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Oscilloscope Repair

Discussion in 'Repairing Electronics' started by pierreretief, May 27, 2009.

  1. pierreretief

    pierreretief New Member

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    Hi guys

    I was wondering if someone can help me. I am an engineering student and a couple of months ago, I bought an oscilloscope. Lately, every time I changed the volts/div knob, I had to adjust quite significantly to get the ground line in the middle (By adjusting the position knob) and now, I cant get the ground on the line at all, when the knob is in the middle, the line is completely off the screen and both channels have very strange things appearing on screen.

    Is there any basic test I can do? I have adequate electronic knowledge so I am sure that if you can get me in the right direction, I will be able to have a look.

    Any help will be appreciated.


    Thanks
     
  2. schmitt trigger

    schmitt trigger Active Member

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    What oscilloscope (brand and model) are you attempting to repair?
     
  3. pierreretief

    pierreretief New Member

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    It is a Hung Chang (aka Protek) 3502
    2 channel
    20MHz
     
  4. dave

    Dave New Member

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  5. schmitt trigger

    schmitt trigger Active Member

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    I'm not familiar with that particular scope, but I still can give you some advice.

    First of all, is the problem identical in both channels?
    If it is in a single channel, that is good news, because You have one functioning channel which you can use to compare voltages, waveforms, etc. But from your post it appears that both channels are mis-behaving.

    Second, borrow another scope. This will allow you to follow the signals on the scope you are fixing.

    Last and foremost, try to get the service manual. If you do find the problem, and repair the unit, most likely your scope will be out of calibration. And in an oscilloscope, there are usually several cascaded adjustments, which interact with each other, so you need to follow the manufacturer's recommended calibration procedure.

    For that last step, you will need to get a Calibration source, which puts out high precision DC and AC signals. Those are expensive to purchase, but you may be able to borrow or lease one. Or if you can't get hold of one, then send the scope to a service center for calibration.
     
    Last edited: May 28, 2009
  6. Rolf

    Rolf Member

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    Try this link for service manual. Even if it is not exactly the same model no. it will more than likely be close enough for some help.

    HUNG CHANG 3500T Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

    Start by checking all power supply voltages. If the trace can be focused then it is not necessary to check hi voltage. Danger!!!! You need special hi voltage probes for that.
    If both channels have the same symptoms then it is more than likely it is a low voltage power supply problem.
    Check for burnt resistors, bad capasators and shorted transistors.

    Good Luck!

    Rolf
     
    Last edited: May 28, 2009
  7. pierreretief

    pierreretief New Member

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    Thank you so much for the quick replies. Am I correct in saying that after I might have fixed it, it would prob be out of calibration? And this is bad since its not a simple matter of tuning a preset pot to get it calibrated again. Ok, but an uncalibrated scope at this point is better than no scope. Since I am a student, I dont exactly have the cash to get it completely fixed right now and I need to measure some waveforms, so I would love to get it up and running. I will check for burnt resisistors and caps. And am I correct in saying that I cant measure the low voltage points without the circuit diagram? Or is there a way

    I had a look at the link, but that is the schematic os some digital RLC meter or scope, so unfortunately that is not going to help me. Thanks anyway for all your replies.

    Yes it does exactly the same thing in both chanels, which will make it prob a low voltage power supply problem. I w
     
  8. Rolf

    Rolf Member

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    All ocopes I ever worked on has a probe calibrator on the front panel. On Tektronix they are usually 1kH and 5V. That is sufficient to verify the rough overall calibration of the unit.
    Factory calibration of oscilloscopes is not (INHO) needed for service work.
    Low voltage power supply repairs should not effect
    the overall calibration by much, if anny.
    Do not turn any trim-pots unless you know what it is for!
     
  9. pierreretief

    pierreretief New Member

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    great, mine has a 0.5V 1KHz calibration point, but the problem is still how do i measure voltages if i dont know where to measure or how much it should be. I have looked everywhere for a schematic, i cant even find a place to buy one. All i manage to find are manuals, which dont have that information in them... Thanks for the calibration info, wont be turning any random pots in the near future!
     
  10. Rolf

    Rolf Member

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    Trace the wires from the transformer to the rectifiers to the filter caps, look at the caps voltage rating and polarities, this will give you an idea of the voltages and polarities.
    There are two low voltage supplies around 15v one positive and one negative, make sure they are equal in potential.
    There will be two others of a much higher voltage, maybe 250v, for driving the sweep and deflection amps, make sure they are equal
    also.
    If there are several PCB's connected with cables then you will find some or all of the voltages at the pins. Be suspicious of voltages that are not common like 12, 15, 24, 250, 300 etc.
    Take written notes!

    P.S. Please fill in your location.
     
    Last edited: May 28, 2009
  11. pierreretief

    pierreretief New Member

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    Great, thank you so much, I am going to try that today

    I am in South Africa

    Thanks again
     
  12. pierreretief

    pierreretief New Member

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    I havent quite gotten around to opening my scope, due to universtiy exams, but I did take a quick video if you want to see exactly what I was trying to explain...

    Thanks alot

    YouTube - Hung Chang Oscilloscope repair
     
  13. user_88

    user_88 Member

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    It is possible for electrolytic capacitors to become defective or to lose their specified capacitance values if they are not used frequently. Sometimes it is possible to rejuvenate the capacitors by simply leaving the main power supply of the unit turned on for a period of time ... a few hours ... a few days.
    The random display pattern that seems to be associated with turning the control dial on the video might be associated with humidity or moisture within the dial mechanism.
    If it is possible to obtain access to the part of the knob control that is behind the front panel, then a few sprays of electrical contact cleaner should improve the electrical conductivity, and also the 'smoothness' of the associated display waveform.
     
    Last edited: Jun 3, 2009
  14. pierreretief

    pierreretief New Member

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    I doubt that that is the problem, since both channel A and B have the position control knob, and it is exactly the same effect on both channels with both knobs
     
  15. PatM

    PatM Member

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    I have had good results with a little tuner cleaner/lubricant applied to potentiometers and switches.
    Had several pieces of test equipment that I bought on eBay that were resurrected to normal operation with that method.
     
  16. jzaghal

    jzaghal New Member

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    Hi,

    First thing to do is clean all contacts with spray and check!!

    Poor electrical switch contacts will cause such problems.

    Good Luck.
     
  17. unclejed613

    unclejed613 Well-Known Member

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    looked at your video........ that garbage looks like power supply ripple. either electrolytics or voltage regulator or both are bad, so check the supplies first, if that's not it, then proceed to cleaning pots and checking the amps for offset. the best way to find ripple in a DC power supply without a scope, is with a meter set on AC volts. most regulated power supplies should have less than 1% noise and ripple on them. so for a 100V supply, there should be less than 1V noise and ripple after a regulator and filter


    before you clean any internal pots or caps, mark their position.

    sounds like your DC balance of the final section of the vertical amp is the problem, since both channels do exactly the same thing. you could have one side of the deflection power supply out, or something amiss in the section of the vertical amp after the front channels get switched in. most scopes have the main vertical amp and the channel input amps, which get fed through a switcher that allows you to either select one channel or the other, or switches them electronically, either chopped or alternated. your problem is after the switcher circuit (unless you're missing a supply rail to the input amps). so the first place you want to look is the supply rails of the input amps and the supply rails of the final amp. if the rails are ok, switch to channel A and put the position pot in the middle, and use the input switch to connect the input to ground. then begin checking amplifier outputs for DC offset from the input amp, through the channel switcher, to the final amp, to the deflection plates. most scopes uses dual supplies (=/- rails), but if the scope uses a single ended power supply, then you're looking for half the supply voltage to be the "zero" reference.
     
    Last edited: Jun 5, 2009
  18. pierreretief

    pierreretief New Member

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    Thank you very much
     
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  19. shimniok

    shimniok Member

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    Thanks for the post. While it looks like ripple, our diagnosis so far excludes power supply ripple.

    If it were ripple it would show up on the top and bottom of the square wave trace (like on my P3502). Also, switching to LINE triggering would stabilize the ripple waveform (letting the other waveform roll). Lastly, ripple would not be affected by vertical position of the trace; it would be present at all positions. Note that both halves of the vertical amp are driven by +120V (unlike, say, an audio amp) as well as +20V etc.

    So, based on the behavior we are seeing, we believe this is caused by a problem with one half of the vertical amp circuit. It's very difficult to tell which circuits within let alone which components. At this point we're trying methodical, in-circuit testing of the trannies and diodes to see if anything jumps out.

    Btw, the next test--- use the DMM to try to measure voltages along the way to see if we can see where the problem is introduced. Will want to power off when switching probes around, and use mini grabbers to avoid zapping anything. ;) I anticipate that you'll be able to measure an AC voltage on the 680Ω resistor leading to Y- ... so basically we'll work our way back until we stop reading AC and we start reading DC. That will tell us where the problem lies. Anyway that's the preview :D

    I'm convinced we are getting closer to solving this. Eagerly awaiting measurement results on the diodes & transistors. :D

    Michael
     
  20. unclejed613

    unclejed613 Well-Known Member

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    unfortunately, i haven't seen a schematic posted yet, so i'm troubleshooting from a few seconds of fuzzy video and experience..... another thing that can be caused by bad filter caps or bypass caps, is oscillation in amplifier circuits, which this problem could be. since it's only in the vertical section, i would concentrate on small electrolytics in the vertical section, such as 1 to 10uf caps used as bypass caps in individual amp stages. these little caps have a tendency to dry out first. since oscopes tend to be a piece of equipment that gets turned on and left on, these small caps can dry out, often before the warranty of a scope is expired. so if you have a way of testing ESR on a cap, start by checking these. they will most likely be 1 to 10uf at 160V if as shimniok said, they are operating at +120V. even a few ohms of ESR (1uf/160V caps average about 5-10 ohms when new, and 15 and up would be considered bad) can cause amplifier circuits to oscillate or "motorboat". if it were my scope i would begin by recapping it, a little bit expensive and time consuming, but it would avoid a lot of problems that would appear over the next few years with a used scope. it would also allow the semiconductors to run cooler, and improve their reliability.
     
  21. shimniok

    shimniok Member

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    Cool, thanks for the tips on the 'lytics. I posted part of the circuit somewhere above btw. Posted a link to the service manual early on...

    Michael
     

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