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2445 “A” sweep Issue


christopherbath@...
 

I am having issues with a 2445 Oscilloscope that is not showing the A horizontal sweep. The “B” sweep appears when the B delayed time base is used. The readout shows some dots on the bottom line of text on the CRT. And the A sweep appears to have a starting dot which is offset to the left of the CRT.

I had wondered if the 2445 has issues with the horizontal output hybrid (as the “A” sweep is offset to the left) or the Z-axis hybrid (as there are dots on the CRT). As the “B” sweep and readout appears however this seems to imply that the horizontal output hybrid may be working? I am not sure is there could be a failure mode of the horizontal output hybrid which results in the “A” sweep not appearing.

I have swapped the “A” and “B” sweep hybrids which did not resolve the issue.


christopherbath@...
 

I have tested the "A+" input to U800 and whilst there is a large DC offset there is no AC component.


Siggi
 

Hey Christopher(?),

inline.

Siggi

On Tue, Sep 8, 2020 at 9:59 PM <christopherbath@...> wrote:

I am having issues with a 2445 Oscilloscope that is not showing the A
horizontal sweep.

This is a base 2445 as opposed to 2445A or 2445B?


The “B” sweep appears when the B delayed time base is used. The readout
shows some dots on the bottom line of text on the CRT. And the A sweep
appears to have a starting dot which is offset to the left of the CRT.
You may need to post a picture of what you're seeing on the CRT, and don't
forget to show the front panel control settings. You'll need to create a
new album in photos (https://groups.io/g/TekScopes/photos) and please link
the new album from your post.



I had wondered if the 2445 has issues with the horizontal output hybrid
(as the “A” sweep is offset to the left) or the Z-axis hybrid (as there are
dots on the CRT).

Depending on how this is manifesting, it could be that the dots are
signifying that the scope has lost calibration. Does it go through power up
self-tests at power-up, or do you need to press a button before you get to
sweeps?


As the “B” sweep and readout appears however this seems to imply that the
horizontal output hybrid may be working?

If you get a readouts and/or a sweep, that'd indicate your
horizontal/vertical output hybrids are working.


I am not sure is there could be a failure mode of the horizontal output
hybrid which results in the “A” sweep not appearing.
You've verified that triggering works? More likely than an output hybrid
would be a trigger or some kind of a control problem.


I have swapped the “A” and “B” sweep hybrids which did not resolve the
issue.


christopherbath@...
 

Hi Siggi,

Thank you for your useful comments. The is a 2445 scope and not 2445A or 2445B.

I believe the scope has lost calibration data. The scope shows a failure of Test 04. With a status of 11. Having a look at the service manual just now I have found the following comment "If the bottom row of the readout contains dots in the normally blank spaces, a wrong calibration constant has been encountered. The instrument must be readjusted to remove the incorrect calibration constant from the EAROM". This matches exactly with what I am seeing on the CRT.

My suspicion is that the hybrid is working however I wanted to be sure as the start of the “A” sweep is off the CRT to the left. I am assuming that if the horizontal hybrid had failed then everything would have been to the left.

The scope does seem to trigger. The Trigged LED illuminates when the Cal signal is applied and trigger level adjusted.

Can the loss of the calibration constants result in the “A” sweep being “lost”?

Thanks,

Chris


Siggi
 

Hey Chris,

it'd be mighty useful to see pictures of how your scope behaves, as I'm not
sure what "the start of the “A” sweep is off the CRT to the left" means. If
you input the calibrator signal to a channel and trigger on it, you should
be able to position the start of the sweep off the right and left edges of
the CRT as I recall. Does the horizontal position control work?
If you enable both A and B sweeps, do they start at roughly the same point
on the CRT?

Also note that the calibrator has a fairly precise frequency that adjusts
with the sweep speed you select to maintain 5 cycles of the square wave on
the CRT (up to point). You can therefore use the calibrator to assess
whether your sweeps are reasonably calibrated and whether they're linear.
Are they?
The 2445 is all drive by wire, where control voltages generated on the A5
board are used to set the sweep timing and the vertical gain of the scope.
The horizontal position control is one of those control signals, though it
should apply to both sweeps equally.
Sweep timing is also governed by control signals that are set through
calibration, so it's possible that some or all of your A sweep timings are
way off.

So far you're not giving us a whole lot of information to point you in
profitable direction though.

Siggi

On Thu, Sep 10, 2020 at 11:09 PM <christopherbath@...> wrote:

Hi Siggi,

Thank you for your useful comments. The is a 2445 scope and not 2445A or
2445B.

I believe the scope has lost calibration data. The scope shows a failure
of Test 04. With a status of 11. Having a look at the service manual just
now I have found the following comment "If the bottom row of the readout
contains dots in the normally blank spaces, a wrong calibration constant
has been encountered. The instrument must be readjusted to remove the
incorrect calibration constant from the EAROM". This matches exactly with
what I am seeing on the CRT.

My suspicion is that the hybrid is working however I wanted to be sure as
the start of the “A” sweep is off the CRT to the left. I am assuming that
if the horizontal hybrid had failed then everything would have been to the
left.

The scope does seem to trigger. The Trigged LED illuminates when the Cal
signal is applied and trigger level adjusted.

Can the loss of the calibration constants result in the “A” sweep being
“lost”?

Thanks,

Chris








Siggi
 

Hey Chris,

Here's a great document on how to troubleshoot your oscilloscope:
https://vintagetek.org/troubleshooting-your-oscilloscope/. It's worth a
read if you haven't read it before. Even if you have read it before, it's
probably worth another skim :).

Inline.

Siggi

On Thu, Sep 10, 2020 at 11:09 PM <christopherbath@...> wrote:


The scope does seem to trigger. The Trigged LED illuminates when the Cal
signal is applied and trigger level adjusted.
Have you tried X/Y mode? Have you seen any kind of trace on the CRT? Does
the horizontal position control work?


Can the loss of the calibration constants result in the “A” sweep being
“lost”?
To directly address this question: I don't think so, but I'm often wrong. I
would be surprised if you'd lose all sweeps, e.g. I'd think there would be
sweep speeds that DO work.

Also, I don't think the B sweep will run without a functional A sweep, so
you may have some kind of a display/intensity problem rather than a sweep
problem. The A/B sweep setup controls are fairly intricate, and I don't
know which settings (if any) have produced a trace for you.