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Tek 465 S/N B299598 Slight Horizontal trace jitter


Craig Cramb
 

Dealing with a 465 scope that is having a condition on CH1 that the trace has a slight jitter to it. Less than .1 but it is annoying to me. Is visible when a signal is displayed or when the Attenuator is in the GND position. CH2 trace is solid smooth in any condition no jitter at all. I have gone thru the A3 CH1 Vertical preamp suspecting the condition would be located there. But reading thru all the test points thru this circuit and comparing CH1 to CH2 test point values they are all the same. No indication thru this board of any slight jitter.
Any thoughts of where this might be coming from that there is a difference in the CH1 and CH2 signal. I'm applying a common square wave to both inputs and selecting Chop mode then tracing thru the CH1,CH2 test points.

There is another issue, the balance on CH1 is not working as it should when trying to get Trace shift to less than .2V I can get it real close in balance but then the position adjustment for the trace won't travel thru the entire graticule scale. Which this might be the trim pot R25. But would like to pin down the other issue first before removing the preamp board to replace this resistor.

Any thoughts would be greatly appreciated.

Craig


Dave Peterson
 

Hi Craig,
Have you stepped through the calibration procedure in the service manual? It would be good to be assured that all the preliminary settings and checks are clear. Like the 55v is driving the first stage bias. It'd be good to know it is good and stable.
To clarify, when you say "jitter" I'm concluding that you mean the entire trace is randomly jumping in the vertical only? Your signal is triggering and the horizontal sweep is stable? This sounds like vertical position pots that are "noisy". That is the vertical position does not move smoothly as the knob is turned through it's range. That is typically improved by repetitive turning of the knob and use. Does the "jitter" respond to touching or moving of the position knob?
Your R25 issue does seem to indicate a CH1 pre-amp bias issue. I would not conclude right away that there's a problem with R25 itself. I would be curious to know how calibration has gone with position centering (R115) and Var Balance (R120). I'm wondering if there's an issue with the position pot itself (R302). Might explain both conditions. I'm a bit surprised you don't see noise on CH1 test points, and that they are matching CH2.
You're applying a square wave to both CH1 and CH2. What is it's frequency, hi and lo values, and is this what you are measuring at the test points? Or a DC value? A DC value might simplify things, but the square wave is fine. You just have to measure both levels, and are you measuring differential or relative to ground? If you're having a bias issue measuring all four values relative to ground might be more revealing than differential values: CH1+ (TP141), CH1- (TP147), CH2+ (TP241), CH2- (TP247).
Portions of the pre-amp are current based signaling - you may not see a voltage noise at the above test points. I'm wondering if there could be issues with, for example, Q304/Q308. The transistors in these scopes are plug-in, and a little schmutz on the leads could cause noise. You might want to carefully identify, remove, and clean the leads on these. So many of these scopes have been idle in hardly ideal conditions. This would not be the first case of a problem caused by connectivity problems with plug-in parts. Corrosion happens.
I'm not an expert. Just beginning this journey myself. But these are the things I'd be looking at. When communicating debug info don't be shy to be very explicit and very thorough. It's hard to know what's going on when one doesn't have the circuit in front of them. A little too much info is better than a little too little. But you've got to start somewhere, and I understand not wanting to do a big data dump when you're just trying to feel out the feedback. Let me/us know how it goes.
Dave

On Tuesday, December 22, 2020, 06:00:49 PM PST, Craig Cramb <electronixtoolbox@gmail.com> wrote:

Dealing with a 465 scope that is having a condition on CH1 that the trace has a slight jitter to it. Less than .1 but it is annoying to me.  Is visible when a signal is displayed or when the Attenuator is in the GND position. CH2 trace is solid smooth in any condition no jitter at all. I have gone thru the A3  CH1 Vertical preamp suspecting the condition would be located there. But reading thru all the test points thru this circuit and comparing CH1 to CH2 test point values they are all the same. No indication thru this board of any slight jitter.
Any thoughts of where this might be coming from that there is a difference in the  CH1 and CH2 signal. I'm applying a common square wave to both inputs and selecting Chop mode then tracing thru the CH1,CH2 test points.

There is another issue, the balance on CH1 is not working as it should when trying to get Trace shift to less than .2V  I can get it real close in balance but then the position adjustment for the trace won't travel thru the entire graticule scale.  Which this might be the trim pot R25. But would like to pin down the other issue first before removing the preamp board to replace this resistor.

Any thoughts would be greatly appreciated.

Craig


 

If you are only seeing the jitter on one channel then it MUST be in the pre-amps before the channel switch.

It looks like the 465 doesn't have an IC for channel switching (I'm only familiar with the 475 which has a channel switch IC): it looks like it's done by two pairs of diodes, CR305/CR307 (ch 1) and CR315/CR317 (ch 2). There are two test points that let you see the signals controlling those diodes (TP374 and TP364 for ch 1 and ch 2 respectively). You could see if the jitter is visible on TP374. If so, I guess that would implicate either the diode pair (305/307) or something around Q374.

-- Jeff Dutky


Craig Cramb
 

On Tue, Dec 22, 2020 at 07:03 PM, Jeff Dutky wrote:


If you are only seeing the jitter on one channel then it MUST be in the
pre-amps before the channel switch.
Jeff Dutky,
Thank you for your intrest.
Went thru the readings as you described ahead of vertical switching TP364, TP374 and after vertical switching TP322, TP324 and made comparisons, I see no jitter in these area's and the signals look very comparable. Used both a digital and analog scope to check and compare the test points It seems to be more prominent during a cold startup. So I will measure these points during a cold start as the instrument is well warmed up now.

Craig


Craig Cramb
 

To clarify, when you say "jitter" I'm concluding that you mean the entire trace is randomly jumping in the vertical only? Your signal is triggering and the horizontal sweep is stable?

Dave,
Thank you for your interest.
The jitter is very small and and yes it is the entire trace but specific to CH1 only. It isn't showing up in area as I went thru testing that Jeff Dutky suggested. As I had assumed that I would find it there. The position control is smooth and there really isn't any dead spots or indication of excessively dirt pot. I haven't started down the calibration path yet as was just evaluating conditions of the scope. The signal I am applying a 1Khz 300mV signal. But can change it to a DC voltage signal as advised. I have removed cleaned and reseated the majority of the transistors if they were accessible without removing and boards. I will get back with you as I go set by step thru your questions.

Craig


adesilva_1999@...
 

Do you see the jitter even without a signal input? Does it vary when changing the position of the volts/div switch? Before you do anything else, check for loose connections/poor solder points in the vertical input area.