Advice on fixing 2465B slow horiz sweep


Grayson Evans
 

My 2465B has been sitting on the shelf for a few months. I notice when I fired it up today that the horizontal sweep is slow, by that I mean that the
calibrator square wave does not line up with the screen scale lines. It sweep starts out on the left correct but by the time it reaches the right side, it is off about 1/2 cm.
I checked the calibrator output on another scope and it is correct. The Delta T measurement shows that the calibrator timing is correct, but it just doesn't line up with the screen scale. Same for B sweep.

Could this be a screwdriver/pot adjustment (too good to be true Im sure), or some software calibration? I have maint manual which seems to reference R860 for horizontal gain, but discusses a CAL 01 procedure in detail, more detail than I can follow.

Any one else have this problem?


Ozan
 

On Thu, Jul 22, 2021 at 03:02 PM, Grayson Evans wrote:


My 2465B has been sitting on the shelf for a few months. I notice when I
fired it up today that the horizontal sweep is slow, by that I mean that the
calibrator square wave does not line up with the screen scale lines. It sweep
starts out on the left correct but by the time it reaches the right side, it
is off about 1/2 cm.
....
---

To rule out the simplest case, is "VAR" knob of sec/div switch in the calibrated position?
Ozan


Grayson Evans
 

Sure, first think I checked.


Siggi
 

On Thu, Jul 22, 2021 at 6:02 PM Grayson Evans <wa4gvm@gmail.com> wrote:

My 2465B has been sitting on the shelf for a few months. I notice when I
fired it up today that the horizontal sweep is slow, by that I mean that
the
calibrator square wave does not line up with the screen scale lines. It
sweep starts out on the left correct but by the time it reaches the right
side, it is off about 1/2 cm.
I checked the calibrator output on another scope and it is correct. The
Delta T measurement shows that the calibrator timing is correct, but it
just doesn't line up with the screen scale. Same for B sweep.
If you're seeing this only for the sweep speed, but consistently for *all
sweep speeds* *and for the cursors*, then possibly your horizontal gain has
drifted.
What's more likely happening is that your scope is suffering from the 2465B
capacitor plaque, where the electrolytics on the A5 board leak and wreck
the DACs reference (among other things). This would affect the sweep speed,
but also a whole lot of other functionality on the scope. If you haven't
already changed the electrolytics on the A5 board, it'd be my bet that
that's your problem.


Grayson Evans
 

I suspect horizontal gain drift since that fits the symptoms and I don't know of any other problem.
I replace all the electrolytics in the power supply and everywhere else I could find one.
Just not sure if the horiz. gain is adjusted using a pot, (R860) or in the confusing CAL 01 procedure.
Anyone done that procedure?


Siggi
 

Hey Grayson,

It'd be helpful if you could describe what you've done and seen in some
detail. Detailed observations are vastly more likely to produce useful
advice than writing up your conclusions.

Inline...

Siggi

On Sat, Jul 24, 2021 at 12:57 PM Grayson Evans <wa4gvm@gmail.com> wrote:

I suspect horizontal gain drift since that fits the symptoms and I don't
know of any other problem.
Like I said, if you have the *same exact* offset at *all* sweep speeds, and
on the cursors, then this could plausibly be horizontal gain. From what I
can see, the 1X and 10X CRT gain are adjusted separately, so you can
further verify that it's the 1X gain that's out if the sweep is OK at 10X.
This is a case where you can describe what you've done and what you've
observed in order to get better advice.

The horizontal gain is adjusted first in the CAL01 procedure with R860 (1X)
and R850(10X).
Note that the sweep speeds are under software control through
control voltages from the DAC on the A5 board. These are adjusted
throughout the rest of the CAL01 procedure.


I replace all the electrolytics in the power supply and everywhere else I
could find one.

Do you have surface mount electrolytic caps on your A5 board? Had they
leaked?


Ozan
 

On Thu, Jul 22, 2021 at 03:02 PM, Grayson Evans wrote:


My 2465B has been sitting on the shelf for a few months. I notice when I
fired it up today that the horizontal sweep is slow, by that I mean that the
calibrator square wave does not line up with the screen scale lines. It sweep
starts out on the left correct but by the time it reaches the right side, it
is off about 1/2 cm.
I checked the calibrator output on another scope and it is correct. The Delta
T measurement shows that the calibrator timing is correct, but it just doesn't
line up with the screen scale. Same for B sweep.

Could this be a screwdriver/pot adjustment (too good to be true Im sure), or
some software calibration? I have maint manual which seems to reference R860
for horizontal gain, but discusses a CAL 01 procedure in detail, more detail
than I can follow.

Any one else have this problem?
-------
There are good suggestions in the other posts too. Given that delta-T measurement has the same wrong horizontal scale factor, horizontal gain could be an issue as you suspected, although it is strange that the gain is off by ~ 5% (0.5cm error in ~ 10cm).

One more test to check horizontal gain is to apply a signal, e.g. 1kHz sine wave, with 8 division amplitude to channel 1. Then switch to X-Y mode with nothing connected to Ch2, and with only Ch2 selected in the mode switches. Do you see a 8 division horizontal line, or is it compressed too? If you now turn on Ch1, do you see a 45 degree diagonal line? I did this experiment on my 2467B (almost same as 2465B).

You can also use the calibrator output as signal generator, then you will see two dots in X-Y mode. You will need to adjust Ch1 volts/div for 8 divisions amplitude or best measurement accuracy.

Looking at the schematic there is some interaction between x10 gain and x1 gain pots, if you see the gain off in x10 mode it could still be R860.

If you touch R860 you may want to mark the original position in case it is something else. Again 5% error all of a sudden is strange.

Ozan


Tom Lee
 

Sorry that I haven’t been following this thread closely, so forgive me if you’ve already explicitly verified that the vertical calibration has not been affected. But if the vertical gain is off by the same amount as the horizontal, though, that would point to a HV problem (in this case, a reduction).

Cheers
Tom

Sent from my iThing, so please forgive typos and brevity.

On Jul 24, 2021, at 9:57 AM, Grayson Evans <wa4gvm@gmail.com> wrote:

I suspect horizontal gain drift since that fits the symptoms and I don't know of any other problem.
I replace all the electrolytics in the power supply and everywhere else I could find one.
Just not sure if the horiz. gain is adjusted using a pot, (R860) or in the confusing CAL 01 procedure.
Anyone done that procedure?