Got a new toy

Discussion in 'General Electronics Chat' started by tom66, Oct 25, 2011.

  1. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Gould OS300 20 MHz analog dual channel oscilloscope (top).

    I bought it to complement my 100 MHz digitising oscilloscope (HP 54501A on bottom), it set me back £60 (included a free probe).

    It does need some preventative maintenance though. The trace "wobbles" when the timebase is slow (and x10 is selected) possibly due to PSU ripple. The focus control works, but the best focus is 2mm diameter trace, and the front panel controls are very position sensitive (needs a good clean.)

    Main reason I bought this was for portability and repairing plasma TVs which use +385V Vset voltages; this thing can withstand 400V (and measure up to 100V/div with the 10x probe I got with it.)

    I do have the service manual for it in paper form.
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This may be nothing to do with it, but it is certainly likely to have a rippled trace if it is lying on top of its friend, and they are both awake!

    Stray fields from one 'scope may be interfering with the other. Try separating them and see if the trace gets any steadier.
     
  3. tom66

    Thread Starter Senior Member

    May 9, 2009
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    The wobbling happens whether or not the bottom scope is on.

    It happens only at 10X zoom and when the timebase is set on a slow rate - sufficient to see the screen flicker. I think it's probably PSU ripple on the +11V and/or -11V lines.

    Looking at the schematic, Y and X deflection uses +/-11V and filtering is performed by C710 (-11V), C708 (+11V), C704 (both). C704 is a 470u/25V cap. If that goes bad or dry, then the trace will ripple...
     
    Last edited: Oct 25, 2011
  4. Wendy

    Moderator

    Mar 24, 2008
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    I have seen that all the time, anything with a CRT throws out a large magnetic field from the neck coils. If you have two they interact. The bigger the CRT the worst it gets. With old large computer monitors it was really bad.

    Different subject, did you notice your oscope probes on the older oscope were not compensated. The bottom scope showed a flat square wave, the top doesn't. If the probes were compensated (a standard procedure before using them) they would be flat. There is a small capacitor tweaker in the probe head itself that will compensate that probe to the scope. Without compensation the freq response will also be slightly off.

    The bottom scope probably goes into standby, unlike the top scope. The only thing I found that solved it at work was distance.
     
  5. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Thankfully, no standby mode on the big scope. The switch on the front uses a large lever which operates a switch on the back, cutting both live and neutral...

    Yes I noticed my free probe wasn't compensated, and it doesn't appear to have a compensation control. Whereas my newer probes from China for my big scope do have compensation controls which I have set up properly.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Can you connect it to a printer or a computer to pick off the image?
     
  7. tom66

    Thread Starter Senior Member

    May 9, 2009
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    It has outputs for a plotter: PEN LIFT, X, Y etc... on the rear along with TRIG OUT and Z INPUT.
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Oh well, it was just a guess. This sort of thing does happen, but of course it's much more convenient when equipment is well enough screened not to have to worry about it.
     
  9. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Yeah, I've had a similar problem with my scope and power supply: When the power supply was switched on while below the scope, the screen would ripple.

    I opened it up and checked the rails and the ripple is in spec. Quality Chemicon capacitors too. Single sided layout, all through hole. Two boards, one for the inputs and front panel and the other with everything else.

    Also got a date code from the ICs on it: 1984, and 1983 (in different parts.) So that means it's probably >27 years old...!

    I did notice something: the ripple seems to be more prominent in the X-axis when on 10X. So maybe it's the 10X gain or the trigger that's getting poor power, or the gain pot is worn.

    Overall, for 27 years old it's doing very well.
     
  10. Lundwall_Paul

    Member

    Oct 18, 2011
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    You may have some ground bounce.
     
  11. tom66

    Thread Starter Senior Member

    May 9, 2009
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    Good point. There is no ground for the cal output, it uses the earth return.
     
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Reminds me of the Scopex scopes I used to use for a similar purpose - the ability to accept high input voltages was great.

    The Scopex had a custom tapped pot/switch for the trace expand and horizontal shift which could pickup stray voltages as it was not shielded.

    You may also find if you have a nearby wall wart it will interfere more with an older scope.

    go well
     
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