Author Topic: What is the "At Rest" Position of an HP 17xx CRT Horizontal Sweep: L, R, or C?  (Read 1221 times)

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Offline techie1234

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So I'm having some problems with the horizontal sweep on my HP 1743A CRO.  Intermittently I'm seeing two different anomalies:

In one, the beam just (correctly) traces up-and-down vertically, but stuck in the dead center of the CRT.  The result is a vertical line, albeit of the correct length/V.

In the second, the trace is shaped correctly and does have some horizontal movement but all confined within the right-most 1/4 of the CRT.


Adjusting the horizontal position all of the way left pulls the trace toward (but never past) the center point.


The problem affects both A and B channels, so it would seem that the sweep generator is having some sort of problem.  But it got me thinking: if the sweep generator is "dead", where would the CRT gun end up pointing in the absence of deflection voltage: at the extreme left, extreme right, or dead center?  I think this will help me better understand what is happening in the sweep generator.

As a side note: the problems can often be stopped by switching around various controls (time/div, V/div, channel, single/chop display, etc.) There's no apparent rhyme or reason to what changes will affect the behavior other than they "move things around".  Also, now that I have the case off it seems less prone to having the problems - until they reappear.  But cooling things down with compressed air didn't in and of itself help any. Hmmm...
« Last Edit: December 10, 2016, 03:04:12 pm by techie1234 »
 

Offline rf+tech

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Hi techie1234,

Beam position of electrostatic deflection CRTs is determined by the *difference* in potential between the respective axis plate pairs. With equal potential on a plate pair, including absence of potential, the beam lands in the center (usually specified to be within a 1 cm circle of exact center).

This information should give you a pretty broad hint as to the cause of the problem. Also, Deoxit is your friend. Let us know what you find.

RF+ Tech
 
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Offline techie1234

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Beam position of electrostatic deflection CRTs is determined by the *difference* in potential between the respective axis plate pairs. With equal potential on a plate pair, including absence of potential, the beam lands in the center (usually specified to be within a 1 cm circle of exact center).
Thank you - that's what I had come to conclude (I'm still very early in my electronics learning curve - had hoped the scope would be more a tool for learning rather than the source of projects...) but I had (perhaps incorrectly) a mental model that the gun rested at the far left and the sweep would ramp from 0V to xV.  Thus, one could "cut" the sweep bias off when the right side of the screen was reached and the gun would then automatically return to the extreme left on its own in the absence of any deflection voltage.  Sounds like that's not quite the case.   

For my learning: is the sweep voltage typically only applied to one plate with the other held constant, or is it moderated between the two plates?  It would seem to this novice that the former would be easier to keep consistent - albeit it would seem to require the ramp circuitry to ramp from -nV to +nV instead of from from 0V to +2nV.
« Last Edit: December 10, 2016, 03:41:51 pm by techie1234 »
 

Online tggzzz

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...but I had (perhaps incorrectly) a mental model that the gun rested at the far left and the sweep would ramp from 0V to xV.  Thus, one could "cut" the sweep bias off when the right side of the screen was reached and the gun would then automatically return to the extreme left on its own in the absence of any deflection voltage.  Sounds like that's not quite the case.   

Get hold of the service manual; it contains a good definition of how the scope works, the schematic and components, and how to fault-find and calibrate it.

You can probably find a scan for free on the web. If not then I would use one for an HP1740A, or maybe buy one on ebay. The scanned ArtekMedia manuals are cheap and veru usable.
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Offline techie1234

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Get hold of the service manual; it contains a good definition of how the scope works, the schematic and components, and how to fault-find and calibrate it.
Yep - bought one when I first got the scope about six months ago.  I originally read through the "Service Sheet 8" section, but in reading further along now I think that Service Sheet 12 is the more applicable.  More to think about...
 

Offline techie1234

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For my learning: is the sweep voltage typically only applied to one plate with the other held constant, or is it moderated between the two plates?  It would seem to this novice that the former would be easier to keep consistent - albeit it would seem to require the ramp circuitry to ramp from -nV to +nV instead of from from 0V to +2nV.
From now reading through Service Sheet 12, it appears that the both plates are varied by the horizontal amplifier - albeit in a positive range of +8V to +110V.
 

Offline rf+tech

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Hi techie1234,

Quote
For my learning: is the sweep voltage typically only applied to one plate with the other held constant, or is it moderated between the two plates?

Deflection is push-pull, by differential amplifiers for both axis. Consider that to move the beam left or right of center requires reversing polarity of one plate relative to the other, as you refer to as -nV and +nV. This is not to be confused with the voltage measured relative to the chassis. Both deflection plates typically sit somewhere between 100 to 250 Volts positive to the chassis.

The horizontal position offset points to one half of the horizontal deflection amplifier not working. By measuring the voltage to chassis at each of the X-axis deflection amplifier outputs, while turning the horizontal position control, it may observed that one of the two outputs shows no response, while the working side shows a voltage variation with the horizontal position control.

The HP1741 manual may be close enough for reference on my end:
http://cp.literature.agilent.com/litweb/pdf/01741-90911.pdf
page 135 shows the interconnecting diagram. W4 is a fly lead from A11 to the CRT neck. Also shown is B+ of 120 Volts, so this sets an upper bound to the horizontal position test described above.

Please check page 125 of the above manual, if it is similar/same as the horizontal deflection schematic in your 1743A manual, and shows +55 Volts typical for the two deflection outputs.

RF+ Tech
 
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Offline rf+tech

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Quote
From now reading through Service Sheet 12, it appears that the both plates are varied by the horizontal amplifier - albeit in a positive range of +8V to +110V.

Yes, this is he expected voltage range relative to chassis when turning the horizontal position control.

What do you measure on the two outputs?

RF+ Tech
 
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Offline tautech

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I had a 1740 that had similar symptoms and it turned out one of the Horizontal output transistors was blown/sick which threw the bias off center like yours. The universal replacement I settled on for output stages in CRO's is BF259 that generally has a better spec than OEM. It's listed as a HIGH VOLTAGE VIDEO AMPLIFIER as all CRO plate drive output transistors are.
DC voltages listed in the schematics can be a great help if you haven't got another scope BUT you must have the controls set in a certain way for those values to be valid. RTFM.
Section 8.32 of the 1740 SM is where to look for the waveforms and voltages.
Earlier there'll be the default setup description for DC values to be correct.

In relation to your thread title the neutral values for the deflection plates is normally just under 1/2 of the HV available when a dot is centered on the display.
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Offline techie1234

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Yes, this is he expected voltage range relative to chassis when turning the horizontal position control.

What do you measure on the two outputs?
Thank you so much for your suggestions - I was coming to the same conclusion and now understand the test procedures in the SM to measure exactly this.  I'm unfortunately under the weather today but will be back with my results in a day or two when I can fully concentrate around HV so I don't  :-BROKE
 

Offline techie1234

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Since I promised to report back on my findings, I feel compelled to do so (albeit months later).  Well, the darn behavior no longer exhibits.  I'll keep this thread in the back of my mind and if it ever re-appears, I'll let you know what I find and how I repair it.  Thanks again to everyone for their help!
 

Offline tautech

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Since I promised to report back on my findings, I feel compelled to do so (albeit months later).  Well, the darn behavior no longer exhibits.  I'll keep this thread in the back of my mind and if it ever re-appears, I'll let you know what I find and how I repair it.  Thanks again to everyone for their help!
Hmmm, suspect the PCB interconnects.
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