Author Topic: Tektronix 2465B oscilloscope teardown  (Read 460134 times)

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Online MarkL

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Re: Tektronix 2465B oscilloscope teardown
« Reply #725 on: April 15, 2016, 03:26:28 pm »
OK, I figured out what's going on. The 2465 options service manual I have only covers options 6 and 9. I have a separate 2465 service manual for the DMM (option 1). Do you know of a source for the Option 10 manual?
The 2465 service manual is equally uninformative; (irr)relevant page attached.

I only have the GPIB Option 10 service manual in electronic form and I don't recall where I found it.  It was at least 11 or 12 years ago.  I will send you a PM to get it to you.
 

Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #726 on: April 16, 2016, 05:30:56 am »
SoundTech-LG -

Yes, I found GQ Support to be unbelievably helpful.   Of course I'm only a sample of one, which does not say much about a continued trend, but it is a good start.   If you wish, feel free to use my name and experience with them as a reference.

Regarding the AT89C51 listed in the GQ-4X4 library, the question that comes to mind is; are the electrical characteristics close enough to your IC's to work correctly.   GQ Support should be able to answer that.   If they are not compatible, it has been my observation that they probably/might be interested in adding it.   They sound like they are always on the lookout for additional IC's to add.

Just for the record, I'm a novice in the world of programming.   Learning as I go.   A neighbor friend has been helping me over the hard spots with the DS and FM IC's and explaining things as we go.   While I now feel pretty comfortable in reading or writing existing programs to IC's, I think there's so much more out there that I don't even know exists yet.

Let me know how things go with GQ. 
 

Offline RF_Pursuit

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Re: Tektronix 2465B oscilloscope teardown
« Reply #727 on: April 16, 2016, 06:24:58 pm »
Hi everybody!

On the road to change as many capacitors as possible in the power supply  PCB, I replaced most of the capacitors except C1016 and C1018 those are plastic (0.068uF X2 250VAC) can I replace each with a RIFA PME 271 M (220n X2 275VAC)?

I also have a some (0.1uF 275VAC) but not X2 can I use these to replace the 0.068uF X2? Which would be suitabel the 220n X2 or the 100n?

« Last Edit: April 16, 2016, 06:26:49 pm by RF_Pursuit »
 

Online med6753

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Re: Tektronix 2465B oscilloscope teardown
« Reply #728 on: April 16, 2016, 06:37:57 pm »
Hi everybody!

On the road to change as many capacitors as possible in the power supply  PCB, I replaced most of the capacitors except C1016 and C1018 those are plastic (0.068uF X2 250VAC) can I replace each with a RIFA PME 271 M (220n X2 275VAC)?

I also have a some (0.1uF 275VAC) but not X2 can I use these to replace the 0.068uF X2? Which would be suitabel the 220n X2 or the 100n?

See attached list.....which I give credit to the Yahoo Tek group. I used the recommended 0.1 uF 275VAC X2 for C1016 and C1018 and they are working fine.
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Offline RF_Pursuit

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Re: Tektronix 2465B oscilloscope teardown
« Reply #729 on: April 16, 2016, 07:42:58 pm »
Hi everybody!

On the road to change as many capacitors as possible in the power supply  PCB, I replaced most of the capacitors except C1016 and C1018 those are plastic (0.068uF X2 250VAC) can I replace each with a RIFA PME 271 M (220n X2 275VAC)?

I also have a some (0.1uF 275VAC) but not X2 can I use these to replace the 0.068uF X2? Which would be suitabel the 220n X2 or the 100n?


See attached list.....which I give credit to the Yahoo Tek group. I used the recommended 0.1 uF 275VAC X2 for C1016 and C1018 and they are working fine.

Thank you, I followed the that list in ordering the capacitors. Unfortunately, I couldn't get all of them as recommended.


And reading the tiny writing on the 0.1 capacitors I found the X2 within. So, that's done with.
 

Offline tautech

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Re: Tektronix 2465B oscilloscope teardown
« Reply #730 on: April 16, 2016, 07:47:42 pm »
Hi everybody!

On the road to change as many capacitors as possible in the power supply  PCB, I replaced most of the capacitors except C1016 and C1018 those are plastic (0.068uF X2 250VAC) can I replace each with a RIFA PME 271 M (220n X2 275VAC)?

I also have a some (0.1uF 275VAC) but not X2 can I use these to replace the 0.068uF X2? Which would be suitabel the 220n X2 or the 100n?
The absolute value is not as critical as the X2 and voltage ratings that must be maintained.
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Offline Micma

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Re: Tektronix 2465B oscilloscope teardown
« Reply #731 on: April 19, 2016, 07:09:29 am »
Hello everyone, I am new to this forum and wanted to thank you all for sharing valuable information!
A friend of mine gave me a 2465B does not work (he wanted to throw it!) and recently I bought another one in like-new condition!
Unfortunately I have to replace the NVRAM. Following the instructions that I found in this forum I have already ordered everything needed and  it should reach me in a few days.
 

Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #732 on: April 24, 2016, 03:12:47 am »
NOTE -

The Excel spreadsheets attached to Reply #703 back on 3-25-16, have been replaced.   The updated version has a couple of corrections, additions and an improved layout - easier to follow.  (4-23-2016)
 Problem:  Page 1 of the new Excel spreadsheets wants to come up in portrait rather than landscape format.   To fix:  After opening click on "Enable Editing" at the top of the page and the formatting will fix itself.   Sorry about that.   Unable to find the cause.   It opens fine on my computer, but opens strange in the forum.

These are the spreadsheets showing the sequence to access the internal tests and Calibration data stored in the scope's RAM.
« Last Edit: May 07, 2016, 03:19:41 pm by Old-E »
 

Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #733 on: May 07, 2016, 02:56:07 pm »
CHASING THE ILLUSIVE MILLIVOLT & NANOSECOND – PART I

My 2465B appeared to be working perfectly (even though the PS caps have yet to be replaced) until it was pressed toward its limits.   Then some anomalies turned up that I felt needed to be understood.
Started by looking at sine waves from my fastest generator which tops out at 264 MHz (Tek. SG 503).   Peak to Peak (P-P) voltage was measured by using the manually adjusted cursors, accessed by pressing delta V.   This indicated 177 mv P-P.

Then pressing the lower red “HELP” button, followed by the “CHAN 2” button, commands the scope to measure the voltage from internal sources.   This indicated 134.7 mv P-P (reads on screen “CH1: PK-PK 134.7 mv”).  This is 24% lower than the cursor set measurement above.

Ok, so which number is correct?   I trust my digital multimeter, but not at that frequency.   So using a 60 Hz generator, the test was repeated.   This time, the internal scope function measured 2.5% higher than the manually adjusted screen cursors, and 1.7% higher than the digital multimeter.

Returning to the high frequency generator, I compared the voltage readings at different frequencies.   The result was; the internal sourced voltage measurements compared more favorably to the cursor reading as the frequency was reduced from 264 MHz.

On a hunch, I used a Tek. PG 502 pulse generator to generate a flat topped pulse train.   The short version of the findings were that the internally sourced voltage measurement needs some dwell time at peak voltage to capture the P-P voltage number.   That dwell time was experimentally found to be about 12 ns.   Problem with a sine wave is that it has very little dwell time at its peak voltage.   Following data illustrates the accuracy vs. flat top dwell time, with 1.00 volts high pulses displayed on screen.
3 ns = .934 v P-P
5 ns = .964 v P-P
9 ns = .996 v P-P
10 ns = 1.001 v P-P
12 ns = 1.011 v P-P
100 ns = 1.010 v P-P


Next question is; what happens to the flat top time requirement when the repetition rate of the pulse train is varied?   For this test the top pulse width was set to 10 ns.   Then the rep rate was adjusted between 10 us and 10 ms.   But no significant difference was found.   So pulse repetition rate does not appear to affect the internal P-P voltage measurement.

Ok, so where does that inaccuracy begin with a sine wave?   Skipping to the bottom line, the following data was collected by using the SG 503, inputting 100 mv P-P at various frequencies, and then commanding the scope to measure the P-P voltage.
5 MHz = 99.9 mv
10 MHz = 99.6 mv
15 MHz = 99.1 mv
20 MHz = 98.7 mv
25 MHz = 97.2 mv
50 MHz = 89.7 mv
75 MHz = 85.8 mv
100 MHz = 83.1 mv

Conclusion: The scope can accurately measure P-P voltage of a flat topped pulse, providing the flat top is longer than 12 ns.   And, same for a sine wave that is <20 MHz, depending on the accuracy one is looking for.
« Last Edit: May 07, 2016, 03:15:15 pm by Old-E »
 

Online med6753

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Re: Tektronix 2465B oscilloscope teardown
« Reply #734 on: May 07, 2016, 10:22:50 pm »
Update from my last entry on April 15th.....


The DMM Option Board is fixed. The culprit was dual FET Q5070. I also had on hand Op-amp U5060 but it's OK. Luckily Op-amp U4970 is also OK because so far that part has been unobtanium. I calibrated the DC voltages with my home built calibrator with the exception of the 500VDC range. I don't want to risk blowing out that FET again. I don't foresee needing to accurately measuring voltages that high anyway. I also recalibrated the ohms. AC volts calibration is pending until I can figure out an accurate calibration constant source of 0.19V, 1.9V, 19.0V, 190.0V, and 500V.

Here's the schematic of the front end of the DMM Option.

And here's a sample measurement to my 10VDC standard.


The last repair pending is the Buffer Board checksum error.   
« Last Edit: July 18, 2017, 10:03:34 am by med6753 »
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Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #735 on: May 08, 2016, 04:13:05 am »
CHASING THE ILLUSIVE MILLIVOLT & NANOSECOND – PART II

While testing the scope’s limits, the next area of interest or anomalies is the frequency measurements.   This was accomplished by using a Tek. SG 503 sine wave generator at its highest frequency setting of 264 MHz.

Comparing the scope to itself, in the 1st photo, I carefully measured the cycle time at 3.82 ns using the manually adjusted screen cursors by pressing delta T.   This calculates out to a frequency of 262 MHz.   Then pressing the delta T & delta V simultaneously reads out the frequency, which also indicated 262 MHz.

In all of my tests, the numbers measured from the manually adjusted cursors were always exactly in agreement.   That includes the period & frequency, and the screen grid or graticule.

Then pressing the red lower “HELP” button, followed by the “CHAN 1” button, commands the scope to automatically measure the frequency from its internal sources (no connection to the cursors).   This indicated 266 MHz, which is 1.53% higher than the cursor measured number.   The frequency meter on the SG 503 generator indicated 264 MHz (only reads 3 places), which is in the middle of the numbers.
   
To determine if the input frequency has any effect on the accuracy,  the above measurements were repeated at various frequencies.   These included one point at 462.5625 MHz.   This came from my FRS Walkie Talkie.   I held its antenna near the scope probe while transmitting.   Can’t say anything about the amplitude, but this frequency is accurate for channel 1.   At this frequency, the 400 MHz scope and 300 MHz probe have rolled off quite a bit in amplitude at 462 MHz.   But, the scope manual says it will trigger up to 500 MHz, and so it can display 462 MHz as seen below.

Adjusting the cursors closer together will indicate a frequency up to 20 GHz (20,000 MHz), but of course the scope won’t actually display anything that high.   Need to find a higher frequency source to see how high it will go.

Letting the scope automatically measure the 462 MHz frequency via. the red HELP button, indicated 474.7 MHz, which is 2.3% high.

Additional lower frequencies were set up with the screen cursors, and then compared to the scope’s internal frequency measurement via. the red HELP button.   They were 100 MHz, 10MHz, 1 MHz, 50 KHz and 60 Hz.   These lower frequency readouts all agreed within the limits of being able to set the cursors which is a fraction of 1%.
 
Conclusion; All the lower automatic frequency measurements appear to agree well with the screen set cursors.   However, they read a few percent high at the higher frequencies.   And given that the cursor settings were right on for 60 Hz and 462.5625 (the 2 frequencies I knew for sure), and they mostly agreed with the lessor known accuracy of the frequency meter built into the SG 503 generator, I believe the cursors are accurate across the scope’s frequency spectrum.
« Last Edit: May 08, 2016, 04:16:16 am by Old-E »
 

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Re: Tektronix 2465B oscilloscope teardown
« Reply #736 on: May 16, 2016, 05:04:27 am »
CHASING THE ILLUSIVE MILLIVOLT & NANOSECOND – PART III

In search of more performance limits, I looked at RT (Rise time).   RT can be theoretically determined from the frequency.   The 2465B has a specified frequency response of 400 MHz (at the -3 DB roll off point).   Using the generally accepted formula of .35/Freq = RT, we have .35 divided by 400 MHz.   This = a rise time of .875 ns.

Should also point out that -3 DB in signal amplitude means the peak voltage at 400 MHz will read 29.3% less!   To maintain reasonable amplitude accuracy, of say -3%, the upper frequency limit would be about .30 x 400MHz, which brings the usable frequency (for amplitude accuracy) down to 120 MHz!   A good Tektronix article on this and the approximation of RT can be found at - http://www2.electron.frba.utn.edu.ar/~jcecconi/Bibliografia/06%20-%20Osciloscopios%20de%20Almacenamiento%20Digital/Understanding_Oscilloscope_BW_RiseT_And_Signal_Fidelity.pdf

Ideally, to measure RT accurately, one needs a pulse generator with a RT of roughly 10x the .875 ns RT of the scope, or 87.5 ns.   Unfortunately, the best I could do was to borrow a Tek SG503 pulse generator.   It’s specifications give a rise time of 1 ns or better, which places it roughly equal to the scope.

When one begins to measure RT, a number of other questions arise.   Tektronix specifies it to be measured between the 10% & 90% of amplitude.   That’s easy in a perfect world where pulses have square clean corners.    But there is the “pre-shoot and overshoot of the leading edge of the displayed pulse.   I wrestled for some time with how to deal with that in making the RT measurement.   In the article above, they illustrate the answer which I like because it simplifies the measurement.

In order to minimize the leading edge distortions, I found the best results by connecting a 40 inch piece of 50 ohm coax directly between the SG503, using its internal 50 ohm output option, and the internal 50 ohm input to the scope.   When using a 50 ohm terminator feed through into the 1 meg ohm port, or a 300 MHz (unfortunately) scope probe into the 1 meg ohm port, the displayed pulse was not as clean.

All this wordiness was needed to qualify the following measurements.

The 1st photo below shows the displayed pulse with the amplitude adjusted to sit on the reticle 0% dotted line and the stabilized top on the 100% dotted line.   The under & overshoots pass outside those limits.

In the 2nd photo, the same pulse is shown with the sweep speed at max using the “x10 MAG”.   Measuring the time between the 10% & 90% crossings on the reticle, the cursors indicate 1.55 ns.   But the 1st photo shows the leading edge of the pulse rounding over before it plateaus across the top.   This will lengthen the measured RT.   Following some thought and experimentation, I came to the conclusion that, this distortion is coming from the source rather than the scope.   If so, then eliminating it would be fair.

The 3rd photo shows the elimination of the rounding and distortion across the top of the pulse.   This was done by reducing the pulse width to its minimum.   But this also shortens the height of the pulse.

The 4th photo shows where the pulse has been increased in height to again align with the 0% & 100% dotted lines.

In the 5th photo the sweep speed is increased to its max and the cursors are set on the 10% & 90% lines to indicate a rise time of 1.04 ns.

Letting the scope measure the rise time via the red “HELP” button, it indicates a rise time of .89 ns or .93 ns, depending on the repetition rate of the pulse generator.   A little discouraging, but again underscores the manually set cursors as being the most accurate.

Using the formula, .35 / 1.04 ns = 337 MHz.   But of course, that is the speed of the combined rise times including the scope, generator and cable.   But we can estimate the individual RT from the composite.   A published formula for this is - the square root of the sum of the individual squared rise times.

In this case – if we assume the cable is perfect for simplicity, it leaves us with 2 unknowns, the RT of the scope and the RT of the generator.   Since the scope RT is spec’d at .875 ns, and the generator is spec’d at equal to or better than 1 ns, we can further simplify by assuming they are both the same.   Using the formula above, then gives us an individual RT of .73 ns.   And since .73 ns is a safe margin below the spec’s of both instruments, I think it’s safe to say that the scope RT is equal to or better than .875 ns.   If we were to factor in something for the non-perfect cable, we would have an even larger margin.

This is not as perfect as I would like, but lacking a better pulse generator, it’s the best I can do.
« Last Edit: May 17, 2016, 12:16:35 am by Old-E »
 

Offline tautech

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Re: Tektronix 2465B oscilloscope teardown
« Reply #737 on: May 16, 2016, 05:46:35 am »
When one begins to measure RT, a number of other questions arise.   Tektronix specifies it to be measured between the 10% & 90% of amplitude.   That’s easy in a perfect world where pulses have square clean corners.    But there is the “pre-shoot and overshoot of the leading edge of the displayed pulse.   I wrestled for some time with how to deal with that in making the RT measurement.   In the article above, they illustrate the answer which I like because it simplifies the measurement.
Tek are stating what is the common interpretation of rise time and what we most use.
However I believe one can overlook pre-shoots and overshoots and focus on measurement of the step function as mentioned in this Wiki article:

https://en.wikipedia.org/wiki/Rise_time

In electronics, when describing a voltage or current step function, rise time is the time taken by a signal to change from a specified low value to a specified high value.[1] These values may be expressed as ratios[2] or, equivalently, as percentages[3] respect to a given reference value. In analog or digital electronics, these percentages are commonly the 10% and 90% (or equivalently 0.1 and 0.9) of the output step height:

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Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #738 on: May 17, 2016, 01:16:43 am »
tautech -

Yes, the link to the Tek article in my previous post clearly illustrates that the pre & overshoot on the leading edge of a pulse is to be ignored when making RT measurements.   Looking back, I neglected to actually state that.   That small difference can make a significant difference in the measurement outcome.

Getting into the sub-nanosecond arena, things starts getting rather spooky.   Things that have little or no effect at the lower frequencies, now can totally upset things.   And to make accurate measurements or properly functioning designs, those fine "spooky" points need to be addressed.

Thanks
 
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Re: Tektronix 2465B oscilloscope teardown
« Reply #739 on: May 20, 2016, 08:31:13 pm »
For information on the EAROMs in the 2445,2465, I opened  a thread  which may be of interest to the "A" and "B" guys....just search for EAROM
 

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Re: Tektronix 2465B oscilloscope teardown
« Reply #740 on: May 20, 2016, 09:12:09 pm »
For information on the EAROMs in the 2445,2465, I opened  a thread  which may be of interest to the "A" and "B" guys....just search for EAROM
Editing your above post and inserting a link would be better.  ;)
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Online med6753

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Re: Tektronix 2465B oscilloscope teardown
« Reply #741 on: May 21, 2016, 06:13:11 pm »
For information on the EAROMs in the 2445,2465, I opened  a thread  which may be of interest to the "A" and "B" guys....just search for EAROM
Editing your above post and inserting a link would be better.  ;)

Here's the link....

https://www.eevblog.com/forum/testgear/tektronix-2445-2465-cal-settings-earom-er1400/

I'm very interested in how this turns out.


More steady progress on getting everything on my 2465 DMS fully functional and calibrated. Since my last update on May 8th I've starting tackling getting the AC volts on the DMM calibrated. The “calibration constants” to calibrate the AC volts are:

0.19V @ 60Hz
1.90V @ 60Hz
19.0V @ 60HZ
19.0V @ 20KHz
190.0V @ 60Hz
500.0V @ 60Hz
(All RMS)

I used a function generator with a pot as a voltage divider to get the 0.19V and 1.90V constant. But the maximum output of the function generator is 20.0V p-p so it can't do the 19.0V. I'm in the early stages at looking at possible designs to boost the generator output to about 60V p-p and above. If someone has some ideas I'd like to hear them. I'm not going to try the 190.0V and 500.0V constants because those are insane p-p voltages.

Here's the DMM measuring a 1.0VAC Sine with the Fluke 87. Not bad but may require further tweaking. The dots on the screen are the DMM logic warning of a potential out of calibration condition. The dots do not appear when measuring DC volts or ohms.


Measuring mains voltage with the Fluke 87. Clearly needs more work. I think if I can get the 19.0V range calibrated it will be much better.


Follow on: I'm going to build a temperature probe since the DMM has temperature capability. And awaiting the results of the EAROM experiment so I can fix the Buffer Board check sum error.
« Last Edit: July 18, 2017, 09:55:34 am by med6753 »
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Online MarkL

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Re: Tektronix 2465B oscilloscope teardown
« Reply #742 on: May 21, 2016, 08:00:13 pm »
...
I used a function generator with a pot as a voltage divider to get the 0.19V and 1.90V constant. But the maximum output of the function generator is 20.0V p-p so it can't do the 19.0V. I'm in the early stages at looking at possible designs to boost the generator output to about 60V p-p and above. If someone has some ideas I'd like to hear them. I'm not going to try the 190.0V and 500.0V constants because those are insane p-p voltages.
...
Ok, here are some ideas...

You could drive a power transformer backwards with the function generator to step up the voltage.  You don't need any current to speak of for the DMM input, so 60V should be fairly easy to hit.  At 20kHz, a power transformer may not work well, but you could try a small audio transformer (like the kind in old transistor radio).

If your function generator does not have enough output power to reach the desired output voltages, you could add an audio amplifier to drive the transformer.  And if it's an amplifier with a high enough output, you might be able to dispense with the transformer altogether.

If using a transformer, use the scope input to make sure the sine wave is still clean before relying on it for RMS calibration.
 

Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #743 on: May 22, 2016, 04:27:14 am »
MarkL reflected some of my thoughts on generating higher voltages.   Another thought might be to feed 120 v ac into a variac for voltage control, then feed that output backwards through a 24 volt doorbell transformer.   That would easily give you 500 volts.   I think the winding insulation is rated higher than that.   Cost on-line should be less than $10.

I recently used the same transformer to step up 120 v to 240 v by using the 120/240 volt primary as an auto transformer (120 into the center tap and one end - 240 out across both ends of the same primary).   The 24 v secondary leads were dead ended (not used).   Similar to your case, I was powering 2 digital panel meters that only draw milliamps.   $6.25 each on-line.

Question - How do you know if it's the scope or the Fluke (or both) that are in error?
 

Online med6753

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Re: Tektronix 2465B oscilloscope teardown
« Reply #744 on: May 22, 2016, 02:23:36 pm »
Guys, thanks for the responses. Here I was trying to think of what was turning out to be a complex solution and a simple transformer solution was staring at me the whole time. Unfortunately this project is going to have to go on the shelf for a few weeks because I'll be involved in other pressing activities. But I'll provide an update when I get a chance.


Question - How do you know if it's the scope or the Fluke (or both) that are in error?

A very legitimate question and I'll try to provide a satisfactory answer. It goes back to the 1980's when I built a voltage/resistance “standard” based upon an article in an electronics magazine. It provides 10.00VDC, 1.000VDC, 100.0mVDC, 1.0VAC as well as some resistance “standards”. I used a borrowed and freshly calibrated Fluke 8050 to set it up. And documented the results. I also checked my then Fluke 77 and documented those results. I still have those documents to this day. And in the 30 years since building this unit I have never had the need to perform any further adjustments.

Around 1997 the Fluke 77 was damaged beyond repair by a leaking battery. So I purchased this current Fluke 87. I also documented how it compared to my “standard” and it was spot on. From about 1998 until very recently I was mostly inactive in this hobby and the equipment sat mostly unused. The 87 was rarely used and was never molested or damaged or overloaded. The only time it was opened was to replace the battery. So about 6 months ago when I got active again I compared the “standard” to the 87 to the 1997 readings and they were the same. I highly doubt both of them would have drifted the same amount so I trust what the 87 is telling me. And I often check both to make sure nothing has changed.

So I used the 87 as a reference point when applying the “calibration constants” to the 2465 DMM. Those constants do have a little bit of “wiggle” room but if it's too far out I get an “Out of limits” message on the CRT. Now will the 2465 DMM meet it's published specs? Of course not. But at least I know I'm close.

Now if I knew someone who had a Fluke 5101B and a Fluke 5205A I could have the entire 2465 DMM calibrated in about a half hour. But what's the fun in that? Redneck solutions are much more fun. “Hold my beer and watch this”.        ;D
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Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #745 on: May 22, 2016, 04:36:37 pm »
“Hold my beer and watch this”.

Wasn't that the statement most often heard just before a car crash in redneck territory?  :)

Sounds like you have the cal issue pretty well nailed down as I assumed you did.   Just curious as to what route got you there.   Good work!
 

Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #746 on: June 01, 2016, 05:50:50 pm »
CHASING THE ILLUSIVE MILLIVOLT & NANOSECOND – PART IV

Looking further into the 2465B’s abilities, the scope can also function as a time domain reflectometer.   For those unfamiliar with this, the scope’s speed is fast enough to measure the transit time of signals traveling through conductors.
 
For this test, I connected a BNC T to the scope 50 ohm input at channel 1.   A 50 ohm cable connected one port of the T, to the 50 ohm output of the SG503 pulse generator.   The addition of the T was found to have no measurable effect on signal level or rise time.   The 1st picture shows the shortest pulse width out of the generator measuring 1.71 ns at 50% of amplitude.   The height of the pulse is adjusted to extend from the 0-100% markings on the scope screen.

Then a 41 inch piece of 50 ohm cable was connected to the remaining port on the T, and the other end of the cable was left open (no connection).
 
The pulse coming from the generator is split at the T which sends 50% of the total pulse power down the open ended coax, and 50% of the pulse power into the scope.   This results in the voltage at the scope dropping to .707 of the original voltage.   If we were reading the current, it would indicate the same drop.   Then when multiplying the reduced voltage x the reduced current, the pulse power would = ½ of the total from the generator.   In the meantime, the pulse going down the open ended coax finds an open (very high impedance).   This mismatch results in reflecting the pulse back toward the T where it splits again, sending 1/2 back to the generator and 1/2 to the scope input, but at a later time.   It is this time difference and relative voltage levels that can used to measure conductor characteristics.

The 2nd picture shows the time delay of 9.0 ns between the primary pulse and the reflected one, and the reduced voltages from each split at the T.   Again, the coax is 41" long.

So how does that calculate out?   Light & electricity travel at 186,000 miles per second, or 11.8 inches per nanosecond.   Therefore; 9.0 ns x 11.8” = 106.2” of electrical length.   But the pulse only travels a round trip through the cable of 82”.   The difference is the velocity factor.   That is; the pulse travels slower through the cable, because of the dielectric materials making up the cable, then it does through free space.   So if we divide 82” by 106.2” we get a velocity factor of .772.   That is; the signal is traveling through the coax at roughly ¾ the speed it would in free space.   Now that I know that number, I can determine the physical length of another piece of coax off the same spool, or find the point of damage along an existing coax, etc.

The same test with a 62 inch piece of coax gave a reflected delay of 15.80 ns.   This times 11.8 inches gives a round trip electrical length of 188.4”, which results in a velocity factor of .665.   Even though they are both 50 ohm coax, they are made up of different dielectric materials resulting in different velocity factors.   The 41” coax is stamped RG58/U, and the 62” coax is stamped RG58C/U.

Same test again with a 13 ¼” length of coax resulted in a reflected delay of 3.4 NS.   This calculates out to a velocity factor of .661.

Looking closer at the peak voltage of the pulses, as mentioned above, the primary pulse should be reduced to .707 of the original when adding a 2nd cable at the T.   In a perfect world, that would reduce the pulse height on the scope screen from 5 divisions (cm) to 3.54.   But the scope is actually displaying slightly less at 3.40 cm.   This difference can be from any number of minor dimensional errors in the coax or connectors.

The voltage of the reflected pulse indicates 2 cm high when it should be 2.5 cm.   The primary pulse power is first split when entering the T, and then the reflected pulse is split again.   That’s a voltage reduction of .707 x .707 which = .500, or 2.5 cm.   Part of that difference can be attributed to the cable attenuation at 400 MHz, and the remainder is probably caused from minor imperfections in the cable and connectors.   RG 58 coax gets fairly lossy at these frequencies, which is why they use RG 8 or better for any significant length.   For example, 100 feet of RG58C will lose (absorb) nearly 95% of the input power at 400 MHz.   RG8 will lose 45%.   A cable loss calculator can be found at - http://www.qsl.net/co8tw/Coax_Calculator.htm

I realize this is pretty basic stuff for many reading this, but hopefully others may find these tests interesting as I did.

 

Online MarkL

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Re: Tektronix 2465B oscilloscope teardown
« Reply #747 on: June 01, 2016, 10:37:04 pm »
You don't need to use power to calculate predicted amplitudes.

In the case of just the scope with its 50R termination, you have a 50R transmission line feeding the internal 50R resistor and measured at that junction.  The amplitude, as compared to open circuit (or 1M impedance setting) can be calculated as a simple resistor divider 50R/(50R + 50R).  So, when termination is turned on in the scope the amplitude is 50% of the unterminated setting.  Most people are familiar with this behavior and there are no lengthy stubs to observe pulse reflection behavior.

When you add the second coax transmission line, you're adding another 50R impedance at the scope's input (and also creating a discontinuity in impedance).  The amplitude of the pulse is now reduced because there's another impedance in parallel with the first two and creates a 3-way divider.  It is 50R/(50R + 50R + 50R), which 33.3% of the open circuit amplitude.  Or 66.6% if you want to use the terminated (50%) amplitude as the baseline.

The terminated pulse height you report as 5cm.  The pulse height with the second coax attached should be 5 * 66.6% = 3.33cm.  You report 3.40cm, which is within about 2%.  That's pretty good.

For the pulse on the way back, it is split again as you describe, but the amplitude should be 66.6% of 66.6% = 44.3%.  5cm * 44.3% = 2.21cm and you report 2cm.  Looking at the screen shots and using your 0 to 100% graticule, it looks like it might even be a little more than 40%, which again is exactly right.

When doing TDR with a scope, I usually leave the scope unterminated internally.  It's less confusing if you don't have the scope as a third impedance on the line.

More reading:

  http://cp.literature.agilent.com/litweb/pdf/5966-4855E.pdf
 

Offline Old-E

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Re: Tektronix 2465B oscilloscope teardown
« Reply #748 on: June 01, 2016, 10:51:19 pm »
Hmmm.   I'll give that some thought.

Thanks MarkL
 

Online med6753

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Re: Tektronix 2465B oscilloscope teardown
« Reply #749 on: July 04, 2016, 01:11:15 am »
Update from my last entry on May 22nd....

June was a busy month with other activities and finally got some time to do some additional work on the 2465 DMS.

The DMM option has temperature readout but I didn't have the Tek P6602 Temperature Probe. In checking E-bay and other sources it turns out this is a pretty rare animal and the prices reflect that. In other words...no way. Decided to build my own and the total cost was about $6 USD.

A scrap collapsible antenna, glued on rubber plug to make it immersible in liquids, Heraeus M1020 PT100 Class B sensor. Some banana plugs and done.


The results. Immersed in an ice bath. Sorry gang but this old school Yank prefers Fahrenheit over Celsius.


More updates to follow..stay tuned.       :D
« Last Edit: July 18, 2017, 06:48:44 am by med6753 »
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