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Tektronix 2235 troubleshooting.

Discussion in 'Troubleshooting and Repair' started by AcousticBruce, Apr 1, 2013.

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  1. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    Hello folks. This looks like a great place to learn and help others in I am happy to be here.

    I checked out this Tek 2235 scope and saw that both traces created a nice 4khz sinusoid no problem. So I bought it.

    I just plugged it up for the first time today and could not get a trace. I pushed every button and turned every knob and nothing happened until I hit beam find. When I hold down beam find I can see the beam for either channel A or B or both. I can position and focus no problem with beam button held down.

    I am not sure exactly how a scope with beam find is supposed to act, but I did hook it to a function generator anyway and could get a sinusoid but only in a small area with beam fund pushed in.

    I would like to trouble shoot this and figure it out. I am very much into learning why this happened and how to fix it. I am more interested in fixing this for educational reasons. If anyone can be so kind into pointing me in the right direction or perhaps run me a few pointers on this post. It would be amazing!

    Thank you for your time.

    Edit: I have a manual as well as a service manual and even a Tektronix oscilloscope troubleshooting book.

    Edit: I uploaded this video for better explanation...

    Edit: I have subscribed to Yahoo Tektronix group but I am a week or two away from being accepted!
     
    Last edited: Apr 3, 2013
  2. shrtrnd

    shrtrnd

    3,701
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    Jan 15, 2010
    My first advice is to get a manual for it, so you can follow the circuits.
    A lot of guys here say there are Tektronix-geared websites available on Google that
    will help also.
    We can reference circuits and give advice, but it'll be a lot easier if you've got a manual
    to know what we're talking about.
     
  3. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    Yes I have all that. I have been looking through the books to see if I can narrow down the problem. I am looking for more of answers like this..

    Oh nothing is working except when you press beam... hmm that might be the power supply or could be....


    Then I can learn a little also. I do have the books.. they are not so straight forward for me.. forgive me I am new to electronics but I am careful and complete when I learn.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,293
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    Jan 21, 2010
    Best to eliminate simple things first.

    My guess is that (being new to electronics) with all the twiddling etc, you've set it into a strange mode.

    Likely you're not seeing beams because you've got some sort of triggering set up on both channels.

    I'd go through the instruction manual and carefully follow whatever setup instructions it has just to be sure that this isn't the case.

    The scope should have a calibration output and you should be able to use that to get something to display.
     
  5. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    Your right Steve. Eliminate the obvious easy stuff first. Just so you know I did do 4-1 in the manual which is a quick start on getting your scope started.

    Cannot get any trace on either channel to show. I flipped and pushed and turned every button and dial just to see if anything at all would happen. Nothing. Only beam find displays anything on the crt. The sad thing is it was working when I bought it. I put it in my jeep and took it home... it didn't work even though it did hours before.

    Any other ideas?
     
  6. davenn

    davenn Moderator

    13,496
    1,830
    Sep 5, 2009
    well the obvious thing that infers is it didnt like the travel... probably got a bit of bumping around. I would be looking at all the internal connections
    fly leads to boards and even plugin boards.... make sure ALL are plugged in firmly

    Dave
     
  7. shrtrnd

    shrtrnd

    3,701
    462
    Jan 15, 2010
    I'd go with *steve*'s advice first.
    Assume one of your controls is locking your signal display out.
    Are you in a Triggered Sweep Mode?
    Do you have a bad probe cable installed right now?
    I've seen a lot of problems with the cheap push-buttons on Tektronix scope not working
    right, but if it WAS working, the odds really are, that something in your control panel
    is locking-up your display.
     
  8. AcousticBruce

    AcousticBruce

    43
    0
    Apr 1, 2013
    I did follow Steve's advice and even did it again just to be sure. I own 3 probes. Tried them all. But even without probes you should still be able to get the traces to show and adjust the sec/div.

    I did pretty well in previous years when I owned a Tek 2445 4 channel. And a Owon SDS6062 before I foolishly sold them because I needed money. I have used Analog and Digital scopes and know the basics pretty well especially when it comes to calculating sec/div and volts/div.

    Just curious... what types of buttons / switches, if activated, will cause nothing to EVER be displayed? I mean NOTHING, even in a dark room, ever happens until I hit beam find.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,293
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    Jan 21, 2010
    Anything to do with triggering can cause this. Also controls that position the beam -- if they have a loose connection (or just poorly adjusted), the beam can be pushed right off the display. Clearly a brightness setting can also do this (brightness may inhibit the beam finder too, so I think that's unlikely).

    If you've already owned scopes then you should have the basic ability to set up the scope in a sensible manner. Your initial posts seemed to suggest you had no (or little) experience and that is why I made the suggestions I did.
     
  10. AcousticBruce

    AcousticBruce

    43
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    Apr 1, 2013
    Yeah I still consider myself a noob though! haha



    Here is a Youtube video I shot about 30 minutes ago.

    So here is a video from Youtube that will add a tone of information for you guys. A couple things I mentioned that may be confusing from the video is the instant start up waveform that goes off the screen. That was NOT happening before I shot this video and hooked up the function generator. Now it is a normal thing (when its warmed up of course)... not sure what happened there.


    Btw I have subscribed to Yahoo Tektronix groups but they are very selective and it takes a few weeks.... sheesh.
     
  11. shrtrnd

    shrtrnd

    3,701
    462
    Jan 15, 2010
    Watched your video.
    You're not sweeping.
    May be one of the trigger controls is bad, maybe a problem in the sweep circuit.
    I'd be looking at the trigger control manual switches first.
    As I said, the push-buttons are particularly suspect in a lot of the scopes I've worked on.
    I have assumed (in error) in the past, that depressing some of those push-buttons was
    engaging or disengaging the circuit, only to find bad switch contacts.
    It ain't much, but that's what I'd start with before I went troubleshooting the sweep circuits themselves
     
  12. AcousticBruce

    AcousticBruce

    43
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    Apr 1, 2013
    Edit: For all those that did not see a video of the scope and how it is acting, here is a youtube vid.

    This is exactly what type of info I was looking for. I will let you know as I learn more.


    thanks again.
     
    Last edited: Apr 3, 2013
  13. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    Question about XY mode. Does this mode need triggering?

    I cant get a signal even on this mode. Would this eliminate a block needing troubleshooting?
     
  14. shrtrnd

    shrtrnd

    3,701
    462
    Jan 15, 2010
    When you use the X-Y Mode, Q737 is biased 'on' to establish a HI on U760 pin 12, so that the A & B Sweeps are disconnected from the preamp outputs.
    When you're NOT using the X-Y mode, Q756 is biased 'on', and the X-Axis signal is
    shunted to ground.
    To USE the X-Y display, you're supposed to obtain a basline trace first. I saw on your video
    that you can display the two dots, and you have position control of them.
    So it's 'possible' this function is (stuck) disengaging the sweep, but there are a lot of
    other things that could be affecting the sweep also.
    For example: the 'A Trigger SGL SWP Reset' Push-button will hold off the sweep until a trigger signal is received. When that trigger signal is received, the next trigger pulse
    initiates the sweep, but only a single trace will be displayed.
    See what all those little push-button switches can do to stop the sweep?
    Yeah, you might have a circuit problem, but I always try to eliminate those cheap
    push-button switches as culprits first.
     
  15. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    In the vertical mode area there is a push button called "Trig View" (S200)

    All contacts have continuity weather pushed in or not. I did not remove this from the circuit board but all power is off and unplugged.

    The push button has 1 terminal and then 3 on the other side. All of them in ANY order have continuity. This doesn't sound right does it? Could this cause a problem?

    here is a picture
    https://www.dropbox.com/s/x8hya3ay91kf2i6/trigButton.jpg

    Edit: Interesting enough when it is on holding the trig view button down changes the display. But I have triple checked and found continuity when it is off. Im Confused.
     
    Last edited: Apr 4, 2013
  16. shrtrnd

    shrtrnd

    3,701
    462
    Jan 15, 2010
    The only way to know for sure, is to desolder the switch, and THEN check it for continuity.
    My computer won't let me access your picture.
    Some pushbuttons are affected by the other push-button on-off configurations.
     
  17. AcousticBruce

    AcousticBruce

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    Apr 1, 2013
    Well thats the next step then. Ill start taking out some circuit boards.

    Will follow up with what I find.
     
  18. shrtrnd

    shrtrnd

    3,701
    462
    Jan 15, 2010
    I've got a 2235 I bought for $1800 in 1981.
    Sure, it's needed some work over the years, but it's a very good scope.
    It's worth some effort to get it going.
    Patience pays off.
     
  19. AcousticBruce

    AcousticBruce

    43
    0
    Apr 1, 2013
    Wow the price has gone down a bit ay.

    I recorded another video showing more front panel tests before I delve into the unit.



    I got some horizontal with p-p auto button, but only a small amount.

    Not sure if this can pinpoint anymore. I have held off from removing circuits for a bit until i get more of an idea of all the places it could be.
     
  20. shrtrnd

    shrtrnd

    3,701
    462
    Jan 15, 2010
    You may have to talk with one of the Tektronix groups when you're able to get to them.
    1) Do you have the Operator's Manual, or the Service Manual?
    2) I'd take the probes off, and use a 50 Ohm coax cable to input the signal, just to
    be sure your probes are compatible with this scope, just to eliminate that as a possible problem.
    3) You're still not sweepiing.
    4)the P-P Auto-TV Line permits triggering on waveforms with repetition rates of 20Hz or larger.
    The sweep should run free without a trigger signal.
    This panel section (P-P Auto/Norm) is for TV servicing.
    5) I checked your latest video, I'll check the service manual I have and see if I can find
    something useful.
    6) You said you're not heavy into troubleshooting, so advice here might be taking things
    for granted that you may not be familiar with. If you try to work with the components,
    make SURE you've got the scope unplugged, and stay away from that anode wire to
    the CRT.
    7) Don't get frustrated if you can help it. This might take a little time and work to isolate.
     
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