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Tektronix 2235 Sweep Generator (horizontal mode) A issue

Discussion in 'Troubleshooting and Repair' started by JRUBIN, Aug 14, 2015.

  1. JRUBIN

    JRUBIN

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    Jul 17, 2015
    It seems like this issue only exists in Horizontal Mode A (and ALT obviously to the extent A is involved with ALT) However when in B mode the scope works fine with no issues. Id hope this would narrow down the cause, the movie below elaborates on the issue.

    X-Y shows steady solid DOT on both A and B.... and two solid dots on both when connected to the reference voltage




    Apparently This setting deals with which sweep generator is used (page 2-6 of the operators manual)

    I believe sweep generator A is BLOCK 3 of the block diagram which may be disassociated with Alternate B in BLOCK 5

    From TM 11-6625-3135-40 page 1-4

    Al MAIN CIRCUIT BOARD ASSEMBLY (C) — Provides instrument working voltages and processes vertical and horizontal signals.

    A5 ALTERNATE SWEEP CIRCUIT BOARD ASSEMBLY (D) — Processes Alternate B Sweep horizontal signals.

    From TM 11-6625-3135-40 page 1-8

    A Sweep Generator and Logic Circuit (fig. FO-8)
    Alternate Sweep Logic Circuit (fig. FO-9)


    A SWEEP GENERATOR AND LOGIC CIRCUIT. The A sweep generator logic circuit controls A Sweep generation and Z-Axis unblinking. When the A TRIGGER mode switches are set to either P-P AUTO or TV FIELD (with no trigger signal present), a reference sweep is produced. In the NORM setting, sweeps are inhibited until a trigger input is present. This is useful for low-repetition-rate triggering. The SGL SWP setting allows only one sweep at a time to be generated. The sweep signal is applied to the horizontal amplifier.


    ALTERNATE SWEEP LOGIC CIRCUIT. The alternate sweep logic circuit controls the alternate and B horizontal mode displays, intensity, Z-Axis amplifier drive level, and includes the B Miller sweep generator and B sweep logic circuitry. It also provides the B sweep sawtooth waveform and generates signals to control switching between the A and B displays.


    From TM 11-6625-3135-40 page 2-21
    Monitor A4R701 (161) pin 1 for approximately -8 vdc ±50 mv, as switch is turned through all TIMING ranges. ● If voltage varies more than 5 mV, replace A4 (para 2-20).


    From TM 11-6625-3135-40 page 2-47
    A4 TIMING CIRCUIT BOARD ASSEMBLY REPLACEMENT

     
    Last edited: Aug 14, 2015
  2. dorke

    dorke

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    Jun 20, 2015
    First, I would eliminate the case of a triggering fault in A mode :

    Try it in A only mode.
    Check if you can make it trigger and get a steady signal on the CRT.
    I mean try all possible A triggering settings(including external,if possible)and playing with the trigger level knob.
    Do you get the "TRIG'd led" green light?

    You can also monitor the "trigger view" and compare it to B only and ALT modes.

    Also,
    what happens in the above case with no signal connected to the vertical input(auto trig,and normal)?
     
  3. JRUBIN

    JRUBIN

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    Jul 17, 2015

    I made this video in the hope that it answers the questions that you had posted


     
  4. dorke

    dorke

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    Jun 20, 2015
    I can not see the video it says: "Sign in to view..."
     
  5. JRUBIN

    JRUBIN

    95
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    Jul 17, 2015
    sorry, i changed the permission to public, it should work now
     
  6. dorke

    dorke

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    Jun 20, 2015
    Regarding the trigger I would also test these cases,
    in A only .
    Is there any influence of the following:
    1.the "hold off" knob
    2.the A trig slop
    3.the trig level knob in "normal trig mode"

    The following regards the A/B sweeps
    In ALT:
    1.Are the "A/B intensity" knobs functioning?
    2.Is the "A/B SWP SEP" (i.e sweep separation) functioning?
    3.Try separating the A/B SEC/DIV knobs(pull out and rotate separately) any effect?
    4.Is the "B delay" dial functioning?
     
  7. JRUBIN

    JRUBIN

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    Jul 17, 2015
    I made this movie today to help answer the next round of question



    added this one later using an FY3200S, i though a sine wave would be better and not block the screen using the reference

     
    Last edited: Aug 16, 2015
  8. dorke

    dorke

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    Jun 20, 2015
    It looks like there is an issue with the "HoldOff"
    and it is related to A triggering.

    Can you try this:
    A only.
    A trig source=INT(and select the CH. you are using).
    A trig=auto.
    Holdoff=normal.
    A/B per division "unified" in a position to show 4 cycles or more on the screen.
    B delayed= 0,0.

    Can you get a good steady signal on the CRT in this case?
    If you can try changing the "Holdoff" ,do you lose the signal or effect it in any way?
    If not try trig "norm" and changing the level.


    It may be the "HoldOff" potentiometer itself(try deoxit on it).

    Otherwise, you would need a 2nd scope
    for debugging this one(hope you can get one).
    The debug method would be to monitor the signals according to the service manual waforms related to the hold-off and A triggering.
     
  9. JRUBIN

    JRUBIN

    95
    15
    Jul 17, 2015
    As per your instructions Ive made 3 more videos. Notice how over the three videos as the unit warms up the condition changes








    The fact that it does it using the ground reference with no triggering.... could it be the CRT? or a circuit that goes to it?
     
  10. dorke

    dorke

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    Jun 20, 2015
    Well,I don't think it is the CRT since when the fault occurs and you switch to "B only" it will work fine,right?

    Just to be clear on the issue:
    If you start with a "cold" scope it works (with Holdff=norm) fine.
    Then, as the scope warms-up it is deteriorating gradually to intermittent and then to permanent failure?
     
  11. JRUBIN

    JRUBIN

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    Jul 17, 2015
    With regard to your first question , Yes you are correct. There is never a fault on B... ever.
    I will try to cold start it today and get you a time if any for it to fail....

    UPDATE:

    Failure happens also on cold start. It would also seem that less V/DIV increases the issue. Also as an observation when its working it seems as though its just on the threshhold of functioning where anything changed to "weaken" the output would cause it to fail, as in the case of var holdoff
     
    Last edited: Aug 19, 2015
  12. dorke

    dorke

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    Jun 20, 2015
    You need another scope to be able to find the problem.
    Have you got one,or can you get one ?
    I'll be happy to help in tracing the waveforms on the service manual.

    Look at the "general" trace of a scope added below.
    I added some lines to explain one possible "A gate" problem that can produce the "shrinking" of the signal on the Horizontal axisis.
    The faulty "A gate" period in RED on the left waveform.

    There are of course many other problems possible...need actual tracing with another scope. A-sweep.jpg
     
  13. JRUBIN

    JRUBIN

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    Jul 17, 2015
    I appreciate your offer to help. The wife has approved the purchase of a modern digital oscope this week. While the name escapes me it is well documented on eevblog with an easy ability to upgrade it to 200mhz by modifing filenames in the firmware. That being said, it wont arrive for a while.

    At the risk of you jumping through the computer and smacking me in the head, would it be at all possible to use the existing oscope in alt mode to troubleshoot itself by observing the signals on the B timing while observing the very same signal messing up on A? I realize the impracticality of this, as opposed to another scope on hand. If not I will keep you posted on the scope purchase and arrival

    Jordan
     
  14. dorke

    dorke

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    Jun 20, 2015
    Well,
    It may be possible,certainly worth a try!!!

    There are limitations of course ,
    like the "dependency" of Time/Div and the sweep time(that we want to observe) etc.

    First thing before opening the scope, be aware of the presence of fatal voltages and take the required precautions !

    Open the scopes cover.
    Locate module A4- timing board.
    Measure with scope probes waveforms 10 and 8 (prefarably simultaneously with both ch1 and ch2).
    A good place is to measure may be on connector P9700.
    I would set the scope to something like0.5uSec.

    wave1.jpg sch1.jpg parts1.jpg

    Results should look like the Tek Waveforms.
    If you can please make a clip.
     
  15. JRUBIN

    JRUBIN

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    Jul 17, 2015
    This thread is being resurrected. I now have the time and resources to tackle the issue. I will be posting with my findings in the coming weeks.
     
  16. JRUBIN

    JRUBIN

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    Jul 17, 2015
    [​IMG]





    [​IMG]




    [​IMG]





    [​IMG]
     
  17. JRUBIN

    JRUBIN

    95
    15
    Jul 17, 2015
    More info

    Ran for seven hours without issue, playing with every knob and switch. Started again out of nowhere. During the time it ran perfect, it ran perfect in both Channels 1 and 2. During time of failure, it failed in channels 1 and 2. This still only happens on A and never on B.

    Reading through the TOA where channels 1 and 2 converge to see where this splits off to A and B to try and localize the fault
     
  18. JRUBIN

    JRUBIN

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    15
    Jul 17, 2015
    All of the test-points in the A4 timing section were checked today [6 through 16] and no indication was found that did not align to the diagram in the book or destabilize with the waveform displayed on the CRT.... onward and forward


    The main block diagram indicated much activity from [4] into [3] so the next section to look at will be A TRIGGER
     
  19. JRUBIN

    JRUBIN

    95
    15
    Jul 17, 2015
    As I read more, The front panel does not have much by way of test points, section [6] horizontal amplifier seems like a good place to work back from and should display the fault to start a traceback..
     
  20. JRUBIN

    JRUBIN

    95
    15
    Jul 17, 2015
    Problem identified........
     
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