faulty parametric measure on Tektronix 2445B

[j ferguson]

I have a 2445B which seems to work ok except for time-base functions using Parametric Measure.  For example, i can feed a 400 Hz sine wave from my FG502 into the channel 1 input using 50 ohm coax and either with or without a 50 ohm terminator and get a very nice display which can be accurately measured for both voltage and frequency using the cursors.

If I try MEASURE, I get a voltage readout which agrees closely with what I got using the cursors.  If I ask for frequency, I get CH1:SMALL OR LOW REP RATE SIGNAL and no frequency readout.  I've read, and (gasp) reread the manual and can find no hint to what I might be doing wrong.  I imagined that problem might be an offset setting so I reset it to minimum, but no joy. I can't get the slope outputs either.

I've concluded that I must have a problem with the circuitry which provides time-based information for the parametric function.

I've asked this question twice on the Yahoo tektronix list-serve and received one suggestion that I check internal 50 ohm termination switch.



I realize this could be a colassally stupid question, and I won't mind hearing this, but it would sure be neat to discover that I'm not the only one ever to have had this problem and get some guidance on where to look for the problem.

Thanks in advance for any suggestions.

john ferguson

[macboy]

I don't have the 2445B but here are a few tips in general:
Go to a slower time base so that 5 to 10 periods of the waveform appear on screen.
Ensure that the cursors span at least 2 periods of the waveform.

[j ferguson]

good idea. I'll try it. thanks

[j ferguson]

I don't have the 2445B but here are a few tips in general:
Go to a slower time base so that 5 to 10 periods of the waveform appear on screen.
Ensure that the cursors span at least 2 periods of the waveform.

Parametric measurement on the 2445b doesn't use cursors.  I tried showing more periods on the screen.  paramteric measurement showed same voltage , but still not frequency, same error as i wrote above.

but thanks anyway.

[Andy Watson]

What trigger options do you have set? Without looking at the manual I don't know how the 2445 derives its frequency information but my guess would be that it uses the trigger. If you can't get a sensible frequency reading, report all the trigger, timebase and channel settings here and I'll try to replicate the measurement on my 2465 (I assume it will use the same methods as the 2445).

[j ferguson]

Hi Andy,
thanks much. will do later this evening.  I assume parametric measurement for frequency does work on your 2465.  my guess is this part of the machines isn't that different, but then I don't know.

cheers, john

[j ferguson]

This is set, IIRC, to 40 mHz.  Pressing "MEASURE" and then the FREQ: button, produces the low signal or low rep rate response.

I have the service manual, so I suppose what i need to do is figure out where to look for problem. 

best,

John

[Andy Watson]

It works for me. I've set the controls as per your picture. It works at both 40MHz and 40kHz (as your scope is indicating!)

One thing that I have noticed is that my scope will do the equivalent of "auto set" whenever a measurement is called for - so even if I deliberately set the controls to odd positions it will find the signal and measure it. I actually find this feature is a PITA because it will un-do delicately set triggers, this is the reason that I rarely use it.

I notice that your timebase "var" control is not set to the calibrated position - on my scope the var setting is overridden when a measurement is called for.

[j ferguson]

Thanks much Andy, I too found that it resets itself in MEASURE mode.  The service manual is 350 pages.  I suppose i should just bite the bullet and print the whole thing.  I just got through reviving my dad's old Nakamichi CR3 - which needed a couple of new electrolytics. ah well...

[rf-loop]

This is set, IIRC, to 40 mHz.  Pressing "MEASURE" and then the FREQ: button, produces the low signal or low rep rate response.

I have the service manual, so I suppose what i need to do is figure out where to look for problem. 

best,

John

This your image is not after you have run measurement.
What it show with this setting tell me that this is not result after auto measurement. (I mean V/div and t/div what can see in image)
Btw, how much you have there DC offset in signal.


But first take signals off. Under vertical V/div knobs there is buttons. Push both upper buttons together for start offs/balance null process and wait it ends.

Also please turn CH1, CH2, Timebase "Variable" to detend position, now they are not as can see in image.. (keep these normally always in detend position until nessessary to change and after then return always back to detend for avoid mistakes, this is normal lab procedure)
After then select CH1 GND
Adjust trace position to vertical center.
Select CH1  DC, 1V/div
Connect your signal generator.  Adjust signal generator  level  to something like 6Vpp (6 div)
Adjust signal generator DC offset for well vertical centered signal (DC offset)
Adjust freg to around example 40kHz
Change scope input DC / DC50ohm and watch signal level change around 1:2
If not, then 50ohm input is defect (this is not very rare)  If it is defect then continue with normal DC input.
Adjust signal generator  level is around 6 divisions (1V/div)
Adjust enough low trace and text brightness for good picture with camera.
Take image from screen and after then do not change anything.
Press measure and after then select frequency
take image from screen.
Press measure and select voltage
take image from screen.

(my preliminary suspect is that CAL is not ok)

[j ferguson]

Good morning rf-loop, this is picture taken after measure - freq.  I did this when I took first picture.  I have not yet tried what you suggest. 



I will do the things you say above and send another photograph and any other results.

thank you for your ideas.

john

[rf-loop]

Good morning rf-loop, this is picture taken after measure - freq.  I did this when I took first picture.  I have not yet tried what you suggest. 



I will do the things you say above and send another photograph and any other results.

thank you for your ideas.

john

If before measuremet there was around 5Vpp signal, vertical 1V/div, 10.3us/div (because Timebase Var) and trigger level -1.1V and then you only press "measure" and then 1 (frequency) and result is this your image now, there is many strange things.


1. after measurement run it have changed to 2ms/div, 5V/div. Why?
2. after measuremet run no any visible signal. Where is signal?

After this failure "small or low ref rate signal"  in measurement it continue showing signal. (This is not only from my memory, just tested it with my 2445B)

But lets look what happend after you test agen as my recommendation.

[j ferguson]

screen before press MEASURE button:



MEASURE FREQ:



MEASURE VOLTAGE:




I think I followed your instruction exactly, I did it three times, each time starting with 2445B unpowered, did balance, then what you said.  all Three tests showed same result.

john

[rf-loop]

First quick question. First image can not see V/div and t/div.
What are they? t/div perhaps 5us/div?

You have done vertical adjustment so that first CH1 select GND and adjusted trace (just straight line) to vertical center line? Yes/No ?

After then select 50ohm input and first image is exatly this result without touching anything? Yes/no?

If it was well centered and if level is around 5Vpp (as in some previous images)
Then it is clear if look this image that something is wrong.
First need trusted signal generator what you really know what is sigal true level and what is true offset. Then look various V/div settings that result on the screen is acceptable, including offset. Also it need chec using manual trigger level adjust with known signal that trigger level displayed left top match with what you see where it trig and where is loose trig. (trig manual, normal, not auto level)
Try take wire or probe and look probe cal output. Signal level and DC vertical position  to GND match.

I have now more strong suspect this scope need run full CAL prosedure. If we look last image measured voltage values.

Before can go to CAL need inspect that all works (all attenuator level steps works in both channels and also that 50ohm are ok. It is important because V and H cal need run from start to end, all steps. It can not do if input attenuators have any failure.
Also CAL need equipments what need be available. In service manual is whole prosedure and it need understand and follow every word there and also working order is important without expetions exept if manual tell what CAL routine can do separately.

If there is NVRAM battery near final then it is better to wait first this is changed. All CAL data is there. CAL without perfect equipments and experience is so "hard work" that you do not want do it agen just because NVRAM was in final and after soon you loose CAL.

[j ferguson]

First quick question. First image can not see V/div and t/div. 

V/div was 1 volt   T/div 5uS/div
What are they? t/div perhaps 5us/div?

You have done vertical adjustment so that first CH1 select GND and adjusted trace (just straight line) to vertical center line? Yes/No ?  YES

After then select 50ohm input and first image is exatly this result without touching anything? Yes/no? YES

Switching from ground to 50ohm made vertical trace half height, just as you said.

[rf-loop]

First quick question. First image can not see V/div and t/div. 

V/div was 1 volt   T/div 5uS/div
What are they? t/div perhaps 5us/div?

You have done vertical adjustment so that first CH1 select GND and adjusted trace (just straight line) to vertical center line? Yes/No ?  YES

After then select 50ohm input and first image is exatly this result without touching anything? Yes/no? YES

Switching from ground to 50ohm made vertical trace half height, just as you said.

Ok, good.

With this simple test we do not know anything enough for real diagnose. But, we know Voltage measurement result. And important there is p-p value is totally wrong. And we know result is not symmetric (1.8V / -11.7V) but we can believe your signal is nearly symmetric roughly around +2.5V / -2.5V IF we trust what we see in image 1 before measurement.  With this kind of scope it need understand that what you see on the screen trace is not at all what measurement system see.


Because do not know what kind of equipments you have it is difficult to recommend next step.
Least you need signal source what can trust. We can not believe "anything" what scope is displaying until we know it display right.


Just like with time. If we have only one good working clock and nothing else to check time we have exact time. If we have  2 good working clock and nothing else to check time and if these clocks show different we do not know what is time and we can suspect also that both are wrong or one is right.

Next step need overall check if somethong need repair. If this full check do not give any "need repair" result then next step is full CAL.

And, in this scope (whole 24xx serie) CAL is not like open case and turn some internal adjustments until ok. There need do some this kind of adjustments and they need do without exeptions just as service manual tell including also order. Also there is told what settings user need do for every step.  Scope FW have CAL system. Step by step user need connect needed signal and use front panel for adjust and every step generate CAL factor what all data it keep in RAM. (more new versions have Dallas NVRAM and bit older have separate Lithium battery (Mostly "Keeper" brand) and normal low stand by current RAM where is keep CAL data.   It need also note that user settings and last state is also stored in this RAM.
If need do full CAL. And because these are old and perhaps never changed NVRAM chip or battery it is good to do before CAL.
If change battery and CAL data is not yet loosed it can change so that use external back up battery so that RAM power do not break when change battery on the board.
NVRAM is more fun. It need first take out and copy data to computer. Using example programmer what can reliable do it and also reliable move later this data to new NVRAM. For this I recommend high guality programmer for avoid hassle with cheap crab toys. One good example is "top class" Batronix 48 Batego II or something similar (if only need copy and program this one NVRAM it can do more cheap version of Batronix what all are still professional class equipments with top class support.)

Have you tested this scope all other things (exept these (automatic) measurements) looks like well calibrated?
There in service manual is explained how to check, step by step.

[j ferguson]

Thank you for these ideas.  I have the programmer and a new NVRAM.  I also copied the CAL settings from the Scope using EX 02 (If i remember correctly).  I have not done this yet, nor did I yet replace the 4 electrolytics on the A5 board - existing ones 'look' good. I was going to replace them and the DALLAS NVRAM at same time.

I have only the FG502 for signal generator, although I also have a 10 kHz crystal generator which I bought to align a HF transceiver I built from kit.

I understand about the two or three clocks which do not agree.  I also want to do one more test. I think (may be wrong) that if I left setting on DC not 50 Ohm DC, I got correct voltage.  I'll check and let you know.

This machine shows 58,000 hours with 1,800 off/on cycles.  The components have 1992 dates.  it seems possible that it has not been calibrated since it was built.  I haven't looked at the CAL requirements to see what I need to buy.  It does look as though I need a Time mark generator and a leveled sine wave generator.  I have to decide whether it makes more sense to send it out to have it repaired, or buy the other equipment and learn how to do it myself. 

I'll check MEASURE voltage output with DC setting and let you know.

Thank you again,

john

[rf-loop]

Thank you for these ideas.  I have the programmer and a new NVRAM.  I also copied the CAL settings from the Scope using EX 02 (If i remember correctly).  I have not done this yet, nor did I yet replace the 4 electrolytics on the A5 board - existing ones 'look' good. I was going to replace them and the DALLAS NVRAM at same time.

I have only the FG502 for signal generator, although I also have a 10 kHz crystal generator which I bought to align a HF transceiver I built from kit.

I understand about the two or three clocks which do not agree.  I also want to do one more test. I think (may be wrong) that if I left setting on DC not 50 Ohm DC, I got correct voltage.  I'll check and let you know.

This machine shows 58,000 hours with 1,800 off/on cycles.  The components have 1992 dates.  it seems possible that it has not been calibrated since it was built.  I haven't looked at the CAL requirements to see what I need to buy.  It does look as though I need a Time mark generator and a leveled sine wave generator.  I have to decide whether it makes more sense to send it out to have it repaired, or buy the other equipment and learn how to do it myself. 

I'll check MEASURE voltage output with DC setting and let you know.

Thank you again,

john

Perhaps good to try analyze bit more this offset what can see in voltage (parametric) measurement.
Voltage measurement use same measurement system what AUTO Trigger use for solve signal min and max.
Have you checked also CH2 result using same parametric meas and also same result with measure voltage. This because trigger start from CH preamplifier hybrid and trigger is in high role in parametric mesurement.

[j ferguson]

I think AUTO does not work either.  I'll repeat test using Channel 2, and show result for MEASURE voltage.  First, I must do chores for spouse.

thanks again.

[j ferguson]

Channel 2 setup:



Channel 2 MEASURE freq:



Channel 2 MEASURE voltage:



Not so good.

[j ferguson]

Since i want to keep this scope, maybe now is the time to replace the DALLAS NVRAM, the four caps on A-5, check the other caps, replace the front switch/pot panel with the panel I bought which has seen a lot less use - pots tight, not loose.  then Print the service manual and run through all of the checks which can be done without serious calibration equipment, and then send it out to be calibrated.  It's hard  to believe that it won't do the Auto setup, or measure frequency at all or voltage correctly and STILL pass the post. 

Fortunately I'm not in a hurry.

[Andy Watson]

I would be tempted to solve the existing fault(s) before potentially generating more

and STILL pass the post. 

Have you forced it through all the tests in the diagnostic list once it is up and running?

Fortunately I'm not in a hurry.

Time to read the service manual then Fortunately, Tek were still supplying full manuals with schematics and descriptions of how the circuits were intended to operate for this series of scopes.
And pay attention to the cooling requirements. It is alleged that some of this series rely on forced cooling to stop the unobtainable hybrids from frying themselves.

[j ferguson]

Hi Andy.  good advice.  my 2445b has the TV option which included more lights on the right side of the panel.  without thinking enough about it, i bought a lightly used panel on ebay.  it came and I saw that it wasn't for the TV option but its pots were so much tighter than those on my 58k hour machine.  I worried that it might be hard to remove the TV option if it wasn't going to work with the new panel. so i took the new panel apart and discovered that the lights were there, just no translucent spots on the front.  so easy fix, just change the front - although mine is really beat up.

I think, as you suggest I'll complete the tests which can be done without disassmbly and if I don't find anything, then i will do what i proposed.  I have a pretty peppy fan for keeping U800 down to a low boil while I'm checking things.

thanks again,

john