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475A horizontal amplifier problem


Jim Hall
 

I am in the process of restoring a 475A scope and have reached a 'roadblock'; the horizontal width of any waveform is only three divisions. The vertical portion of the waveform is shown correctly for all volts/division. The following are my observations in trying to determine the cause of this problem:
• My first thought was that the Beam Finder was the problem, but it is functioning correctly and compresses the waveform more than three divisions;
• All of the LV and the HV supplies are correct;
• When the X10 magnification switch is engaged the display width is about six divisions;
• The compressed waveform appears on the right side of the screen and the position control only moves it slightly to the left of center;
• With the maximum horizontal gain, the voltage on the right deflection plate can be adjusted to 52 V using the position control, but then the voltage on the left plate is only 40 V; if the voltage on the left plate is adjusted to 46 V, the voltage on the right plate is only 46 V;
• Comparing the waveforms and voltages in the horizontal amplifier circuit of this scope to those in a correctly functioning 475A, did not uncover the cause of the problem;
• Also, swapping the horizontal amplifier's two output transistors for the right plate with those for the left plate resulted in no change.

Any suggestions as to where to look next will be greatly appreciated.
Thanks, Jim


 

Check the resistor values in the H- output ckt.

Regards

On 10/4/2020 8:37 PM, Jim Hall wrote:
I am in the process of restoring a 475A scope and have reached a 'roadblock'; the horizontal width of any waveform is only three divisions. The vertical portion of the waveform is shown correctly for all volts/division. The following are my observations in trying to determine the cause of this problem:
• My first thought was that the Beam Finder was the problem, but it is functioning correctly and compresses the waveform more than three divisions;
• All of the LV and the HV supplies are correct;
• When the X10 magnification switch is engaged the display width is about six divisions;
• The compressed waveform appears on the right side of the screen and the position control only moves it slightly to the left of center;
• With the maximum horizontal gain, the voltage on the right deflection plate can be adjusted to 52 V using the position control, but then the voltage on the left plate is only 40 V; if the voltage on the left plate is adjusted to 46 V, the voltage on the right plate is only 46 V;
• Comparing the waveforms and voltages in the horizontal amplifier circuit of this scope to those in a correctly functioning 475A, did not uncover the cause of the problem;
• Also, swapping the horizontal amplifier's two output transistors for the right plate with those for the left plate resulted in no change.

Any suggestions as to where to look next will be greatly appreciated.
Thanks, Jim



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Jim Hall
 

Thanks for the suggestion Tom. I have now checked the resistors and compared the values to the parts list and those in the good scope. They all appear good. The waveform extends only one division to the left of center when I maximize the horizontal gain and use the position pot to move to the left. Something is preventing the beam from moving further left.


 

Was the scope dropped? I have seen where the CRT internal glass support rods have broken causing similar problems. How do the waveforms look on  the H defection plates? They should be the same but inverted.

Tom

On 10/5/2020 2:38 PM, Jim Hall wrote:
Thanks for the suggestion Tom. I have now checked the resistors and compared the values to the parts list and those in the good scope. They all appear good. The waveform extends only one division to the left of center when I maximize the horizontal gain and use the position pot to move to the left. Something is preventing the beam from moving further left.



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Jim Hall
 

Since this was one of the scopes that I have purchased on eBay, I don't know its history. The waveforms look correct on the deflection plates. However, compared to those on the plates of my good 475A, they are compressed horizontally.


 

In that case, it might be well worth pulling the tube and inspecting for broken bits of glass in the neck. The 475A is well worth saving.

Regards

On 10/5/2020 4:36 PM, Jim Hall wrote:
Since this was one of the scopes that I have purchased on eBay, I don't know its history. The waveforms look correct on the deflection plates. However, compared to those on the plates of my good 475A, they are compressed horizontally.



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Tom Lee
 

Sorry that I haven't read the entire thread, but to localize the problem, you should first verify whether the outputs of the horizontal amplifier are swinging the amount that they should. If they are, then your problem lies in the interconnect to the crt, or the crt itself. A revealing test is to engage the 10x magnification function to see if you are able to overscan the screen. If you are, then you have sufficient swing capability in the output amp, and the finger begins to point toward a gain insufficiency.

-- Tom

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Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
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Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/5/2020 13:36, Jim Hall wrote:
Since this was one of the scopes that I have purchased on eBay, I don't know its history. The waveforms look correct on the deflection plates. However, compared to those on the plates of my good 475A, they are compressed horizontally.




Jim Hall
 

PROBLEM SOLVED - After checking the components, voltages, and waveforms in the horizontal amplifier circuit and verifying that the outputs of the amplifier were correct, I took the advice of Tom M. and Tom L. to consider the CRT. The problem was that the connection to the CRT's left plate was disconnected. Much effort and patience was required to reattach the wire to the CRT through the hole in the interface board. . Many thanks to both Tom M. and Tom L for their help in locating the problem.

Jim


toby@...
 

On 2020-10-06 8:28 a.m., Jim Hall wrote:
PROBLEM SOLVED - After checking the components, voltages, and waveforms in the horizontal amplifier circuit and verifying that the outputs of the amplifier were correct, I took the advice of Tom M. and Tom L. to consider the CRT. The problem was that the connection to the CRT's left plate was disconnected. Much effort and patience was required to reattach the wire to the CRT through the hole in the interface board. . Many thanks to both Tom M. and Tom L for their help in locating the problem.
Well that's great news. Another TekScopes success we can all learn from...

--Toby


Jim





Simon
 

I don’t know how you got access to the X deflection pins on the CRT through the hole in the interface board. Keyhole surgery? It might have been easier to remove the CRT after unsoldering the X amp output wires (red and green go through the hole). Attach a puller wire to them so that you can pull them back through the hole after checking the X deflection connections. Even so that is fiddly as you have to remove the 4 Y deflection wires, and they are not easy to put back.
Simon


Dave Wise
 

It's usually simple... once you know what it is.

Dave Wise
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of toby via groups.io <toby=telegraphics.com.au@groups.io>
Sent: Tuesday, October 06, 2020 6:17 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 475A horizontal amplifier problem

On 2020-10-06 8:28 a.m., Jim Hall wrote:
PROBLEM SOLVED - After checking the components, voltages, and waveforms in the horizontal amplifier circuit and verifying that the outputs of the amplifier were correct, I took the advice of Tom M. and Tom L. to consider the CRT. The problem was that the connection to the CRT's left plate was disconnected. Much effort and patience was required to reattach the wire to the CRT through the hole in the interface board. . Many thanks to both Tom M. and Tom L for their help in locating the problem.
Well that's great news. Another TekScopes success we can all learn from...

--Toby


Jim