A Drop-In Replacement for the Tektronix U400 for 24XX 'Scopes

[Dr_Ram]

Dear folks,
Finally I managed to get round to completing my design for a replacement of the Unobtainable U400. The result is viewable at the following link:

https://youtu.be/EWU4GnehzLM

Would appreciate feedback and questions :-).
Cheers - Ram

[coromonadalix]

wow   kudos   

[Dr_Ram]

Thanks @coromonadalix and @edavid.

As of now this is not an open-source project, so I have not [yet] made the schematic available. I will probably do it at a later date.
Cheers - Ram

[jonpaul]

How tested and qualified over all 2465/7/A/B models?

Over all op modes and full signal/sweep ranges/

j

[Dr_Ram]

How tested and qualified over all 2465/7/A/B models?

Over all op modes and full signal/sweep ranges/

j

To the extent seen in the video, on my 2465A. The timeline describes the various tests done. I assumed, as I did with my U800 replacement project, that if it works in the 2465A, it works more or less as well in the others. I may be wrong.

[jonpaul]

Ram, bravo for the effort and initiative. Cost? Availability? Response in modern late model 400 MHz scopes?

Your Video shows poor transient response, ringing, overshoot. on the TEK PG506A CAL and leo Bodnar 40 pS pulser.
See TEK CAL procedure for transient response adjustments.

The common fault in these channel switches is poor linearity. Did you test vert linearity? Channel crosstalk? BW? Transient response?

If yes please send us data and scope shots (NOT a video)

Notice that TEK 2465/7, A, B are   different. A "drop in" needs more than a video taken on just one model ....
Every relevant spec and CAL needs to be checked.

The 2465/7 B are the preferred later models. Most TEK analog scope aficionados have these.

if you can send us samples of U400, U800 gratis , we could  test and evaluate in our six calibrated units, 2467, 2465B, 2467B.

Bon Chance,

Jon

[Dr_Ram]

Thank you @jonpaul, for the inputs!
I have not yet made the RU400 available for sale - trying to get a sense of how interesting it is to people before I make a batch. The RU800 has been available on ePray for a while, have sold about 20 pieces for $75ea + shipping so far, to people who have 2465s, 2467s, and 2445s. They seem pleased enough.

Let me begin by tagging your sentence "The common fault in these channel switches is poor linearity." My project is definitely not meant to target the likes of experts like you who have found fault with the original chips!

The primary purpose of my replacements is not even to provide the same level of accuracy, performance or precision as the original. I don't even have a EE degree to attempt that . It's just to allow one to keep their original away safely for a rainy day and use this "most of the time" which purpose it seems to serve. The only alternative at this point seems to be to shell out $$ on ePray for another chip and hope it lives...
I have received several offers to evaluate my RU800 but I have not had it professionally evaluated, and don't plan to for the above reason. The phrase "drop-in" is meant to indicate only that no modifications are required to the scope to be able to use it, and that existing adjustments such as horizontal gain/position etc work as they do with the original chip. My apologies if it conveys anything over and above that. My ePray listing for the RU800 actually carries lots of disclaimers so that experts like you don't place too many expectations on it  .

Yes I agree there is less than perfect transient response on my scope. My comparison was a relative one - "on my scope, do I get more or less the same performance as with the original U400, all other things being the same?"
No I did not check most of the stated parameters explicitly. Take for example, vertical linearity. Two high-speed current-feedback opamps (OPA2694 and THS3202) handle the entire signal path, so I don't really have reason to believe I would get terrible vertical linearity from them unless they near the output or input swing limits which they do not. Channel crosstalk will certainly be limited by the performance of the switches (DG611E) and of course by the PCB itself. BW as the video shows is decent upto 300MHz as tested.

Best - Ram

[jonpaul]

Dr Ram: To clarify:

1. Linearity is speced at 1...2% ie a 1 div signal should be very close to 1 div regardless or position on screen as POS control moves trace from top to bottom.

2. Typical U400 failure is nonlinear odd shaped waves and compression of trace at top or bottom of screen.

3. Indicates blown U400

4. Test yours like that very easy.

5. Only the early 2465 have 350 MHz 2465/7B are 400 MHz

6. Transient response is critical and looks bad on your Leo B 40 pS shot.

7. Video is not a good indicator, post stills of the various tests.

Look at your  PCB, my impression is a nice effort but the use of off the shelf opamps will NOT reproduce the GBW of the original TEK made parts. The lack of a heatsink means the opamps dissipation is low .



Just the ramblings of an  old retired EE

Jon

[Dr_Ram]

Thank you Jon, all inputs acknowledged with gratitude  . Will keep these in mind as I attempt further refinement.
Cheers - Ram

[jonpaul]

Ram:

Examples of CALed and well behaved fast  transient response
 Leo Bodanr 40 ps  >>50 Ohm >>scope

1/  Yokogawa DL7440 digital 500 MHz 2 GHz sample  scope
https://cdn.tmi.yokogawa.com/IM701450-01E.pdf


2/ TEK 7104 + 7A29 1 GHz scope after transient response allignment
https://w140.com/tekwiki/index.php/7104

Compare to your transient resp of your U400 clone with same leo Bodanr 40 ps gen. Notice underdamped ringging and overshoot.

Enjoy,

Jon

[Kleinstein]

Much of the ringing was there with the original U400 already. This ringing makes it a bit hard to see the difference with the replacement circuit.

[jonpaul]

Dear Kleinstein!

After    many 2465/7B repairs and CALs, including the transient adj. ......

Never used a substitute U400, only replaced with a Qservice pulled part,  original TEK or swapped from other 2465/7B scopes.

I adjustd the transient response and BW, flatness within or well beyond the TEK spec according to the TEK CAL

Tweaking TEK scopes  transient resp is more of an art or learned technique,  rather than a cut and dry procedure

If ringing is seen at the U400, it  may be an artifact of the probing technique or probes used.

Enjoy!


Jon

[Dr_Ram]

Much of the ringing was there with the original U400 already. This ringing makes it a bit hard to see the difference with the replacement circuit.

Which actually tempts me to make a stupid statement I was hesitating to make. But hey, I don't have an EE degree so laugh or groan away!

Begin Stupid Statement
If the square wave has a high frequency input transient, the U400 will reproduce that in a certain way and we want to see how well the RU400 reproduces that using the U400's reproduction as a reference. If there was no input transient, you would actually have less information to make a judgment of the RU400's HF response. So while the input transient is not a positive statement of the oscilloscope's calibration status per se, it is nevertheless useful to determine the RU400's performance. As I said above, "all other things remaining equal".
End Stupid Statement

Of course if there was an input transient that the RU400 smoothed out nicely I would not post this video here at all!

Cheers - Ram

[Kleinstein]

The ringing staying about equal confirms the performance at that one ringing frequency. However it also hides some other effects. If one looks careful there is something different with the replacement board, a bit like an extra step before reaching the final value. The ringing part makes it a bit hard to see.

[jonpaul]

Ram (Doctor?)    square wave has a high frequency .. U400 ..in a certain way .. how well the RU400 reproduces ...If there was no input transient, you would actually have less information ... input transient is not a positive statement of the oscilloscope's calibration status per se, it is nevertheless useful to determine the RU400's performance."

Klienstein: "the ringing staying about equal confirms the performance at that one ringing frequency."

I try to clear some  misimpression re scope  transient response.

A few moments please:

0/ In our use of TEK scopes over decades since 1970s we often need excellent transient response for testing and development of high speed pulse transmission, cables and transformers.

With baud rates to 25..50 MHz, the fast rise transient test is very revealing. Read signal processing theory texts for the frequency content of a fast rise pulse edge.

Taub Principles of Communication Systems 
 Millman and Taub:   Pulse and Digital Switching Waveforms ...1965

See TEK Circuit concepts books on Vert Amp design and use. Many Chinese scopes cheat on specs and ignore transient resp. IT IS VERY IMPORTANT!

1/ Ram, "acid"   test of a "drop in replacement" (form/fit/function/every spec) U400:
Does the scope pass a FULL CALL with TEK recommended procedures and equipment especially for BW and transient resp, linearity?

2/ lost the 2465/7/B calibrated transient scope shots,  posted the 7104 . The2465/7/B  result was similar to the 7104 except slower risetime.

  a few % overshoot and specified risetime as per TEK specs and TEK CAL procedure.

Your RAM U400 rings like a bell.

3/ RAM and Klienstein: I never looked at the U400 pins as they were not at issue. I did swap out one defective. No doubt sloppy probing will induce ground/earth return lead inductance and ringing into the robed signas.

Note the very tight Zo control and use of cascode/diff amps in most TEK vert amps.

4/ The results we measure on are NOT at U400 but on the scope CRT where it counts.

Bon Chance,

Jon

[MathWizard]

Cool project, looks fine for plenty of stuff.

What's the future of all these old scopes ? They weigh a ton, and more and more parts will need solutions like your RU400.

In 2023, what advantages do these old , but quality, scopes still have over modern ones ? I still like them, I'm just wondering tho.

[jonpaul]

MathWhizard BRAVO for your question:

"What's the future of ..old scopes ? .. weigh a ton, and more and more parts will need solutions ..In 2023, what advantages do these old , but quality, scopes still have ..

There are several reasons for usibg and restoring old (analog) scopes

1. Nostalgia..I used TEK 500 valve/tube scopes in 1968..1970 and TEL portables 465/475/485 in 1970s..1980s.

2. Excellence of design: TEK 2465/7/A/B are still working fine despite their age. Easy to maintain, lots of spares and parts donors available, and the original service docs.

One can only marvel at the brilliant electronic CRT and mechanical desing.

Read the TEK Circuits concepts books.

3. Superior to modern digitals in some appliactiohs.

4. Easy to use, simple intuitive user controls.

5. Learning  by doing many  CAL and repairs,  satisfacton to find and fix bugs.

6. No SW/FW updates to brick it, nor impossible to fix digital stuff like the modern Chinese

7. Cheap! Have boght Hameg German made 10/20 MHz, 1/2 ch 1980s analog scopes for EU 7/25/40. Still going strong, easy to service.
Our 2465/7/B were $150..350 ea.

Many more ..can send some pix if anyone is  interested,

HAVE AN ABSOLUTELY FANTASTIC DAY!


Jon

[Dr_Ram]

Ram (Doctor?)   

1/ Ram, "acid"   test of a "drop in replacement" (form/fit/function/every spec) U400:
Does the scope pass a FULL CALL with TEK recommended procedures and equipment especially for BW and transient resp, linearity?

Your RAM U400 rings like a bell.

4/ The results we measure on are NOT at U400 but on the scope CRT where it counts.

(Dr?) In mathematics,  not in medicine or EE

1. --> Agree

"Your RAM U400 rings like a bell. "
--> Poor conclusion. The scope has a transient overshoot, yes, you cannot conclude from the CRT that the RU400 does, simply because...

4. --> ...the results I measure are not meant to evaluate the scope, they are meant to evaluate the RU400.

Clearly @jonpaul, you don't like what you see, that's fine. You are the only one out of about 30 so far, Tek Groups included, who have commented negatively on my prototype. Of course all negative feedback counts, but not the illogical ones. I take it in good spirit, but I will advise you exercise more logic and less emotion in making evaluations. That way you will also realize as a corollary that the quality of a pulled U400 does not depend on who pulled it, Qservice or otherwise

I gracefully withdraw from this discussion, again with gratitude.

Cheers - Ram

[jonpaul]

Ram no offense intended. My commnets are meant to be informative and to assist less experienced EEVbloggers.

Your efforts are fine, BRAVO for the initiative.

Still needs some work and improvement to be the titled "drop in replacement'

See TEK service manual for your model and serial: Use  TEK CAL 06 procedure for transient response.

 Does it CAL OK?

Suggest to post same on tekscopes

https://groups.io/g/TekScopes/topics

 and

tekscopes2 https://groups.io/g/TekScopes2/topics

on groups.io, many experienced TEK repair guys and ex TEK techs and engineers....

have an ABSOLUTELY FANTASTIC DAY!

Cheers!


Jon