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HP 1741A oscilloscope repair help

Discussion in 'Troubleshooting and Repair' started by Pioneer621, Aug 26, 2012.

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  1. Pioneer621

    Pioneer621

    13
    0
    Aug 3, 2010
    Hello, hoping for a little advice on an oscilloscope problem I have.

    It powers on, I can get a trace on the screen & get it to lock on waveforms, but there is a small portion of the screen where the waveform is not displayed correctly. I’ll show some pictures of the waveforms below.

    I have measured all the power supply voltages my meter will allow & they measure ok. I found a leaking capacitor on the main sweep board, but that did not solve the issue. At this point I don’t really know where the problems may be. I think it may be a sweep problem of some sort as if it set a slow sweep, say 2 SEC, it sweeps normal until it hits the mentioned problem area then greatly slows down for almost 2 divisions until it resets, then starts over.

    After sitting for a while & turned on the trace is deflected below the screen and the position control cannot get it up, the beam find shows it. After several minutes it comes up though. Maybe more bad capacitors?

    Also, from 10 uSEC down the trace does not sweep fully across all horizontal divisions, but from 5 uSEC up it does.


    Thank you for any help on this, it is greatly appreciated.

    Here is the service manual PDF for reference: http://cp.literature.agilent.com/litweb/pdf/01741-90911.pdf

    Here are some pictures of the waveforms; maybe this will indicate where the problem may be: (sorry the quality is not that great)

    20khz input; time/div 50 uSEC:
    [​IMG]

    20khz input; time/div 20 uSEC:
    [​IMG]

    20khz input; time/div 10 uSEC:
    [​IMG]

    20khz input; time/div 5 uSEC:
    [​IMG]

    Here it is with an HF signal; time/div 2 uSEC:
    [​IMG]

    Again but set at 1 uSEC:
    [​IMG]
     
  2. Pioneer621

    Pioneer621

    13
    0
    Aug 3, 2010
    Update! I know the thread was just approved, but made some progress a couple hours ago in the meantime. Still a question or two though.

    While going over the service manual & schematic some more, my other thought was it could be a problem with the Horizontal preamp / output board; so I started checking some parts and came across two transistors that gave funny readings. The schematic shows them as PNP transistors which are Q2 & Q5. I took them out to test with my multi-meter on diode check & the ohm setting & they tested “open circuit” & from my experience with typical PNP transistors that is not normal.

    I replaced them with some transistors I had on an old scrap TV board, I had to cross legs one & two as the pin-out differs, but it works now & shows proper horizontal waveforms! So I guess that was that problem.

    What would cause those two transistors to go “open”? Nothing else on that board appears to be defective.

    Also, the original transistors were marked:

    M712
    3-354

    The M I’m sure is the Motorola logo (looks like it), but I could not find anything on the accompanying numbers at my typical datasheet sites or google. Can anyone give me an idea what specs those transistors have?

    I installed KSA539 transistors in there place (with crossed legs as mentioned). From the voltage those are dealing with I “think” those will be ok, but not for sure on that.

    The trace still hesitates to come up when first powered after sitting off for a bit, so will have to figure that out yet. And maybe some other problems still, but were making progress at least!




    Thanks again & sorry for typing so much!





    Here are some pictures of the same waveforms now; after replacing those transistors. (sorry for the bad pics, and I’m using my computers sound card for the 20khz sinewave)

    20khz input; time/div 50 uSEC:
    [​IMG]

    20khz input; time/div 20 uSEC:
    [​IMG]

    20khz input; time/div 10 uSEC:
    [​IMG]

    20khz input; time/div 5 uSEC:
    [​IMG]

    An HF signal time/div 1 uSEC:
    [​IMG]

    Here is the CAL reference squarewave:
    [​IMG]
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,250
    2,703
    Jan 21, 2010
    Looks a lot better to me.

    Looked like some significant non linearity in the horizontal scale.

    The linearity looks pretty good now.

    Perhaps you can post a section of the schematic so we can see what these transistors do.
     
  4. Pioneer621

    Pioneer621

    13
    0
    Aug 3, 2010
    Hello Steve, thanks for the reply!

    I knew I should have tried to post a picture of the schematic of the area in question, took me forever to find various sections in that service manual.

    Anyway here is that particular section of the schematic, again all I replaced was Q2 & Q5, I checked virtually all of the other parts on the board “in circuit” & they seemed fine, but I know that is not always indicative of a bad / out of spec or leaking component. If any other section is needed let me know, think this is all that's need though.

    Thanks again!

    Edit: just learned how to use the snapshot feature in Adobe Reader & thought I should include the whole schematic on that page. The first pic is where the parts I replaced are located. Just realized that whole schematic should be included for reference. New pic is at the bottom.

    [​IMG]

    [​IMG]
     
    Last edited: Aug 27, 2012
  5. shrtrnd

    shrtrnd

    3,680
    457
    Jan 15, 2010
    I didn't check your manual, you can do that. HP usually has pages of the parts
    installed, so you can look them up to be sure.
    The typical HP parts numbering system is 'xxxx-xxxx'
    The transistors are probably 1853-0354, that's a proprietary part number I don't see
    a cross for. The 'M' probably is Motorola, and the '712' is the date code 1971, 2nd week
    of manufacture.
    It's not unusual in a scope this old to see Horizontal or Vertical transistors go (without
    other damage). The heat gets to them after while.
    Nice job getting the scope going again with limited information.
     
  6. Pioneer621

    Pioneer621

    13
    0
    Aug 3, 2010
    Thanks Shrtrnd,

    Yeah I did see the pages & there HP part numbering system when testing the transistors. I went over that section a couple times & the only reference to Q2 & Q5 I seen showed NPN transistors & two different ones. I assume those are located on another board & use the same designations, it did not appear to mention which board they were on.

    I “think” the parts I put in there will work, so far so good on that, guess I’ll see how it goes.

    Hey thanks again, I greatly appreciate all the info.
     
  7. Pioneer621

    Pioneer621

    13
    0
    Aug 3, 2010
    Well figured I’d update this a little.

    I put this project to the side since my last post & thought I’d get it out a couple months ago & try to figure out the vertical deflection problem where it would deflect off (below) the screen as mentioned in an above post. Thought it only did it on start up but found it would do it randomly while on as well & started doing it quite often & for long periods.

    Anyway, traced the vertical deflection circuit to Q17 & Q18 which seemed to be where the problem was. They tested ok out of circuit with the multi-meter (obviously not a definitive test of the transistor operating within specs). So I figured I’d go ahead & change both of them. The collector voltage of Q17 was pretty far off & I “think” it was the problem transistor of the two.

    Q17 & Q18 were more of the M712 3-354 transistors (like found bad in the horizontal stage), this time I subbed them with 2N3906 (pinout differs so I crossed legs obviously). I assume they just went out being there old / worn like shrtnd mentioned.

    Now the trace stays where I put it on start up & after sitting. I did this a couple months ago & it has been working, so think that was that problem.

    Channel “A” jumps up / down vertically “slightly” from time to time, not real bad, but should not do that. Channel B stays right where I put it though & steady, so the problem must be in the channel A area somewhere, preamp /vertical section maybe? So still some problems to sort out, but at least its somewhat usable now. This is just something I tinker with from time to time, so it’s a slow project. Doubt I’ll ever get it fully functional, but at least maybe useable for basic waveform viewing, which it will work for that is it sits now, haha.

    Not sure if this will help any others with similar problems, but thought I’d post what I found.
     
  8. shrtrnd

    shrtrnd

    3,680
    457
    Jan 15, 2010
    It might be something else on Channel 'A', but the most likely culprit is the potentiometer. When pots get old and dirty, they're suceptible to jerky displays when
    you adjust them or they experience vibration. You can check this by watching the
    display and turning the pot clockwise and counter-clockwise, to look for 'jumping' of
    the display signal.
     
  9. Pioneer621

    Pioneer621

    13
    0
    Aug 3, 2010
    Well, thought I would update this. Might as well put this up with the other 1741A threads :).

    Anyway, other then the last problem I noted; channel A jumping up & down very slightly, it has been working well otherwise. I do not have this scope anymore and a friend now has it & has been using it for the last few months. It has been working fine & he simply uses channel B. Even with the small problem on channel A he would likely still be able to use it for the basic waveforms he views, and again channel B has been fine.

    I moved the vertical position potentiometer as shrtnd suggested (many times actually), but it moved very smoothly on the display & did not seem dirty like I’ve experienced with other pots over the years. I did not notice any jumping of the display, I have an old Heathkit scope that had very dirty pots as well as my old B&K so I am familiar with what to look for. However, I did not clean it or attempt to clean it & I can’t rule it out as the possible problem at this point.

    Since I no longer have the 1741A I likely won’t be trouble shooting it anymore, so this very well may be my last post on it. Again a friend has it now & if any problems develop I “may” see this again to try to repair it, otherwise this is it.

    All the best!
     
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