Tek2440 Mini teardown, NVRAM replacement and calibration

[LazyJack]

I recently managed to buy on eBay and subsequently resurrect a Tektronix 2440 scope (PSU caps). Although the NVRAM chips were still working, they had a datecode from 1993, so I decided to replace them. I ordered two DS1230AB-70 for replacement (those suckers cost nearly as much as the scope). I desoldered the old chips, which was a fortunately a fairly easy job.

Since I installed sockets for the new chips, before I transfer the content from the old NVRAM, I had the opportunity to confirm or bust the myth showing up on many forums that loosing the NVRAM content will screw you, unless you have the expensive equipment to recalibrate the scope.

Actually, there are two NVRAM chips (DS1230AB-120) U350 and U664, both on the CPU board. U350 is used to store the saved waveforms, so if the battery goes flat in that, the only inconvenience is that the waveforms wont be saved when the scope is off. Also, it will enter extended diags at every boot, as the waveform data is invalid.
U664 is however more important. It stores the calibration data and settings. The service manual is not entirely clear on what happens when this data is lost, but it is implied that a self-cal and an ext-cal will bring the scope back to spec after a cold-start. All the other scary adjustments (many involving turning pots) in the service manual are only required when replacing stuff like the CCD modules. So I figured that the pots are there where they need to be, I'll be able to recalibrate the scope.

I put in the new chips and fired up the scope (not forgetting to remove the calibration enable jumper first). It indeed stopped at the self test. Surprisingly it was smart enough to figure that the NVRAM is empty, it did not even try to run the above 6000 tests, as they would likely fail without the calibration constants. Instead it told me to run self-cal and ext-cal once the scope warmed up.
While waiting for the warm-up, I did experiment with it. The scope was badly out of spec, most evidently the attenuator was all over the place, there were gross errors, on one of the pictures you can see where the scope thinks the GND is and where it is really.
After warm-up, self-cal was run, as the service manual warned, it did take much longer with no initial calibration constants. Then I set up for ext-cal. The attenuator and trig cal is easy, you just need to apply specific voltages to the ch and trig inputs, the scope walks trough you on this.
The channel delay and the CTE cal is more tricky. The service manual doesn't really specify what signal the scope expects, other than it is from the fast rise output of a specific calibrator. This could probably be figured out from the manual of the calibrator.
So I used my pulse generator. The delay time calibration is pretty simple, the scope was satisfied with a 100kHz square wave. For the CTE calibration it is more picky on the amplitude and rise time, but after some fails, I eventually managed to pass.
With warm-up time included I spend under an hour on all this. I know, because the cold-start resets the hour counter in the scope.
Obviously I do not know if the scope is really in spec, for that I'd need equipment which I don't have, but some initial test don't indicate anything out of order, for my hobby purposes it's good.

So the TL;DR of all this is it's not the end of the world if you loose the NVRAM content in a 2440, with some fairly simple equipment you can restore it to working state.

Attached are some (sorry bad quality) photos on the internals of the scope and some other pictures relevant to this post.

[chihaxinh]

Hi LazyJack

I have one tek 2440, and after i run cold start, i need ext calibration but a cant pass for atten test. i apply 0.2v 2v and 20v dc into ch 1 and next step ch2 . Allway display fail . sefltest nvram pass !

Pls help me runing pass ext calib
Thank so much !

[LazyJack]

Make sure your setup is correct. You have to apply the rigth voltages with the right polarity. You shoudl do it one channel at a time, so do not connect the two channels. This test fails if there is too much difference between tha voltages and the internal 10 volt reference. So check that the 10 volt refetence supply is ok. It's in the service manual, in the section where the supply voltages are checked.

[LazyJack]

One more thing. Did it pass the trigger cal?

[chihaxinh]

Hi Lazy Jack

Triger Faill when runing 3 test. short to gnd, 0.2v and 2v . 2440 have 2 triger out bnc . i connect to triger 1
I checking 10V Ref and ok .
I follow guide on service manual in 6-42 section . Calibration Problem, and mearsurement 3V square wave signal not present .

So in guide note : Make sure not made the mistake viewing the Calibration signal output with 10M ohm probe and have the channel in 50R termination.

I using other osc to check with 1M ohm probe and 1M ohm setting in OSC ( not using 50R ).

before my osc runing with pass all , but some time fail, some time pass so i runing coldstart.

I check DAC TP650 and TP660 ok  - 0V and +1.25V ( System DAC OK )

[LazyJack]

For trigger cal, you have to connect BOTH trigger inputs to the voltage at the same time. For atten cal, you have to do it one by one. Check the voltage with a DVM at the input. I used a bench supply and a dvm to do the cal. Probably not the most accurate, but ok for me. Is your nvram ok? You can get strange errors when it starts to fail.

[chihaxinh]

Hi Lazy Jack

When i using bnc cable 50R connect to PSU Bench , i pass Atten Cal , for Trigger i using BNC Tee and 3 BNC cable match lenght . Using PSU supply 0.2V 0.5V and 2V but allway fail trigger ! Sefttest pass all exception 9000 Trigger .

NVRam it's OK . no have errors when start

[LazyJack]

The trigger cal needs 0V or (0,2mV), 0.5V and 2V for trig, if i remember correctlx. Follow what the scope says, it may be different from the service manual. (And different from the atten voltages) The manual is older, and it asks for 0.2mV as that is the lowest on the tektronix calibrator. (note: milivolts!) for the first voltage. Gnd is ok instead and for example my scope already says gnd  for the first step.

[chihaxinh]

Hi LazyJack

It's my mistake ! Now when i short BNC Triger , i pass triger . But i cant pass CTE Calib . Allway report not center , How to running CTE with 100khz and wave form center in display CRT ?

[LazyJack]

Ok, CTE is a bit difficult. Apply the square wave an enter CTE cal. The scope will show the rising edge of the pulse up on theötop of the screen. Use the vertical pos for both channels to center both traces (they should be about on each other, so you at the end see one thrace) and on the pulse or function generator set the signal to be 6 divisions peak to peak. So it should be 3 div above and below the middle. You may need to use an attenuator because the scope sets itself to a fairly high sensitivity and you need a low level signal. But do not set anything on the scope other then the vertical positions.
The scope will tell you if it has problems with centering or amplitude or frequency, but it can still fail, if there is high overshoot or ringing on the pulse. So you need fairly good quality pulse. According to the service man, it should have 1ns rise time. My pulse generator only could do 5ns, but i still managed to pass. I had to use a 20dB BNC attenuator to have low enough signal, with still good rise.

[chihaxinh]

Use the vertical pos for both channels to center both traces (they should be about on each other, so you at the end see one thrace) and on the pulse or function generator set the signal to be 6 divisions peak to peak.

I can't move trace to center, potentimeter of pos vertical only move in 1 divisions. Only way to center by enter programer button and init display ( all trace go to center )

But if runing mode init display , menu of CTE gone !

[LazyJack]

It should move more then one div, just keep it turning.

[chihaxinh]

Hi LazyJack

Ok i now a can move trace to center but scrap display ! Do you thing nvram have error ?
https://youtu.be/1WosHxpJBDg

[LazyJack]

Hmm. This may well be power supply issue. Should check the voltages and the ripple.

[bob91343]

I have no clue regarding replacing these parts. I have a 2440 which I bricked when attempting to replace the Dallas chips. Now I am up a creek without a paddle and there is no one to help.

So I found this thread and am hopeful someone can direct me to get this thing going again.

[LazyJack]

What do you mean by "bricked"? Does it not power on, or does it not pass the self test?

[bob91343]

Neither.  It comes on and the display is nonsense, unaffected by any controls.

[LazyJack]

Try reseating and checking all the connectors. Look at the ribbon cable connectors on an between the two large foldout boards and also the side board. I assume you have removed these when replaced the nvram. I once had a similar experience, one of the connectors were not properly in place and got a garbled display.
Also check the power supply, the PSU in the 2440 is known to have problems with caps leaking or drying out. I'm not a fan of recapping for just the sake of it, but in this case it may be legitimate. Check the powers for value and riplle on the side board test points. Although if it was good just before the nvram replacement it would take a huge coincidence of just going bad now, but it is not impossible.
I also assume that the replacement nvram came from a reputable source and not some aliexpress fake chip. Hopefully you used sockets, so you can try putting in the old ones, the scope should start and ask for a calibration even if they are discharged.

[bob91343]

I do suspect counterfeit chips.  That's why I tried putting the old ones back but one of them got damaged and I didn't take care to see which was which.

Power supply is okay.  Connectors have been reseated several times, although I don't think the GPIB line makes any difference since it just goes to an outbound connector.

[LazyJack]

You could try and use a programmer to read the chips and see if they are ok. The TL688 programmer reads them, I ve tried it. Or just connect it to an arduino.
Or you could even make an adapter to use some normal sram chips instead, just to try if the scope starts up.
Other than that, I would also suspect fake chips.
If you have only one good old chip, you could use it in place of U664, that is the system ram, the other is the waveform save ram. You would probably get some startup error, maybe it would not even get to the point of producing a display, but it should flash the startup error code on the trigger leds. BTW if you remove bot nvram chips, do you get any acivity on the trigger leds? A fake chip could disable the cpu completely, but a missing one should at least enable it to start the basic tests.

[bob91343]

It's been a while but as I recall the screen starts with its usual nonsense then locks to a worthless display that responds to no controls.  Same for lamps on the control panel.  If I am feeling energetic I might pull it on to the bench and start mucking with it but it's best I wait for the chips I ordered.  That should be in several weeks.

[LazyJack]

Why don't you post some picture or a video of the startup?

[bob91343]

I may do that.  Right now the bench is overflowing with in-process projects while I await motivation to get things going.  Growing old really sucks, in case you wondered.

[berreizeta]

TEKTRONIX 2440
NVRAM changed.
EXT CAL all right except :

CTE CAL = INPUT FREQUENCY IS TOO FAST

No way to continue....
[ Attachment Invalid Or Does Not Exist ]

[LazyJack]

What is the frequency of the input signal? Does it have a 'nice' edge? CTE cal needs a relatively clean edge, without much overshoot. What is the signal source? Is it terminated properly?