Re: 468 sweep offset issue


 

On 23 Feb 2016 19:13:28 -0800, you wrote:

I have been slowly working on getting this 468 fixed, back into spec, and calibrated.
Physically, it is mint. There's no damage anywhere.

I had to replace 3 of the PSU capacitors: the 55V, the 5V, and the -8V.
In the time where I hadn't the slightest clue what I was doing, I replaced lots of parts that I didn't need to - It has all new capacitors. New tantalums, new lytics, new poly HV caps... even new ceramics. I was very, very careful to make sure that I used the correct values for everything - and not to mix anything up.

There are still a couple of things left to resolve:

- There is a voltage offset between the horizontal deflection plates, which I believe is causing the vertical elements of the traces on the far right 1/4 of the CRT to have a slight bow to them. The Y axis and Geometry adjustments are not able to resolve this. I have not yet tracked down the source of the issue here.
I am not sure what you mean by voltage offset. With no sweep, there
is always a voltage offset because the blanked beam is located at the
left side of the CRT.

Is the bow on the right 1/4 of the CRT visible in your photos?

- The B sweep and A sweep do not sync at the start of the sweep. The sync pulse is correct. I have checked all of the waveforms numerous times trying to isolate the problem, but I believe this is a problem in the timebase section (according to "Tek: troubleshooting your oscilloscope") - making it a royal pain for a beginner like me to track down.
**This is the issue that is giving me all of the trouble**
Are you referring to the start point of the A and B sweeps not being
exactly aligned?

The alignment depends on circuit tolerances and there may be no
adjustment for it. At high sweep speeds, it is normal for the start
of the sweep to creep to the right somewhat.

I can probably suggest where it can be trimmed out if we determine
that there is no actual fault.

I have swapped out the trigger board and also the timebase board in an effort to at least isolate the fault to a specific block. The problem must reside on the main board. My guess is that it's going to be in the sweep logic circuitry.

I have been using the "Tektronix: troubleshooting your oscilloscope" book. I have read the theory of operation for this scope a couple of times. I have even read the circuit description section of my 465 manual to try and get a better understanding of the basic functionality of these scopes.

I'm hoping that I will have an easier time troubleshooting since I bought a new digital scope.
I'm confident that I'll be able to track down the problem with the horizontal amp , but this sweep start offset is driving me nuts and I just cannot figure it out! Will someone please point me in the right direction?

Pics of the problem:
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1461836945/lightbox/1384398865?orderBy=ordinal&sortOrder=asc&photoFilter=ALL https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/1461836945/lightbox/1384398865?orderBy=ordinal&sortOrder=asc&photoFilter=ALL

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