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Service manual - Protek P3502 (Hung Chang) Oscilloscope

Discussion in 'Datasheets, Manuals or Parts' started by shimniok, May 3, 2009.

  1. shimniok

    shimniok Member

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    I could use a little help...

    I have this Protek P3502 (aka Hung Chang 3502) oscilloscope that has a couple of power resistors that got cooked.

    I don't have the service manual or a schematic so this is a nightmare to figure out. :)

    Anyone have a schematic or service manual?

    Btw, I think these resistors are part of either the horizontal or vertical amp (ultimately the signal finds its way to the tube and it looks sort of like a symmetrical circuit... ???)

    Thanks!
     
  2. pierreretief

    pierreretief New Member

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    Oscilloscop

    Hi, I have the same scope, Hung Chang 3502, Mine is doing weird things, cant get a clear picture and get things to move around ground.. Have you had any luck??
     
  3. shimniok

    shimniok Member

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    Funny thing -- it turns out that the Protek site has a P3502C service manual! It's the same scope as the P3502 except for one added feature for component testing. I bet this manual is just what you're looking for.

    http://www.electro-tech-online.com/custompdfs/2009/06/Man_P3502C.pdf

    Can't get a clear picture? You mean you can't focus the beam with the focus knob? If I had to guess... there are some service adjustments for focus covered in the manual I think. Also could be a problem with the focus circuitry. Could be a tube problem.

    Not sure what you mean by being unable to "get things to move around ground?" Let me know if I can help. I am starting to get a better handle on how these things work so maybe I can steer you in the right direction. Meanwhile here is a great troubleshooting guide that applies to all scopes even tho it is written by/for Tek:

    http://www.electro-tech-online.com/custompdfs/2009/06/troubleshooting-scopes.pdf
     
    Last edited: Jun 4, 2009
  4. dave

    Dave New Member

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  5. pierreretief

    pierreretief New Member

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    Thanks

    Wow man, thank you so much... No the intensity and focus is fine. Its what I see one the screen.

    If I flip the channel A to ground, your suppose to see a straight line, right? But I dont, well, not until I move the position knob almost all the way down... and even then I can only get the line to about half way down the screen before it starts making crazy waves...

    I just made a video of it, can I maybe mail it to you or how do I post it..

    Thanks again
     
  6. shimniok

    shimniok Member

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    Hmm... ok... that pdf I posted about troubleshooting will help a little bit. Read thru that and also feel free to make a video and post to youtube and post the URL here and I will take a look -- be sure to also film ALL the front panel settings so I know how it's all set up. I'll take my best guess at what might be wrong.
     
  7. pierreretief

    pierreretief New Member

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  8. shimniok

    shimniok Member

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    Cool thanks. I couldn't make out the settings but that is ok. Interesting behavior!!

    Take a look at that PDF and just kind of skim through it and get the gist. Their approach is--diagnose from the front panel. That's what we'll try and do.

    So, step one, diagnose the faulty section. The o-scope is made up of several main sections. The power supply of course provides various voltage to all the other sections. The vertical section controls the position of the beam on the vertical axis. At present I'd put my money on a problem with the vertical section. The horizontal section is what sweeps the beam from left to right. It also is responsible for triggering when to sweep, blanking the beam when sweeping right to left to start another sweep.

    You seem to be having a problem getting the beam to show up below a certain point on the screen. The vertical position adjust is part of the vertical section circuitry. So maybe that has something to do with it. In the PDF take a look at the chapter called "Troubleshooting the Vertical Section"

    Let's see if we can isolate down to the block, circuit and then component levels. I am kind of winging it here.

    First off, does this behavior only happen on channel A? Or does this also happen on channel B?

    Second, when you set Channel A coupling to AC and adjust the position knob, what happens? What about when you set coupling to DC?

    Third, when you set Channel B coupling to AC and adjust the position knob, what happens? What about when you set coupling to DC?

    Michael
     
  9. pierreretief

    pierreretief New Member

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    I went through the manual, and also pin pointed it to the vertical section, well kind of, they never really mention this kind of behaviour...

    It does exactly the same on both channel A and B for AC DC and ground fo both of them
     
  10. shimniok

    shimniok Member

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    Ok cool, nice work. So whatever the problem is, it's common to both channels and all the coupling. That helps because it eliminates a bunch of blocks from the 'suspect' list. ;) Here's the block diagram from the service manual.

    [​IMG]

    There's two obvious things common to both CHA and CHB, all coupling -- the CRT itself and the vertical amplifier (the "Output" triangle below). This seems like an electronics problem, not a CRT problem.

    The other thing common to both channels is the block labeled "Mode Control Logic / Vertical Select / Trigger Select" -- I'm not 100% sure what these do, but the problem could lie there as well. I'll go look at circuit diagrams and see if I can get a sense of what these do.

    Do you have an oscilloscope probe? If so, could you (1) plug the probe into the ChA input and then (2) hook the probe to the calibration pin on the front panel and (3) film the waveform and show what happens when you move the position knob and then post it to youtube for me to look at?

    I'm curious to see what it looks like when displaying a waveform.

    Thanks,
    Michael
     
  11. shimniok

    shimniok Member

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    PS: Read section 4-2 in the service manual for a little overview of how the vertical section works...
     
  12. pierreretief

    pierreretief New Member

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    Ok, I will do that tomorrow, since I am writing a huge suject that I have to study for today.. One I am repeating... So Il do that tomorrow.

    Thanks again for your help, I feel confident that together, we are going to fix this baby!
     
  13. shimniok

    shimniok Member

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    PPS: when you display the waveform, see what it looks like and how it behaves with Volts/Division at different settings from low to high.

    What I predict is going to happen is that the part of the waveform appearing above the top half of the screen will look normal while the bottom half will look wonky. If you adjust the position and volts/div so that the trace fits in the top half of the screen, it'll look ok. But if you then change volts/div so that the trace takes up more vertical space, or if you reposition the trace lower, the bottom half will be wonky. Let's see if I am right... :D
     
  14. shimniok

    shimniok Member

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    Happy to help! Plus, this is fun. :) I agree--we'll get this figured out!

    I don't remember if I mentioned it but I did get my 3502 working great once I got the service manual. :D It was simply a couple of burned up resistors in the horizontal amp.

    Best of luck in your studies!

    Michael
     
  15. pierreretief

    pierreretief New Member

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    Just a bit off the topic. Can you maybe explain to me what that z axis bnc port on the back of the oscilloscope is for???

    And the x-y setting if you take the sec/div all the clockwise? Was just curious since I have never used it before...
     
  16. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Z-axis is the brightness of the trace.
     
  17. shimniok

    shimniok Member

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    Yup, just put the time/div indicator on X-Y

    Normally the horizontal sweep is controlled by a periodic, regular timing signal. In X-Y mode, CHB becomes the source for the sweep. That is to say, CHA deflects the trace in the Y axis, and CHB deflects the trace in the X axis (or maybe it is the other way around, but you get the idea).

    The result is you plot Lissajous patterns on the scope which is useful for diagnosing components, for example. You'd feed a frequency into a component and measure the signal on the input (CHA) and output (CHB) sides to measure phase shift.

    The P3502C user manual actually goes into this in detail since the C model includes the frequency generator to input the signal. Otherwise, on the non-C model, you have to have a separate frequency generator.

    So the lissajous pattern shows phase shift between two signals:

    [​IMG]
    (from oscilloscope Tutorials.)
     
  18. shimniok

    shimniok Member

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    I captured some stills from the video just for my own reference. This weird waveform appears ONLY when the position is adjusted below about 1/3 from the top of the screen.

    [​IMG] [​IMG] [​IMG]
     
    Last edited: Jun 3, 2009
  19. pierreretief

    pierreretief New Member

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    yeah, its strange huh...
     
  20. pierreretief

    pierreretief New Member

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    mmm, it seems people have posted things on you tube, they think its a ripple in the power supply. looks like this is a weird fault.. I still think we have the right idea. I am going to write my exam in an hour, then Il come back and do the things you said...

    cheers
     
  21. shimniok

    shimniok Member

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    Good luck with the exam!!!

    I saw the youtube comment...

    Think about when the waveform appears and doesn't appear -- only when the trace deflects to a certain point. And then consider the power supply is always supplying power regardless of what knob you select or where the trace is, etc. I think the fact that the trace goes haywire lower on the screen suggests something wrong with the voltage supplied to the Y- plate. Take a look at the vertical amp circuit diagram -- some amplifiers (audio, for example) get a + and - supply but the vertical amp only has a +120V so there's no negative supply rail that could present ripple to Y- without also affecting Y+ -- that is, you'd see the ripple everywhere. Finally, what ripple looks like is a 60hz sine wave where there should be a flat trace ...
     

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