465B Square Wave problem
|
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?
/g/TekScopes/photo/56677/12?p=Name,,,20,1,0,0 /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? |
|
I am in the process of accumulating these devices and plan to learn to calibrate my scopes. Although I know they wont be certified they at least will be usable.
At the moment I have a 60mhz function/arbitrary wave generator. I am using that for my square wave. I have other scopes that are putting the same square wave up correctly. |
|
Thanks for your input
I am using a short 50ohm bnc to bnc and it displays the correct wave on my other scopes. I just tried a second 2ft bnc to bnc and get the same result. The attenuators such as c36 and c37 will not correct this much deviance. Nor will the compensation pots on the preamp. I have not tried much else. But are you saying this could all be corrected with calibration? I am working on getting the correct hardware to calibrate but I wold like to get the square wave visually square, even if it is not calibrated. If there is still a issue, I believe there is, then I wont be able to calibrate until it is corrected. |
|
Hello Timothy,
toggle quoted message
Show quoted text
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:
|
|
On Mon, 16 Jul 2018 11:28:00 -0700, you wrote:
Everything done in order as laid in the manual.This is important. You can ignore the attenuator compensation to start off with by working only with the 5mV/div range where all of the attenuators are switched out. If you cannot get the transient response of the 5mV/div range calibrated, then stop there are fix it. You should be able to do the low and medium frequency transient response adjustments with your 60mhz function/arbitrary wave generator since you have another oscilloscope for comparison purposes however the high frequency transient response adjustment will require a faster clean source and a 50 ohm feedthrough attenuator. |
|
The termination is not required for the low and medium frequency
toggle quoted message
Show quoted text
compensation which is just as well if a x10 probe is used because then adjusting the x100 attenuator in a 465B requires a high level signal for 5V/div. If a normalizer is used instead of a x10 probe, then this is not a problem because the sensitivity will be 1V/div. On Wed, 18 Jul 2018 00:25:19 +0000 (UTC), you wrote:
Timothy, |
|
Couple of questions and a comment.
One, why can't I get the print at w140 ? I went to that Polish site and got it and it is scanned terribly and there are tings missing. Then I went to BAMA and it appears to be the same file but also on the list was "missing pages". None of them seem to have the end of the front end. Where is a good manual for this ? I don't see any TP1 or 2, the PDF I have is 47.4 MB. Is that the same as you got ? If so, can you tell what pages those test points are ? Might help. Two, your pictures show 2 waveforms, one severely overpeaked and the other severely underpeaked. What is the difference in the settings between the two ? Looks like the attenuator is set the same and both CH 1, are they at different frequencies ? Now, one of my general rules about troubleshooting is that when it doesn't work right look at the last thing done to it. you did the multiplier which should have no bearing on the peaking. you changed the vertical output board. not if both channels are affected the same way that means the problem is there. If it is different between CH 1 and CH 2 that is a different story. But you only changed the output right ? If both channels display the same there is one thing they have in common. Both David and Fabio are right, the 5 mV range is where to start. If it displays the same at that setting then get out of the attenuator. (which I call the front end) Forget any adjustments there. If you find the display the same on both channels down there, whatever compensation is on the output board is the most likely culprit except for the outputs themselves. Vertical deflection plates on a scope can affect focus, but that means there is an imbalance in the output stage. When you stick a probe on there you just put 10 meg to ground. (I hope you use 10 X except in rare cases when you need all the sensitivity) If that causes a DC offset of only one plate, or a common mode offset in both plates that could be the cause of the problem as well as the compensation problem. But to determine that we need the settings used on your 2 quite different waveforms. there are o clues, no readout on the screen and we can't see the time base, all I think I saw (pretty sure) is that both waveforms are from Channel 1. If by chance is it intermittent, then it is an intermittent cap that goes shorted and then open when it feels like. Not the most uncommon failure mode in the west. That's where the frequency might help to determine the cause, the higher pF value caps seem a bit more likely to do this. And when it comes to frequency comp they are not using tantalum. I hope this is of at least some help, so I put it out there. If not just ignore it. but that's the way I would proceed, go to 5 mV and see what that looks like, both channels. And don't try to align it anymore. That is far too much error to be simply in need of alignment. |
|
I had to put this aside due to other things but during that time I kept working on other electronics and have really learned a lot. I had been over that scope with a fine tooth comb, I checked every component. I needed a break. This scope like most 465s have been sitting. I had tried cleaning the contacts with automotive MAF cleaner previous to all this, all I had on hand that seemed reasonable. The problem was corrosion in the switches. I finally broke down and bought a can of DeoxIT D5. This fixed the traces corners. I didnt even need a calibrator to adjust them after that. Although I now own a working cal pak. WooHoo!
I have this theme going where when sometimes I fix something, something else breaks. My Beam Finder switch was damaged so I replaced it with a donor. Now my traces are flickering intensity badly. not sure if its related. Off to troubleshoot... |
Timothy
Craig Cramb
Fabio Trevisan
David Hess
Jeff Urban