Tec 475 Oscilloscope questions

Discussion in 'General Electronics Chat' started by spinnaker, Dec 5, 2009.

  1. spinnaker

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    Oct 29, 2009
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    Got my Tec 475 off of eBay a few days ago. I have been playing with it, trying to refresh my scope skills that I have not used in 25 years.

    My scope has a 2nd Volt Div setting. They are both on the same knob. One is at the 11 o'clock position. The other at 1 o'clock. The one at 11 is lit and I turn the knob to that position and it sets the scope to the selected voltage position.

    But the knob only turns so far. There are additional settings that only come in line with the one at 1 o'clock. But I can't seem to get that position to lite. How do I get that position to light.


    I have a cheap 10X probe. If I set it to 10X for calibration and dial down the voltage division settings so I can see a square wave, I am seeing "noise" all along the tops and bottoms of each transition. It is probably only a few micro volts but it is there. Is this normal?

    I assume it probably has something to do with the quality of the probe?


    If I ground the probe, I do not see the "noise".
     
  2. VoodooMojo

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  3. spinnaker

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    Thanks. I have a copy of the manual but I was not finding info on the voltage setting.

    I was looking in the wrong manual. I found more details on operation in another manual. Apparently that voltage division lights up when a special connector is connected to the scope.


    Yes I have a trim adjustment. But this adjust the shape of the square wave which I have done.

    I am talking about what looks like noise all along the tops and bottoms of the square wave.

    The attachment is a picture taken of my scope display with probe set to 10X attached to the calibration port. Scope was set to 10mv per division.
     
  4. SgtWookie

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    Jul 17, 2007
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    Either your focus is off, your astigmatism is off, or your intensity is set too high.

    You won't be able to see the vertical transitions very well; they happen very quickly.
     
  5. spinnaker

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    Focus is fine. I can't say as much for the photo though. But it does pretty much depict what I am seeing at the tops and bottoms of the waves.


    Intensity is set low though, the adjustment is a bit rough, it can be adjusted to something decent. Not sure what the pot looks like in there so I am not sure if I want to try and get into replacing or cleaning it.


    I tried adjusting the astigmatism but that was no help.


    The output of the calibration port is only 300 micro volts.

    I probably won't be working in that range anyway.

    I am also having trouble seeing the leading a trailing edges of the wave transition (the actual transition trace itself) . They are barely there. I have to jack up the intensity to really see them at all.

    Is this normal?



    I hook it up to a PIC demo board and output a square wave.

    But I see the same problem with the transition trace.


    Thanks for all of your help.
     
  6. beenthere

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    A Tek probe has a shorting pin that contacts the shiny ring around the BNC connector. That causes the sensitivity indication to change automatically.

    The cal output should be .5 volts @ 1KHz. Check your manual. Use that 10X probe to measure a battery. If the deflection is not reasonably accurate, that may be the problem.
     
  7. spinnaker

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    Thanks good to know. I assume this would be a high voltage probe?




    Yes I think I wrote cal is micro and I guess I meant to write 300mv.

    I connected a 1.5V battery as you suggested.

    At 20MV per division ( I need to adjust the horz position to see the trace. I am seeing what looks like about 1mv or so of noise. The trace is not a nice sharp trace. It kind of looks like it is out of focus but it is not.
     
  8. VoodooMojo

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    this is the typical trace you will see for calibration.
    40mv @ 1KHz. on my scope.Transition time is very short.
    try wiring direct using wires instead of your probe.
    this will verify if there is a problem with your probe.

    the 475 is a nice unit. Want to sell it?
     
  9. ELECTRONERD

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    May 26, 2009
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    I have the exact same scope! ;)
     
  10. VoodooMojo

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    Dont you just LOVE it?
     
    Last edited: Dec 6, 2009
  11. SgtWookie

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    I'm wondering if you might have a cap going bad in your power supply, or more particularly in your vertical deflection circuit?

    Replacing all of the electrolytic caps in an old 'scope is a very good PM item. If a cap shorts when it dies, it could take a number of components with it.

    Buy replacement caps from a high-volume authorized distributor, like Digikey, Mouser, Newark, Allied, etc. You don't want to use surplus caps from someplace like E-bay; you don't know how long they might've been sitting on a shelf someplace.
     
  12. spinnaker

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    Thanks Sarge. That is exactly what I was thinking too (drawing from my old TV repair days).

    I popped the cover. The could news is that there are only 6 large electrolytic caps. There seem to be none of those really small electrolytics that litter many circuit boards.

    The bad news is that it would be a huge challenge to replace them. They appear to be soldered to a PC. To get to the PSU PC, it looks like, I need to first remove a large PC that is mechanically coupled to at least 6 front panel controls.

    While I am confident I can replace the caps themselves, I am not confident that I can get the whole thing put back together again.

    And I am not 100% sure that will fix the problem.

    And some more bad news. I cannot seem to cal the probe on channel 2 when it is DC coupled. I can get a nice sq wave if using AC coupling but nothing even close to a sq wave on DC.


    Not sure what to do here.


    1. Live with it as it is. I probably won't be dealing with millivolts anyway. But risk a cap blowing out. And not really sure how useful it really is.
    2. Try to fix it and risk not being able to get it back together.
    3. Pay to have it fixed.
    4. Sell it and probably take a loss. I don't think I could in good conscience sell it without disclosing what I know.
     
    Last edited: Dec 6, 2009
  13. SgtWookie

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    Hmm. Well, it would help a great deal if you had a service/repair manual for the 'scope. I seem to recall people saying that some Tek front panel controls can be "a bit fiddly" to remove/replace.

    Do you have a digital camera? Taking a detailed series of photos during disassembly will help a great deal when it comes time to put it together again.

    Nope. However, you will be in a much better position to give the chassis a thorough cleaning with isopropyl alcohol. And you certainly know that eventually, electrolytic caps will fail - it's just a matter of time.

    I have a Tek 2215 with a dead channel 2; haven't been worried about it as my other 'scope works OK on both channels. I did find a service/calibration manual for it though, so when I finally decide to "dig in", I'll have all the info I need to forge ahead.
    I think your very first step should be to locate a service & calibration manual for your particular 'scope. Just going through the calibration routine may help a great deal in narrowing down the problem area(s). There will also be a troubleshooting section which will help you get there quickly.

    Since you have a really obvious problem with channel 2, that would be a good place to start with the troubleshooting. It could be that the problem with channel 2 is coupling noise onto the supply for both channel amplifiers.
     
  14. spinnaker

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    I do have the repair manual on PDF.


    Hopefully it is just calibration as you said. IBut I do not have access to other equipment. The scope is all I have right now. Wouldn't I need other equipment?



    If it does need actual repair, I wonder if this is a bit much for me to tackle.

    Remember I have not touched this stuff for 25 years. If you count true component level troubleshooting more like 30 years.

    Plus I do not have another scope.

    A friend does have a scope. And some troubleshooting experience. The problem is that he lives about 30 miles away. We actually have not talked in almost two years. :) We are supposed to go out for dinner Wednesday night and he will be stopping over before hand. I'll run it by him and see what he thinks.
     
  15. BillB3857

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    Feb 28, 2009
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    To check out the "fuzzy" top and bottom of your calibration signal, turn the sweep speed down and go to LINE SYNC. It could be that you are seeing 60HZ interference with the calibration signal. The fact that it goes away if you ground the probe is a clue to your problem.
     
  16. spinnaker

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    Thanks Bill. Where would I find "LINE SYNC" ? I am not seeing it on my scope. There is a setting below .5 seconds that appears to say "x y". That is all I see.

    BTW I checked on the calibration instructions and the second thing it asks for is a calibrator. :)


    Oh and everything appears to be very clean inside. Not sure about corrosion though. :)
     
  17. BillB3857

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    There should be a selection for LINE somewhere in the lower right corner of the unit. That is where you would select the trigger source, such as CHAN 1, CHAN 2, EXT, and so on. LINE SYNC is pretty standard on a scope and is obtained from an sample of line frequency within the scope. The TRIGGER LEVEL control can be used to select level within the sample that will start the sweep.

    If you set your sweep speed to 5ms after you have established LINE SYNC, touch your probe to the CAL test point. If my guess is right, you should see three positive excursions and three negative excursions of the calibration wave form, sort of like a fat worm.
     
    Last edited: Dec 6, 2009
  18. spinnaker

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    Found it! Yikes I remember when I could read a scope panel with no trouble. They must be making the printing on those scopes smaller these days. :)

    If I put it on 5ms, I am seeing a sq wave but I cannot sync it. No matter what the wave rolls across the screen. The trig light is on but it still rolls.
     
    Last edited: Dec 6, 2009
  19. BillB3857

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    Lower your vertical sensitivity and use the vertical position control so you only see the tops or bottoms of the square waves. See if they look like they are riding on a roller coaster. If the roller coaster track is steady, it is your 60 hz.. The reason it is rolling is that you are syncing on the 60 hz line and your cal signal is 1khz. Not an exact multiple.
     
  20. spinnaker

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    They are "fuzzy" but square just like normal sync with the exception that I cannot sync the display.

    I am not seeing anything like your attachment. Is that a scope with a 60hz problem?



    Should I be able to sync with line?

    Boy talk about buyers remorse. :) I hope I don't have a $200 boat anchor here.

    I have repaired televisions before but this is a whole other animal.
     
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