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Lip* Lip* is offline
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Default Oscilloscope Tektronix 2235 Short Unstable Horizontal Trace

On Jan 18, 3:35*pm, mike wrote:
On 1/18/2012 10:54 AM, Lip* wrote:

You are correct, Jeff.


If the Horizontal Position adjust (R726) can move the "squishing"
trace to the right or left, beyond the dead zone, the amplifier is
working and has enough voltage to show the trace in that region.
Perfect debugging thought. *Will do it tonight.


Look at a signal.
Does the trace stay the same when it squishes? *Or does the trace
get truncated? *That should tell you whether it's a sweep timing issue
or a gain issue.









If the XY works all over the screen, for example, a 60Hz sine
generating an almost perfect ring on screen, it eliminates voltage,
amplifier, plates, etc. *Will be around the trace sweep generator, or
something like that.


I already identified all the scope points on the diagram and their
position on the board printout, so it will be easier to follow this
thing.


The only place in the diagram where I can see different circuits
(bipolar) for left or right side of the screen (half trace) is the
output of the U760, the guy who does the sweep controls, and it is
called "Horizontal Pre-amp". *It has +SWP and -SWP outputs that drive
directly the output transistors. *The input of U760 is not separated,
it comes from one linear ramp from Q742 that represents the whole
screen trace, feeding the U760 "A SWP IN".


So, the fact that the right half of the screen is more "damaged" and
"squishing", lead me to think about connections between those boards,
or something at the output transistors driving the right plate. *As
far as I remember, the right side failing was not steady, but as if
there is a lot of noise in the right side sweep ramp.


I need to find it out... it seems easy. *The first test was to make
sure +100Vdc is steady and Okay. *Then the Horizontal Position knob to
make sure the "sqhished" trace can be moved to the left or to the
right of the screen. * Next, the XY test, to eliminate forward or
backward circuits in the path. *There is also a -5V regulator (U750)
specifically feeding the U760, and I guess this negative rail is only
used to generate the negative sweep ramp to control the output
transistors for the right plate.


It will be nice to have this 2235 back to life, even with the Tek2246
available, I always choose the 2235, it is smaller, lighter, faster to
operate and I can transport it anywhere I need it, without pulling a
muscle in my back.


Thanks, will report.


Wagner.


On Jan 18, 12:54 pm, Jeff *wrote:
If the position control will move what trace you do have to the right,
that might eliminate the output stage. Look for it on the print, in
some scopes it is a dual pot wired in llike an "X" formation and may
feed the CRT plates "directly", but I don't think Tektronix does this.
(they have good reason on an engineering level)


I would definitely check it in XY the very next thing. If it works
properly in XY you've eliminated the entire amplifier section and can
proceed directly to the oscillator. However if whatever you try to
display compresses on the right, you know it's that amp, and whether
the position control moves the flattened portion or not is the next
test. If the whole squished trace moves, it is before the position
control, if the trace squishes (love those technical terms) without
reaching the right side then it is after the position control.


If the problem is one of the output stages to the plates, the non
linear portion of the trace whould exhibit defocussing or astigmatism,
I think. It may not with certain CRT designs. To go any farther I'll
have to get the print, but this should be a good wham bam eliminate a
bunch of stuff. In other words, quichly find out what ituisn't/u.


JURB


Another good tip Mike, will look at it.
Thanks a bunch.
Wagner