Oscilloscope GOS-3110 not displaying right

[betwixt]

I suspect there is another fault.

Firstly, the spikes indicate excessive high frequency response and your home made probe has the opposite effect of reducing high frequencies passing through it. Wait until you new probe arrives or you may be hiding the fault you are trying to fix.

Regardless of the probe quality, you should see the size of the waveform change as you turn the Y attenuator (volts/div) control. If it doesn't change there is probably a fault in the input amplifier and it could also be responsible for the signal overshoot. The trouble with this kind of fault is you need a second oscilloscope to follow the signal in the first one!

Try these two tests and tell me the results.:
1. Can you confirm the displayed waveform is still very small if you use a short wire to directly link the calibrator output to the center pin of the input socket.
2. If you set the switch to GND the waveform should disappear, leaving just a straight line. Set the Y position so the trace follows the black line across the middle of the screen. Now in the DC position, the square wave should sit on top of the black line, in the AC position it should be sitting half above and half below. Let me know if this is what you see.

Brian.

 

[Windmiller]

I suspect there is another fault.
Firstly, the spikes indicate excessive high frequency response and your home made probe has the opposite effect of reducing high frequencies passing through it. Wait until you new probe arrives or you may be hiding the fault you are trying to fix.

But the spikes are gone in any case

Regardless of the probe quality, you should see the size of the waveform change as you turn the Y attenuator (volts/div) control. If it doesn't change there is probably a fault in the input amplifier and it could also be responsible for the signal overshoot. The trouble with this kind of fault is you need a second oscilloscope to follow the signal in the first one!

Yes, the size of the waveform do change in size as I turn the Y attenuator (volts/div) control. I must have been misstaken when I wrote that the waveform didn't change as I turn just anything, I meant that there was nothing inside of the oscilloscope that was able to make the waveform higher than a half square in eight like in the picture while the Y attenuator (volts/div) control was set to 50mV like the manual says. But if I turn the Y attenuator (volts/div) control the size will change like it should. The manual said that I should be able to set the Y attenuator (volts/div) control to 50mV and bring the waveform up to the sum of 5 squares, but I didn't make it higher than about the sum of 1 square at that moment. But now I turned the gain pot inside the oscilloscope while turning some other pots, which made me bring the waveform up to 3 squares in hight but still not 5 like tha manual says.

Try these two tests and tell me the results.:
1. Can you confirm the displayed waveform is still very small if you use a short wire to directly link the calibrator output to the center pin of the input socket.
2. If you set the switch to GND the waveform should disappear, leaving just a straight line. Set the Y position so the trace follows the black line across the middle of the screen. Now in the DC position, the square wave should sit on top of the black line, in the AC position it should be sitting half above and half below. Let me know if this is what you see.

I will test these steps, and get back to you.

Thanks
/Morgan

 

[Windmiller]

I made these tests now, pardon my english in these movies.

Result with probe..

Short wire..

Regards

/ Morgan

 

[betwixt]

The English is perfect - a million times better than my Swedish!

The oscilloscope seems to be working as it should. In DC mode, the voltage is going from zero to a small positive level and in AC mode it is centering itself equally above and below the center line which is exactly what it should do.
There is a slight error in the waveform but it is small. I suggest you use the wire link and put the volts/div switch in the most sensitive position (biggest signal on the trace). If necessary, adjust the vertical position so you can see the top or bottom of the waveform. If you can see both at the same time that's OK. Now adjust VR206 to get the corners as near to square as you can. One way the corners will become rounded, the other way you will get spikes. The adjustment is quite important, if it is wrong the readings you get will only be accurate at one frequency, for example a 1V signal at 1MHz might show as one division high on the scale but 1V at 10MHz might show two divisions high.

When you get the x10 probe you do the same thing by adjusting the tiny screw in the side of the probe but it would be wrong to use the probe adjustment to compensate for an error in the oscilloscope, you really need to get the scope as accurate as possible before touching the probe control.

Brian.

 

[Windmiller]

Hehe thank you, or I hope.. because if you mean that you can't say a single word in swedish which would result in 0 * 1000000 that is 0 times better then

Well, I did what you said and made corners out of the curves. First with the short wire and then with my new probe. Yes, I've got the probe in my possession now.
I believe it's working good now still need more callibration though in other ways. I recently used the new probe to check my PIC PWN project with it and I can see the pulse. But it's moving a bit back and forth, like it's shaking. Maybe this is just because I'm not using capacitators in my circuit just yet, making it unstable?

Regards

Morgan

 

[Windmiller]

By the way here's a picture taken from the testing of PWM servo signal, with some questions. I don't know if this is right..

Is this normal or is it still overshot? This ugly trace only happens in one setting (50mV). Otherwise the trace is looking much better with sharp corners.

 

[Windmiller]

Ok, I think I got it now.. it's working like I want it to, so I must say thank you Brian for ALL the help you have been giving me from the start of this thread. I bought the thing for only 4.5 pound, so I believe it was a bargain

Thanks again

Regards

/ Morgan

 

[betwixt]

Sorry for not replying sooner. Well done at some excellent diagnostic and repair work and for getting a bargain oscilloscope too!

My Swedish - well I did start learning many years ago when "Radio Sweden" English service started lessons but I missed most of the programs and I've forgotten the ones I did hear. It must have been about 40 years ago!

Brian.

 

[Windmiller]

Hello again Brian.

You're not obliged to answer all my questions so that is ok with me, but I'm very glad you take the time to do so.

That was a funny story, I didn't even know about the Radio Sweden until now. So they had lessons in swedish back then? Maybe they still have, I took a quick glance over their webpage, but back then it must have been 100% airborn radio broadcastings.

Regards
/Morgan

 

[brosskgm]

I have one of the NRI 2500's, I have the instruction to calibrate this unit it needed. I'd be able to scan it in this weekend if it will help. Then maybe I can get some help to get mine working. I never used it after I built it and it was stored all these years wrapped in plastic with no dirt or moisture. A couple month's ago decided it would be good to start learning the electronics repair all over again. I was able to get it to calibrate once, and then it acted up and now has lost certian areas during the calibration stage to read the 110v wave from inside the unit. I've pulled it apart a few times to check solder joints etc, but nothing has helped. If I move the AC GND DC switch fast or turn or move another switch quickly it brings the wave on the screen only breefly and then blank. I then tried to set the two dots setting but that didn't work either, so what I've read here, the caps could actually be bad. I haven't a working ESR meter yet if they woudl even be able to test those that are this large.

Thanks
Bob

 

[betwixt]

Electrolytic capacitors have a relatively short life span, especially the low grade types that were probably used at the time the oscilloscope was made. They last much longer if they are used occasionally so storing it for a long time may have reduced their life.

It sounds from your description though, like the switch itself may have corroded. That switch doesn't do anything clever, it just blocks or allows DC from the input socket from reaching the Y amplifier. It sounds like it may be open circuit with the contacts occasionally touching as you move it. It's quite likely the contacts are flashed with a silver coating which has turned black over time. If that is the case, the best method I have found to fix them is to carefully dismantle the switch, clean it with a domestic silver polish, make sure all the cleaner is removed, then smear the contacts with a thin film of silcone grease to prevent them oxidizing again.

Brian.

 

[Windmiller]

I have one of the NRI 2500's, I have the instruction to calibrate this unit it needed. I'd be able to scan it in this weekend if it will help. Then maybe I can get some help to get mine working. I never used it after I built it and it was stored all these years wrapped in plastic with no dirt or moisture. A couple month's ago decided it would be good to start learning the electronics repair all over again. I was able to get it to calibrate once, and then it acted up and now has lost certian areas during the calibration stage to read the 110v wave from inside the unit. I've pulled it apart a few times to check solder joints etc, but nothing has helped. If I move the AC GND DC switch fast or turn or move another switch quickly it brings the wave on the screen only breefly and then blank. I then tried to set the two dots setting but that didn't work either, so what I've read here, the caps could actually be bad. I haven't a working ESR meter yet if they woudl even be able to test those that are this large.

Thanks
Bob

Hello Bob!

I recognise this partly from the beginning of this thread, when I got a similar problem. When I had turned almost everything inside the oscilloscope too much, and I noticed later that it didn't have to be much before the trace would dissapear. I didn't know then which one I had turned too much because I had turned everyone, but while switching and turning I could see the trace for a second too dissapear into a blank screen. I know now that it was the VR???? pot. that I had turned too much (I must open the oscilloscope later to see which one it was). But if you know and already have been trying to turn them without any positive result, then it must be something else maybe like you said that the caps are out.

I hope Brian will look into this also, because he's the one who tought me almost everything about how to fix an oscilloscope

You got the calibration manual? That would be very nice to have a copy of, I have been looking at a completely different calibration manual. That is not working for me at all..

Regards

/ Morgan

 

[brosskgm]

Thanks for your reply Brian.

I'll start looking for those caps to start with. With the switch, I can even move the wire plugged into the J105 and the 6 sine waves will appear momentarly. I have been able to get this calibrated a couple times and then the day I started to test a 5v setup on the lab breadboard is when it gave nothing and this problem started.

Bob

---------- Post added at 04:04 ---------- Previous post was at 04:01 ----------

Hi Morgan,

I don't have the manual, I have the assembly instruction, and calibration instruction, I will work on getting that scanned in. it's 26 pages. The book is held together with staples. So much easier than what they do today.

 

[brosskgm]

Hi Morgan,

Send me a private message or how it's done here with your email address and I will get it to you. It went a little quicker than I thought it would. This is an old book but it was only 26 pages. I scanned each one individual and will zip it.

 

[Windmiller]

Hello Bob, thank you very much for the effort to scan and zip it

 

[brosskgm]

Hi Brian & Morgan,

I have a question that I'm sure will be kind of crazy to ask, but here goes.

Will getting the numbers off the capacitors I ran across 3 that are 0.5uf 1500 WVDC
What is the "W" in WVDC?

They have a name on them I have never seen or reconize Oil-Con. Is this a special type of capacitor. Most of these have the leads
on the oposite ends also.

Thanks
Bob

 

[betwixt]

I think Oil-Con is just a manufacturers name. There are such things as oil filled capacitors but they are usually found in very high voltage applications such as national power distribution grids.

The W in WVCD is a term not used very often these days it means "Working voltage DC". Modern capacitors are rated with a maximum voltage, one that you shouldn't exceed, the ones you have are rated to be suitable at 1500V without telling you they may be able to withstand more than that.

Brian.

 

[brosskgm]

OK, thanks Brian, did you want a copy of the assembly/calibration steps for this Oscilloscope? I zipped it up and put if on a download server. It's just over 43 megs.

 

[betwixt]

No, but thanks for the offer. I do not have one of these oscilloscopes so it would not be of much use to me and my internet connection is so slow it would take hours to download.

Brian.

 

[brosskgm]

I just had a big shock. One of the 0.5uf 1500vdc capacitors are $50/ea and there is three of them in it. MAybe it's time to work on getting or trying to build one to run on the laptop I guess.