Topics

2246 no display/odd behavior

Matt
 

Most esteemed,

My 2246 presents with the following symptoms: At power on the fan runs but nothing on the display is visible and none of the front panel button lights illuminate. I can get the front panel to come to life by repeatedly pressing random buttons. Otherwise, it will not come to life on its own no matter how long I let it sit. The screen remains blank, however, no matter what I press.

My suspicion is the SMPS. I measured ripple on all the rails. See here: https://imgur.com/a/wEGp9Yf Note that I change the time scale on a few measurements. Assuming I've measured these correctly, does the SMPS seem to be a likely culprit? Or if not, other suggestions on where to diagnose?

Roger Evans
 

Do you have the service manual? You can find it at w140.com/tekwiki. Your measurements of ripple are only slightly greater than the specifications in table 6-4 which also lists the limits on the DC values of the supplies. The service manual also explains the function of the front panel buttons in the case of a diagnostic error at start up.

Roger

chipbee40
 

PSU is most likely culprit, you will have fun digging down to get to it. Search the archives for info on this, there were many threads regarding the way to test the PSU out of the scope (carefully) and likely suspects for replacement. Personally I had no problem with the 3 legged caps in the PSU (You will see which when you get in there) however others on this board disagreed. There is also a battery on the digital board which leaks (depends on version) that can cause problems. Its been a while since I repaired any of these so working from (poor) memory here.

Matt
 

Thank you for the manual link. I will review the front panel function and see what I come up with. The trigger lights to not flash any codes on startup (if I do a reboot after I get the panel to come to life).

I took the SMPS out for a visual inspection. Nothing obviously fried and no signs of electrolyte on the board. I think my next step will be to measure ESR of the electros and see what I find.

Battery on the logic board measured ~75mV. I think that counts as dead. But can that be the cause of these issues?

Any further suggestions welcome as well.

tom jobe <tomjobe@...>
 

Hi Matt,
Did you check the various low voltage supplies before you took the power supply out of your 2246?
Knowing what the individual voltages are, and knowing how much ripple is present on them will go a long way toward pointing you to the actual problem(s).
The Tekscopes Message Archive has lots of information about these 224x scopes and their common problems.
Do a search of the message archive on various terms like 2246, 2246A, 2245, etc
tom jobe...

On 3/13/2019 5:40 PM, Matt wrote:
Thank you for the manual link. I will review the front panel function and see what I come up with. The trigger lights to not flash any codes on startup (if I do a reboot after I get the panel to come to life).

I took the SMPS out for a visual inspection. Nothing obviously fried and no signs of electrolyte on the board. I think my next step will be to measure ESR of the electros and see what I find.

Battery on the logic board measured ~75mV. I think that counts as dead. But can that be the cause of these issues?

Any further suggestions welcome as well.


Matt
 

Ok so some updates:

I haven't found any previous posts that address my issues.

All voltages on the low side are in spec. Looking back at the ripple measurements in my first post I think they are all OK but a second set of eyes would be helpful. The p-p measurements seem high but there are very few points. The bulk of the points seem within the specs but there's some pronounced waveforms there that I don't know about being allowable.

I checked the ESR for all the electros on the power supply board - everything looks good (I used the Blue meter and tested in-situ).

Under the board I found a broken (lifted?) trace between C2244 and R2231. See here: https://imgur.com/a/bBWioVT I ran a jumper and re-assembled and no change in behavior. Still not display apart from a brief flash at power off and still the off behavior from the front LEDs where they don't power on unless I fiddle with the buttons.

There is a RIFA cap C2217 that looks a bit rough but not blown. I'm open to further suggestions or testing approaches. Still not sure if the two problems may be related or which I should focus on first. FWIW I can now get that PSU in and out in about 10 minutes flat... practice :D

 

How can you edit a post on this system?
Anyway-
Have you replaced the battery yet? Should do that first and reboot the scope several times to get the ram initialized.

Pay attention to the adjustments section, especially the power supply and beam (grid bias) adjustments.

What are the _exact_ low voltage measurements?

The ripples shown on your pictures all look good.

Can you measure the -2.7 kV line safely? That can be accessed by looking at the cathode or filaments on the CRT.

Does the Beam finder button do anything?

Regards

Matt
 

Ok. I've removed the battery - have a new one on order. When reassembling I found that J2208 was installed backwards initially. Corrected but no change in behavior. All further testing is done with the battery removed:

Exact voltages:
+131.34
+58.201
+15.052
+7.4913
+5.2186
-5.2245
-7.5541
-16.005

These all seem to be in spec.

I have a 6kV probe on order - should be here in a few days. I will probe the HV line at that point.

Beam finder does nothing. There is no display at all - no text, no beam. No change whether the front panel is powered up or not.

 

Take note of where the grid bias R2719 is currently set by following the setup in the manual. You will do this in XY mode. See if you can get a dot displayed.

I agree that all the power supply rails look good.

The battery can be any 3.6 volt lithium cell with leads just for a test.

Good luck and regards

Matt
 

Ok - some progress.

I replaced the battery. No change in startup (front panel controls still hang until I fiddle with the buttons for about 45 seconds). It's now returning to the previous state after a reboot though. No codes displayed in the trigger section.

I adjusted R2719 and we have a dot! Here's a shot: https://imgur.com/dySu4aT

If I deviate from the procedure and turn the Readout knob up I can also see the standard display and even a waveform I fed in.

Now I'm confused because the manual procedure concludes with "adjust R2719 so dot is no longer visible" which then kills the whole display. Turning the readout knob back up after adjusting R2719 'correctly' results in no display or trace.

Still some lingering questions in my head too like how J2208 got installed backwards and how the trace between C2244 and R2231 'disappeared' but this is definitely progress.

Suggestions on how to proceed welcome.

Matt
 

Update: front panel issue is fixed. Or rather, it was never an issues. When the scope goes into self-calibration on startup the front panel does not come alive until it completes or fails. Since the display was not working I couldn't see that and my random button pushing was eventually dumping it out of self-cal.

Display issue still ongoing...

mark audacity romberg
 

I remember finding that disconcerting on a 22xx scope I owned briefly—can’t remember the exact model.
“Oh no, why does it go dead on startup? What’s wrong??”

Matt
 

I've done some more testing. The unit passes all self tests and all features seem to function. What does not function are the A Intensity, B Intensity, and Readout adjustments. All three have no effect on the display when the grid bias R2719 is adjusted down. When it is adjusted up, A intensity and B intensity still have no effect. Readout will turn the background off at it lowest level but otherwise has no effect throughout the rest of its range.

Matt
 

Measured some of the test points in the Z-axis section. 7B, 7C, 7D, 7E, and 7F are all shown here: https://imgur.com/a/eDLAg84

B, C, and D are all way out of spec. E has no variation at all. F looks OK. Does this point a finger at any specific components as culprits?

Matt
 

Starting at the pots, I ran the exercise pots test from the test menu and A INTEN, B INTEN, and READOUT are all reading fine from 00 to FF. I think this rules out the muxing portion of the circuit. On the analog side, I checked the voltages in and out of the pots, all fine (-2.5 to + 2.5).

Moving to the first test point on the A INTEN circuit I get 4.2-4.3 volts at either end of the A INTEN pot travel where the diagram specifies 3.1-4.2.

Here is my marked up schematic: https://i.imgur.com/pgGF15p.png

Does this suggest possibly faulty Q1001/2/3/4/5? Further, any recommendations on how to test these in circuit instead of shotgunning it?

Matt
 

I measured voltages at all test points: https://groups.io/g/TekScopes/photo/90936/0/schem1.png

Green text: measurement in spec
Red text: measurement out of spec
Blue text: unknown spec but I measured

Hoping someone may be able to make some sense of it - I'm out of ideas. Thanks.

chipbee40
 

CR2701-4 are culprits usually for problems with intensity. They will likely measure ok in circuit and usually cause excess brightness but I guess they could be your problem.
1N4936 are suggested replacements in a previous thread.
https://groups.io/g/TekScopes/topic/7650359#83874

Matt
 

Thanks. I will start with replacing these. They did all test fine in circuit but I understand that doesn't mean much for these particular diodes.

One question on this main board: do most people remove it to work on it or just solder from the top side? Looks like quite a pain to remove but, then again, soldering on the top is not fun either.

Lastly, errata from my previous marked up schematic: on the far right "+42 to +42" should be red, not green.

chipbee40
 

For transistors and 2 leg components etc definitely work from the component side. way too much trouble otherwise. Cutting leads may be a good idea before pulling or maybe tacking to existing leads.

Matt
 

Successfully replaced CR2701-04 with 1N4936RLG - no change in behavior. Not worse, not better.

For anyone else who may wonder: The diodes definitely have the legs bent under the board. I first cut the diode out close to each side of the body. Then I flowed each remaining leg and wiggled out with tweezers. Then wicked the remaining solder which left a clean via to work with. Installation was about a 4 second heat application at 600 F tip temp.

Anyway - still open to further suggestions.