PS: I'm really sad because this is my first scope and i've been looking for one to buy for cheap because i'm just a student with really low budget, and i wanted to use it to learn new things and expand my knowledge and experience, but i can't do so with a broken scope, and i don't have the knowledge yet to repair the scope that would have helped me understand how circuits work and how to repair them... so, yeah, i need the help of someone, please..
It may even just be your lack of detailed understanding of how to use an analogue scope.
The sweep signal on the back is an output of what the timebase is feeding to the X axis. What you are seeing is a straight diagonal line, nice and linear, which is what you would expect - the voltage rises as the dot sweeps across the screen. You would only expect to see an actual sawtooth wave (multiple cycles) if you were looking at it with another scope, set to a slower timebase.
It is picking up stray mains voltage fields, it would be very rare to see a perfect sinewave under these conditions.
1) Here you can see that the sweep is reeeally slow https://imgur.com/a/zEhz7Tq
The sweep signal on the back is an output of what the timebase is feeding to the X axis. What you are seeing is a straight diagonal line, nice and linear, which is what you would expect - the voltage rises as the dot sweeps across the screen. You would only expect to see an actual sawtooth wave (multiple cycles) if you were looking at it with another scope, set to a slower timebase.
oh, ok.. on the pcb there is written sawtooth so i expected to see a sawtooth (https://imgur.com/a/YtRglgm), good to know.
Anyway something strange happens when i increase the time/div, https://imgur.com/a/mmz1Wmh , the line that should be a perfect line.. curves. what?? why?
2) Mmmh.. i don't know, watch this: https://imgur.com/a/gbA5HpD . The amplitude changes randomly by fixed values and not so fast, as if the potentiometer does not make contact completely, but there's this extremely amount of jitter and noise, are you still sure that's only a dirty pot?
3) I'm a good antenna i suppose https://imgur.com/a/hfonpyJ
It is picking up stray mains voltage fields, it would be very rare to see a perfect sinewave under these conditions.
not a perfect sine but almost, sure this is just picked up from somewhere?
4) Here you can see that i cannot achieve perfect compensation by just adjusting the probe, https://imgur.com/a/41DvwUb , the square wave is never against the graticule, that's why i thought that some trimmer inside the scope needed to be adjusted.
1) Here you can see that the sweep is reeeally slow https://imgur.com/a/zEhz7Tq
It would be better to include any still images as attachments to your posts to save having to jump out to links, some members also don't like clicking on links in posts, so you might get more input. Videos are more difficult of course, there's no real option there.
One thing that I immediately notice is that the 'uncal' light above the time/div knob is lit. This indicates that the small variable knob above is not at its click stop 'cal' position, where the main knob indications are valid. It might be as simple as that.
Set the HOLD-OFF knob to MINimum and the VAR to CAL, then see how it looks.
Hmm. It is difficult to tell in the ms/div ranges due to the sweep rate beating with the video refresh rate but it does look slow, although each setting in the s/div range looks to be in correct proportion to the one above and below. I can't clearly see the knob positions (decimal points) where you are viewing the stray mains pickup but using this as a reference, how many ms is it reading for a full mains cycle (for 50Hz it should be 20ms of course).
I viewed a video on youtube of a guy that have the same problem on a tektronix oscilloscope, and he commented that he resolved the issue by replacing a resistor that went bad in the sweep generator circuit.
QuoteAnyway something strange happens when i increase the time/div, https://imgur.com/a/mmz1Wmh , the line that should be a perfect line.. curves. what?? why?
I think this is because you have the channel input coupling set to 'AC', so the position is changing as the average DC level changes. Try it again with the input set to DC - It is normal to always leave the input coupling set to DC unless you are trying to look at a small AC signal superimposed on a higher DC voltage.
It does look as if you have a defective pot on Ch2. If I am seeing it correctly, this is the fine amplitude adjustment knob. These are very rarely used (hence they get noisy) - they are normally just left in their 'cal' position and the amplitude adjusted by the main V/div knobs. The fine adjustment pots (voltage and frequency ones) are very little use as they prevent you from being able to interpret voltages and time intervals directly off the CRT graticule.
Quote3) I'm a good antenna i suppose https://imgur.com/a/hfonpyJ
Yes you are.QuoteIt is picking up stray mains voltage fields, it would be very rare to see a perfect sinewave under these conditions.
not a perfect sine but almost, sure this is just picked up from somewhere?
Yes, it is all around you - at high impedance.
Yes, it does look as if you may have some problem there, but again it may also be that you have the input coupling set to AC, so you may see a small amount of slope on the flat tops due to roll-off. Try the same thing with the input set to DC coupled and see if it improves. It looks as if the probes do have sufficient range to compensate the scope input as you are able to achieve both undershoot and overshoot.
[EDIT: Actually, after looking at the video again the slope is on the wrong direction for it to be just due to AC coupling - It looks as if at one point you can almost get it flat with slight roll-off on the leading edge, this is preferable to overshoot for now. It may require internal adjustment to improve it later.]
1)One thing that I immediately notice is that the 'uncal' light above the time/div knob is lit. This indicates that the small variable knob above is not at its click stop 'cal' position, where the main knob indications are valid. It might be as simple as that.Set the HOLD-OFF knob to MINimum and the VAR to CAL, then see how it looks.
It looks exactly the same.. That's the first thing i noticed when i got the scope by the way, that playing with holdoff and var, the sweep in certain situations was just a little bit faster but.. nope.
2)It does look as if you have a defective pot on Ch2. If I am seeing it correctly, this is the fine amplitude adjustment knob. These are very rarely used (hence they get noisy) - they are normally just left in their 'cal' position and the amplitude adjusted by the main V/div knobs. The fine adjustment pots (voltage and frequency ones) are very little use as they prevent you from being able to interpret voltages and time intervals directly off the CRT graticule.
Thanks for the info! In the meantime i found EEVblog #803 and #804 where Dave repairs an HP1740A that has, beyond other many problems, this same amplitude problem on ch2. I'll watch it and make an update in case.
[EDIT: it was a bad rectifier that cause all the problems, but not related to the var knob, no mention of that problem at the end actually, so yeah probably a noisy pot also in there]
4)Yes, it does look as if you may have some problem there, but again it may also be that you have the input coupling set to AC, so you may see a small amount of slope on the flat tops due to roll-off. Try the same thing with the input set to DC coupled and see if it improves. It looks as if the probes do have sufficient range to compensate the scope input as you are able to achieve both undershoot and overshoot.
[EDIT: Actually, after looking at the video again the slope is on the wrong direction for it to be just due to AC coupling - It looks as if at one point you can almost get it flat with slight roll-off on the leading edge, this is preferable to overshoot for now. It may require internal adjustment to improve it later.]
It's the same with ac and dc, there is something going on with volts/div:
10 v/div, undershoot: [ Attachment Invalid Or Does Not Exist ]
5 v/div, overshoot: [ Attachment Invalid Or Does Not Exist ]
It would be a good idea to first check the low voltage power supplies as detailed on Manual page 5-2. Looking at the PSU PCB layout (page 5-3) it looks like the test points are all clustered around the TP2 end of the board. NOTE: Stay well away from the other end of the board, which has the -2kV and other high voltage supply components on it! Measure the low voltage (up to +24V). It is unlikely that there is a problem there (all fixed regulators) but it is sensible to just check. Measure them relative to the chassis.
Check carefully around the pot (VR936 on manual page 7-7) for poor connections, missing ground connection (particularly if the 'Holdoff pot next to it is also not behaving). Check the resistor values and joints in that area too (i.e. R935 and R937).
You ought to be able to do the checking around the Var pot with the power removed, but it would be useful to know if the voltage at Base of T902 (safer to pick it up at R933) varies with the VAR pot. If possible, follow it through to the Emitter of T901 - this is the common point feeding all of the Timebase range selection resistors, so is the last 'common' point before the range knob.
There is a Timebase adjust preset (VR938) in that part too, but I doubt that it would have sufficient range to correct the issue that you're seeing (maybe though, particularly if it has failed open-circuit).
It is hard to see the connections back to the sweep generator circuit (page 7-5), but as it is horizontally sweeping, I think the problem is located on Page 7-7.
Ok, maybe somebody has been tweaking the variable capacitors in the attenuator network before you got it then. Do your scope probes have a switchable X1 / X10 setting? If so, it would be useful to set them to the X1 position and repeat the test. The probe compensation trimmer will have no effect in the X1 position but it will enable us to tell whether it is the scope input capacitance is changing on different V/div settings or whether it is later in the attenuator (Manual page 7-2).
I clearly misunderstood what you were saying about the Var and Holdoff pots, sorry. My best guess is that something in that area has shifted (solder joint?) to throw out all of the timebase speeds. Have you actually quantified the difference, I remember some videos, but it would be helpful if you could actually quantify it, ie. Using the graticule, what setting does is the timebase think it is on when it measures 20ms for a full mains cycle (ie. it ought to be 4 divisions wide at 5ms/div, 10 divisions at 2ms/div)). Same with the 1kHz cal (although this frequency is only approximate). How long does the spot take to traverse the full width (8 or 10 divisions?) at the slowest setting.
Not absolutely sure from the videos but it does look as if your sweep is non-linear at the end. It looks as if the sawtooth is good, although too slow, for 8 divisions then slows for the last 2 divisions.
You could check this with a variable frequency square wave input counting how many cycles you fit per div. at beginning and end of sweep in different timebase settings.
It would also give you a better idea if the timebase is defective on all settings.
That just looks like the ground of your probe is not connected...
The spot seems to take right about 10div*time/div seconds to traverse the 10 divisions on the screen (viewing it in the seconds per div range, in the ms/div range i cannot be precise but it looks right there too), wich is right, right? But then, why is it so slow? It cannot be right this way, it's so slow that one cannot even visualize the waveform past the 50 ms/div because it's just a point moving on the screen.
And worst of all, when i visualize the two channels at the same time, the beam has to halve the cycles, one cycle for each channel, so it is even worse..
MAJOR PROBLEM (encountered while writing this post and doing the tests for you guys):
I would have done more tests BUT having taken the front panel apart to inspect the timebase board (which is now floating mid air) and having it disconnected and reconnected to the sweep board, something has changed. Now channel 1 is funky and channel 2 beam becomes more curved with higher time/div settings... plus i don't know what other problems. In short, pretty much everything about visualizing the cal wave became screwed, the controls work, its just the channels and the timebase and the deflection and i don't know what..
So.. i guessed something snapped? Moved? Too much problems to be just gravity and interference. Ooooh god what just happened...
https://imgur.com/a/iOZlTCU
Now I'm really confused - I thought you were saying in your description and videos that the sweep speed was too slow compared to the timebase knob setting (which is not really visible in the videos). It seems now that you are measuring them at the correct sweep rate. ie. the time for 10 divisions of sweep is 10 times the time/div setting. On a 50ms/div, the time to traverse 10 divisions is 0.5 seconds, so the spot will clearly be visible. To be viewing a waveform (fitting one complete cycle across the width of the screen) you would be looking at a 2Hz waveform at this setting, which is a very low frequency. To see one complete cycle of your picked up 50Hz mains waveform across the screen width the timebase would be 2ms/div and the 1kHz calibrator, 0.1ms/div. Is this what you are seeing? If so, then the horizontal sweep speeds are correct.
Even on a digital scope, below around 50ms/div, they normally default to roll mode where the screen scrolls a little like a chart recorder with the new voltage coming in at the right and scrolling across to the left. Analogue scopes do not have this feature, so you see the dot on the CRT traverse the screen from left to right. For slowly changing voltages, you monitor the dot position against the graticule.