Re: 465B Square Wave problem

Fabio Trevisan
 

Hello Timothy,
You don't give much details about the conditions and scope settings you used...
What is the frequency of this signal you pictured or, alternatively, what is the Horizontal sweep (time / div)?
It would be useful you take a picture at a fastest sweep speed that you can, that can illustrate the first 2/3rds of the signal transition (and also report what sweep is ir)... There seem to be a distinct speed transition before the signal reaches 2/3rds of the amplitude, from the last 1/3rd and usually there's no single mis-adjustment that can cause this kind of waveform, but without more details of how much time this slow transition represents, and not knowing what is the transition speed of the fastest part, it's hard to tell.
To improve the diagnostic, there is a few tips / advice:
1.When you don't know anything about the step response of a scope (i.e. if there are defects and/or mis-adjustments), you want to eliminate as much possible problem causes in the beginning.
In that line of thinking, the first thing you want / need to eliminate are the input attenuators so, until you know that the pre-amplifier and output amplifier are working correctly, you need to feed a signal that's between 20 to 30mVpp and set the oscilloscope input to 5mV/div. That setting will by-pass all the attenuators and leave you only with the pre-amplifier and output amplifier.

2. To further eliminate possible sources of step-response problems (all the input circuitry and all the pre-amplifier), you can try to follow the instructions of the "Check/Adjust Low Frequency Compensation" which is the section 16, of the Vertical Calibration chapter, on page 4-32, steps g and h. (of the 465B User and Service manual (e.g. PN 070-2757-00).
This is a very handy procedure for folks like you (and me) who don't have the proper generators.
What this step g. instructs you to do is, essentially, using the scope's own channel alternation / chopping circuitry, as an internal square wave generator.
Here follows a brief description...Under normal operation when the scope switches the signal back and forth between CH1 to CH2 in alternate mode, you cannot see the transition (from one channel to another) because this transition is performed at the end of every sweep, when the trace is already blanked.
Following this procedure, by pulling the cable out from J4387, and injecting into this plug a TTL level signal (and use this same signal to trigger the scope), you actually cheat the scope into switching from CH1 to CH2 (or vv) in the middle of a sweep.
For that to work, you need to adjust the total horizontal sweep time (10 divs) to be >= 2x the period of the square wave you're feeding into J4387.
For this test you will keep both CH1 & CH2 inputs set to GND (no need to feed any signal to the CH1 and CH2 inputs), and the resulting square wave you're going to see on the screen is controlled solely by the position controls of CH1 and CH2 (CH1 position will control, say, the upper part of the square wave, and CH2 position will control the lower part, or v-v).
Adjust CH1 and CH2 position so that you get a 6 division p-to-p waveform on the screen.
If the rounded square wave corner still shows in this mode, you will have eliminated the pre-amplifiers and everything that is before the channel switching circuitry, as a possible cause.
If you see a relatively good square wave, without the rounded coners... then your problem will surely be on the input circuitry / attenuators / pre-amplifiers.

Hope this helps you advance some steps.

KRgrds,

Fabio

On Mon, Jul 16, 2018 at 02:28 PM, Timothy wrote:


I have a 465B that had a intermittent beam. 90% of the time the screen was
blank. I ended up changing the multiplier (although I am not sure that was the
problem) and the vertical output board and now my trace is back but there is a
problem that is the same on both channels. The problem is on all divisions to
a degree. I have changed some of the attenuator settings and even tried
adjusting the compensation from the preamp board but it has little effect.
Does anyone have any familiarity with this issue or can at least point me in
the direction I should look first?

https://groups.io/g/TekScopes/photo/56677/12?p=Name,,,20,1,0,0
https://groups.io/g/TekScopes/photo/56677/13?p=Name,,,20,1,0,0

Something strange I have noted is when I probe say test point 1 (the resistor
coming out of the attenuator box) or TP2 @ CR1193 on the vertical preamp the
trace on the broken scope blurs so much you cant even see it anymore, the
screen just becomes brighter and the test scope gets a blurred out trace, its
there, just fat. When I test the diode at point 9 I get a clean trace and the
damaged scope does not blur. Im not sure if this is related or if I am doing
something wrong. I was probing a 465 (not B) last night and did not have any
of these problems.

Another question about the manual. The arrows to the test points in the Figure
8-11 diagram dont seem to point to a definite spot. i have been looking at the
schematic to try and determine where exactly I am supposed to probe. Am I
doing this right?

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