Although I could verify and adjust for the 10V level, it may have not been stable enough to succeed.  However, after three or four tries, it accepted it!

This scope, now that it is working, is shaping up to be my favorite.  It is new enough that it has great analog performance and very sophisticated triggering capability.  However, it seems just old enough, that even I should be able to keep it running for years to come.  My TDS digital models always scare me that they are going to suddenly lose calibration and be unrecoverable.  I feel like I can use the 2465A as a "daily driver" and enjoy it.

Since this is the "CT" model, it has high-resolution time interval measurement, plus GPIB (woo-hoo!).  It may not be state of the art, but it still impresses me.  I assume I am not the only person here that remembers how expensive this kind of equipment was in its day, and appreciate the capabilities that a $20 purchase can bring today!

The "CT" also supports the P6407 "Word Recognizer Probe".  To satisfy my curiosity, I had to pick one up to try, and low and behold, it works!  For those who are not familiar with this feature, the Word Recognizer is a simplified logic analyzer trigger probe.  It has 16 TTL inputs, an additional "Qualifier" bit, plus a Clock input.  It allows digital events to be used as a sweep trigger, arm an analog trigger, reference for a time interval measurement, etc.

I know I have dragged on too long, but I do have one more item that might be of interest.  It has a Tektronix Property sticker, so at some point it was an internal asset.  Once I got it up and running, I discovered that the Setup Memory is full, with 30 saved settings.  What intrigues me is the Settings saved in location #1 is named "LEROY G", and position #5 is named "ROY".  Other set names are more functional, such as "TIMEDLY" and "FUJITSU".  This makes me very curious to know if there was a Tek engineer, named Leroy (Roy) G, who may have worked on Fujitsu gear.  Does anyone here know Leroy?  Sorry, but he can't have his scope back!

Brian
« Last Edit: March 01, 2021, 12:03:57 am by bnordlund »
Beaverton, Oregon
 
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Offline LazyJack

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Re: Tektronix 2465A CT lessons and Leroy's Breadcrumbs
« Reply #1 on: February 27, 2021, 08:31:45 pm »
I'm pretty sure someone at the Tek group can tell you who Roy was. https://groups.io/g/TekScopes
 

Offline bnordlund

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Re: Tektronix 2465A CT lessons and Leroy's Breadcrumbs
« Reply #2 on: February 27, 2021, 11:51:11 pm »
I had tried to post the question there, several days ago, and it has yet to show up.  I don’t know if there is an extra long review process right now, but I will keep my eyes open for it, and hopefully an answer.

Thanks!
Brian
Beaverton, Oregon
 

Offline bnordlund

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Re: Tektronix 2465A CT lessons and Leroy's Breadcrumbs
« Reply #3 on: March 01, 2021, 12:03:19 am »
I just saw my post appear on TekScopes (suspiciously much like this one).  I apologise for the duplication, but, after three days of radio silence, I had given up on it being posted over there.  In the end, maybe it won't be such a bad thing to involve a broader audience, as long as those who search both groups don't get irritated.

Thanks
Brian.
Beaverton, Oregon
 

Offline LazyJack

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Re: Tektronix 2465A CT lessons and Leroy's Mystery
« Reply #4 on: Yesterday at 05:40:54 pm »
If you have just joined recently then I think that group has some initial period of moderation for new members. That could explain it.
 

Online MarkL

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Re: Tektronix 2465A CT lessons and Leroy's Mystery
« Reply #5 on: Yesterday at 06:12:24 pm »
...
Since this is the "CT" model, it has high-resolution time interval measurement, plus GPIB (woo-hoo!).
...
One nice thing you can do with the "woo-hoo" GPIB interface is backup, and restore if needed, your calibration constants:

  https://www.eevblog.com/forum/testgear/tektronix-2465b-oscilloscope-teardown/msg1904177/#msg1904177

Grab all 256 calibration words and you're done.  Beats the video method, IMO.

I have a 2465 with the CTT and the word recognizer.  The CTT is impressive in its capabilities and very well integrated into the scope's regular operation.  I wish all my scopes, even newer MSO/DSO models, had the same capability.
 

Offline bnordlund

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Re: Tektronix 2465A CT lessons and Leroy's Mystery
« Reply #6 on: Today at 06:58:25 am »
Thanks for pointing me to the discussion on backing up and restoring via GPIB.  That is a some great information!  This is my tenth GPIB instrument and I am now waiting for one more cable so they can all be controlled at once.  If it was 30 years ago, I would be king of the world, or at least my basement!

I was just playing with the CTT delta measurement and am also very impressed. I learned how to do basic timing measurements, long ago, using the DMM on my 465, and the capabilities of the 2465A CT are such a big step up, I am just thrilled with this thing.

Thanks again,
Brian.
Beaverton, Oregon
 


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