Help evaluating 2nd oscilloscope

Discussion in 'General Electronics Chat' started by spinnaker, Dec 22, 2009.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I just got my 2nd oscilloscope from eBay. If you recall the first had a problem with what looked like noise at the tops an bottoms of the calibration sq, wave on the first one that I purchased (since returned). Plus the first scope had no vertical deflection, when touched to a battery, when set to DC coupling and I could not adjust the probe for a good sq wave on channel 2 (probably related to the no DC deflection problem).

    Anyway I got the new scope and I am afraid that I am seeing some of the same problems but not sure. I am getting a good square wave on both channels. And I get deflection from both channels on DC coupling when hooked to a battery. This scope is a Tek 475A with DMM option. Of course it is an analog scope.

    But I am still seeing what looks like noise at the tops and bottoms of the waves when using a 10X probe under certain conditions.

    There is a control on the scope called Vert Mode / 100 or 20MHZ BW.

    The manual says that this switch limits the bandwidth of the vertical amp.

    Here is what I found with that setting. All tests were done with vertical deflection set to 5MV / division and the probe attached to the calibration port.


    If I set the limit to 20MHZ and the probe to 10X I see a good clean wave form. A nice crisp line at the tops and bottoms of the square wave.

    If I change the probe to 1X, I can see what looks like small spikes crawling on the tops and bottoms of the square waves. They are 1 mv at best but they are there. I noticed that I also see these crawling spikes when I touch the probe while it is attached to the calibration port.

    If I change the BW limit to 100MHZ I and 1X probe, I can see the "crawling spikes" around 5MV with the 1X probe. The noise is still there with the 10X but not nearly as bad. On the scale it is maybe 3mv but of course tnat is really divided by 10.

    I see a similar behavior on the "unlimited" bandwidth setting.


    I see a similar behavior when attached to the

    Trace looks nice and clean on all settings when probe is grounded.


    Is this normal? Maybe it is just a case of cheap probes? If it was line noise (power supply), shouldn't I see it on the 20 MHZ limit setting?

    Maybe it is my nearby PC (10feet) away). I will shut down everything except for the the scope and post back after I submit this.



    Other questions.

    How easily should I be able to see the rising and falling traces of the square wave. I can barely see them. I have to jack up the intensity pretty far to see them. I cannot remember, is this normal for analog scopes?


    Should I be able to sync on Line trigger? I don't seem to be able to do it when the probe is attached to the cal port. The sq wave keeps scrolling no matter how I adjust the trigger level. I can get it to slow a bit but that is about it.
     
  2. spinnaker

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    Update. I shut down everything nearby with the same results.


    Plus I noticed I see those "crawling spikes" if I grasp either channel 1 vertical deflection knob. I see this even in the 20Mhz BW limit / 10X probe where I saw no noise. I see no such noise for other controls.

    Could my magnetic personality be at fault here? :)
     
  3. someonesdad

    Senior Member

    Jul 7, 2009
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    I haven't used a 465/475 since the 70's, so I can't help with much. But I've got some opinions on your last two questions:

    1. It can be hard to see the "vertical" lines in a square wave; it's harder the faster the transition is. Consider: even if it's a slow transition (relatively speaking) like 1 μs, the electron beam is not on the phosphor for very long in the vertical part, meaning relatively few photons per unit length get produced than, say, for the regular part of the trace. In other words, what you're seeing is normal -- I have the same problem on my analog scope. The digital scopes fix this by (optionally) drawing lines ("vectors") between each sampled point.

    2. Not being able to get a stable display of the calibration waveform when using the line trigger just means the calibration waveform isn't derived directly from the line frequency. It's nothing to worry about.

    I'd recommend doing some testing with a triangle wave at various frequencies (a triangle wave has only odd harmonics like the square wave). Your eye can easily pick out where things don't look right.

    As to the "crawling noise" problem, you should try to isolate systems to see where the problem is. Put the probes on another scope and see if the problem goes with the probe. If you can get your hands on a function generator (or build a 555 oscillator) , set it up to output a square wave and see if the problem persists. These things will give you more clues. They might even be things you can simply ignore.

    If worse comes to worse, it would probably be worth it to find someone to put your scope into good working shape (or you might try it yourself if you have the documentation). Those models of scopes were (and still are) workhorses and would be worth fixing (as long as the cost isn't too steep). Of the digital scopes I've used, I've not found one that triggers as well as most of the older analog scopes.
     
  4. spinnaker

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    Oct 29, 2009
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    Thanks for the input.

    I don't have another scope to try or I would have tried another scope.

    I don't have any 555 timers on hand. But something just as good. A microcontroller. I get similar results as on the cal output. I just created a square wave. I think they can also do triangle waves but not sure how to do that.

    Also saw the same problem with a 1.5 VDC battery.


    Why is it that this "noise" or whatever it is goes away on the 20MHZ limit???

    Again the "noise" or whatever it is only appears at the lowest mv vertical deflection setting.

    I doubt I will be working much in that range anyway.

    Another question. Assuming I keep this one. The seller said two knobs were "chipped". Well "chipped" is an understatement. The channel 2 vertical deflection knob has a huge chunk missing out of it. It works but it would be nice if I can replace. Looks easy enough. Any idea where I can lay my hands on a knob? It is just the outer plastic grip that is broken.
     
  5. lmartinez

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    Mar 8, 2009
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    Are you the buyer or power seller?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    A manual for your 'scope can be downloaded here:
    http://bama.edebris.com/manuals/tek/475adm44/

    I wouldn't be upset with a little bit of noise.

    It is a good bet that you have some (if not all) caps that are rather advanced in age. You need to expect this kind of thing with used test equipment.

    The first thing I would do is locate a service and calibration manual for it - that link might be it.

    I would plan on replacing all of the electrolytic caps. That alone will protect your investment for many years, and will likely clear up the noise you're seeing.
     
  7. spinnaker

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    I am the buyer. I want to keep the scope if it is worth keeping. I sent the first one back with a similar problem but worse. I could not get anything that came near a square wave on channel 2 when using DC coupling. No vertical deflection at all when connected to a battery. That is why I sent the first back.

    Since I have the same problem with what I think is "noise" on this scope as the first makes me think it is either my inexperience or it is some other source. I used a scope extensively in my former line of work. But that was almost 30 years ago. I have not touched a scope since. I wish I had more experience. Just getting back into the hobby now.
     
  8. spinnaker

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    OK tks Sarge.


    I guess I will keep this one then. It works a lot better than the first.


    You suggested cal on my first scope too. But the cal I was looking at required a special calibrator. Are there procedures that don't require such equipment? Maybe I was looking at the cal procedure they might do at a cal lab?




    You gave the same advice on the first. Replacing the caps worries me.

    First as I mentioned, they are really buried in the scope. Your advice on taking pictures was an excellent one.

    And there are so many of them! Around 8 large cans if memory serves. I would think expensive? Maybe as much as the cost of the scope? But more important, the chance for me to make a mistake! :)

    I do have a friend that has torn down his Tech before. I guess I can consult him for advice on how hard it would be.


    But my question still stands. Why does the noise appear to go away on the 20MHZ setting?


    Also is anyone knows of a source for that knob. I imagine they are pretty standard, Seems an easy thing for a noob like me to replace. :)
    I might just post another thread at a later time on just that subject. It might get more notice.
     
  9. lmartinez

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    Mar 8, 2009
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    You can easily buy AN ANCIENT 300MHz bandwidth oscilloscope from ebay at a very low price without putting any investment into it. As a buyer you have plenty of options..I would advise you to get educated on how an oscilloscope functions before buying it.
     
  10. spinnaker

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    I pretty much know how they function. What I don't remember is all the fine detail. I already bought one so it is pretty much moot at this point but actually using one is the only way to learn.

    And I have been monitoring prices for a few months 100-200MHZ are running right around the same in dollars.
     
  11. someonesdad

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    From what I've read, SgtWookie's advice is good: electrolytic caps have the poorest reliability over time of the common electrical components. It probably would be a good idea to replace them, but, as you mention, that could mean lots of parts, time, and expense. If I was faced with this problem, I'd look into making a circuit to estimate the ESR (equivalent series resistance) of those capacitors and only replace the ones that were obviously bad.

    I suggest you google "build esr meter" and similar search terms -- spend a bit of time learning about how electrical troubleshooters use ESR to help find bad caps. Basically, low ESR is good and high ESR may indicate a bad cap.

    The attached picture shows the basic idea -- R1 and R2 form a voltage divider and the cap under test is connected as shown. The measurement (it should be a reasonably high input impedance) monitors the voltage across R2. When a cap with the desired low ESR is connected to the tester, you can see that it will effectively short out the voltage across R2 -- leading the measurement tool to indicate a small voltage. Conversely, a bad cap with a high ESR will not affect the measured voltage very much.

    You can, in fact, make this temporary ESR meter with a function generator and any tool that can measure the AC voltage across R2. Reasonable choices for the signal from a function generator would be a 0.1 to 5 volts peak to peak sine wave in the frequency range of 10-100 kHz. Use the lower voltages to avoid turning on semiconductor junctions. Possible starting values for the resistors are R1 = 1 kΩ and R2 = 5 Ω (use noninductive resistors); use what works for you. A scope makes a good measurement device, but you won't have one while you're troubleshooting your scope. An AC voltmeter or VTVM could be good choices.

    A good capacitor will have, roughly, an ESR on the order of 0.1 to 1 Ω (or much less). You can "calibrate" this ESR tester by using some noninductive 0.1, 1, and 10 Ω resistors.

    This simple circuit can be put into a box and have two probes coming out of it. You can then probe the suspect capacitors while they are still in-circuit; a good capacitor will show a signifcant drop in voltage and a suspect capacitor will show little drop in voltage.

    A shorted capacitor will appear to have a very low ESR, but will still need to be replaced (i.e., it's a false negative).
     
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  12. maxpower097

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    You can usually find a couple on craigslist too. Since they are so heavy and most people end up with them from buying out storage units or auctions they usually want nothing for them.
     
  13. spinnaker

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    Thanks for the advice. I realize I have a very nice scope here and with a little work (thanks to advice from you, Sarge and some of the others), I could have one that is next to new.

    But I am wondering if I should not have taken Sarge's advice on the old caps from my first scope as a warning and perhaps looked at other alternatives.

    If the scope were totally dead, I would have no problem tearing it apart to try and getting it to work. Hey there would be nothing to lose anyway. But I have a working scope here.

    I am just getting back into the hobby. I was an electronics tech eons ago but have not touched anything since and as a tech, I dealt with mainly digital circuits with some minor analog troubleshooting.

    So as you may understand my apprehension of tearing the scope apart.

    I am just wondering if I should not keep it for a little while, reestablish my former scope operation skills, while thinking of reselling it.

    I was looking at one of these. But what worries me about this scope is it looks like the SW is from China so who knows how good the English would be or if there would be continual support. The one big adantage of one of these is no large filter caps! :)
     
    Last edited: Dec 23, 2009
  14. someonesdad

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    spinnaker, I'll give you my opinions; YMMV. You'd probably be fine to just leave the scope as is if it appears to be mostly working. A little noise may not be that big of a deal, especially if it's not a wideband noise (i.e., it's easy to recognize).

    A number of years ago I dreamed of having my own electrical measurement bench at home. I was spoiled by working at a company in the 1980's in a division that pretty much let us buy whatever instruments we wanted. I knew there was no way I could afford those fancy instruments, so I pretty much filed the dream away in the same place with the dream of living in the girl's dorm in college (that one was way before the permissive mores we have today...:p).

    Fast forward to the 90's when ebay showed up. It made me realize that it might be possible to realize my dream by buying used equipment. In fact, that's what I did over a period of 6-8 years. I learned how to find and buy the things I wanted -- basically, you study to see about what things are worth, then decide what you're willing to pay for something. Then you bid when you see an appropriate item. You might have to bid on many items before getting one of them (one instrument I have took over 6 years of bidding to get, but I got a superb one worth 10 times what I paid for it).

    Anyway, like you, I wanted a scope. In the 1990's I was a firmware engineer and we had an HP scope in our group that I'd occasionally need to check out something in hardware. I came to really like that scope because I could figure out how to use everything on it without having a manual. That led me to buying a similar HP 4 channel scope for $600 -- WAY more than anything I had bought before on ebay. But it was a great purchase -- the scope works great and I've used it for years. While I still like the triggering abilities of analog scopes better than the digital scopes, I doubt I'd ever go back to using an analog scope -- digital storage and the ability to get the waveform to the computer are just too convenient.

    Now, if I had to do things over again, I'd consider spending $750-$1500 for a new digital scope. The scope is just too important an instrument to scrimp on -- yet, I know that it can be hard to raise that kind of hobby money. Thus, I'd sell some other things and/or do some part-time work and save the money to acquire one. I know I'd have to be patient, as it might take a few years to save the money.

    I haven't used one of those USB scopes. If I needed to travel and troubleshoot things remotely, it might not be a bad thing to have. But I'd NEVER want one on my bench, as I find the scope with display and knobs just too familiar and convenient a paradigm.

    I would bet that most of these are made in China and, frankly, I've seen some pretty poor examples of test equipment that came from China. The real problems are: a) documentation and software that can be nearly useless and undecipherable and b) the worry over being able to get service/parts. Saving a bunch of money only to need service a few years later and finding out the thing can't be fixed will obviously be false economy after the fact.
     
  15. spinnaker

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    Thanks

    As I said a case of buyers remorse. I bounced it off a couple of other people and they said the same as you keep it but don't mess with it.

    In the short time I have been on this forum, I have learned that Sarge is one of the most helpful and patient people on here (not there aren't a number of others because there are plenty) so I know he is giving his best advice and I know it is good advice. But I know my limitations too. So I'll keep it for now an leave it as is for now.

    I think I might add a usb logic analyzer to collection at a later date. I have been doing some reading and there are some decent ones (basic hobby wise) out there for a fairly low price.
     
  16. VoodooMojo

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    Nov 28, 2009
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    now your going to get the sgt bucking for an E-6! :)
     
  17. someonesdad

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    I agree -- he's a total asset to this forum and has my highest regards!
     
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