my o-scope

Discussion in 'General Electronics Chat' started by CaliusOptimus, May 25, 2007.

  1. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    i just received my first o-scope from the ups guy today, and fooling around with it, i noticed (instantly) that the trace will display fine one second, then jump down to where it is only half visible and somewhat blurry. it jumps back and forth from these states randomly. seeing as how this is my first scope i dont have any idea what would cause this, let alone how to fix it. all of the controls work fine, and seemingly independent o the problem with the trace.
    it is a protek p-3502. any suggestions or guidance on what to do would be greatly appreciated!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    I would exercise the warranty. The sooner, the better.

    hgmjr
     
  3. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    for the price i paid for it shipping to send it back would almost leave me where i am now, out 50 bucks. :)
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    Hook the probe to the calibrate connection and view the wave form. If it's stable on that it should be ok.
     
  5. Tube Tech

    Active Member

    Jan 11, 2007
    46
    0
    Learn how the trigger control works first. Then panic. Might just be twitchy triggering.
     
  6. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    lol, im familiar with all of the adjustments. (edit: on the outside of the case ;)) the trace shows a perfectly steady waveform half of the time (1st picture). usually the problem occurs a minute or so after being turned on, then the trace jumps back and forth at random intervals from where it should be, to halfway below the bottom most division (2nd pic). if i adjust the position, it goes up to about the center division, appears to 'squish', and moves no further upward (3rd pic). as for under the case, i dont know where to begin. i noticed 2 resistors and 2 transistor getting extremely hot. (both 6.8K and the transistors i didnt check, 4th pic) one of the resistors has burnt the pcb and a nearby connector (upper right, blue resistor, pic 4). it might be from someone adjusting the wrong pot, being about 30 of them on the main board, i could see it happening. i didnt receive schematics or a manual, and google was no good. as always, any suggestions might help!

    http://img292.imageshack.us/img292/5185/39351299fr3.jpg

    http://img329.imageshack.us/img329/2632/83087652fk8.jpg

    http://img329.imageshack.us/img329/1840/52771664es8.jpg

    http://img329.imageshack.us/img329/8927/67885385ev1.jpg
     
  7. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Wow. The resistor appears to be slightly discolorized, but it can be from the photo. You can fix the connector without even having to cut the wires. It is fully replaceable.
     
  8. Tube Tech

    Active Member

    Jan 11, 2007
    46
    0
    Looks like a vertical deflection circuit problem. Could be anywhere from the Vertical amp to the deflection drivers.

    Unfortunately, what you need to troubleshoot this circuit effectively is a functioning o-scope.
     
  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Is seems that more potential difference is being added to the vertical plates (deflecting the ray further). The problem is that we don't know if the plates are symetrically driven or unbalanced. It's difficult to sort out.
    The gain of the vertical amplifier seems to be ok, since the waves don't change their amplitude. So it seems that we have a voltage addition/subtraction somewere.
     
  10. Tube Tech

    Active Member

    Jan 11, 2007
    46
    0
    "The problem is that we don't know if the plates are symetrically driven or unbalanced."

    Actually, we do. Unbalanced deflection circuits don't work.

    O-scope deflection circuits are driven by a push-pull amp. In the old days, when they used unbalanced amps, the dot would be pin-sized in the top left corner of the screen, a fuzzy horizontal line in the top right, a fuzzy vertical line in the bottom left and a fuzzy blob in the bottom right corner.

    Push-pull amps literally push one side of the trace and pull the other side, keeping it all nice and compact.

    A friend had one of those old-days scopes. Paper capacitors, fabric covered wiring.
     
  11. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Yes, unbalanced circuits were used in some old tubes, where a pair consisting of one X plate and one Y plate used the same terminal. Balanced circuits are better.
     
  12. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    You can start where the wires connect to the vertical deflection plates on the CRT (might be pins on the side of the CRT; Also check that they're not loose.) and work your way backwards through the vertical circuitry, to find out where the main vertical amplifier sections are. There should be two sets of virtually-identical parallel circuits, one positive-going and the other negative-going, IIRC.

    With no signal applied, you can use your multimeter to measure DC voltages and there will be quite a few places where, when you turn the vertical position knob, the voltages in the two circuits should move in opposite directions, IIRC. Probably only one of the two circuits has a problem. Comparing the two circuits should enable you to find the problem.

    It might just be something loose or corroded. First, I'd try re-seating all connectors and socketed components.

    Be careful of high voltages. The voltages near the CRT may be very high. If you don't have a HV probe, be careful not to zap your meter (or yourself).

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html

    -
     
  13. CaliusOptimus

    Thread Starter Well-Known Member

    Aug 14, 2005
    59
    0
    it just so happens i borrowed a nice digital scope from a friend today :) unfortunately i dont have a hv probe, and i dont need two broken scopes. (side question: would a voltage divider on the probe accomplish the same thing?)

    ill give a thorough check on all of the wiring, if that doesnt help ill check all the solder joints on the pcb. then ill consider getting a hv probe and getting into the gritty details. thanks again for all of your suggestions!
     
  14. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    Theoretically, a voltage divider would work. But you're talking about "high voltage". So there are some important practical and safety considerations. (I hope someone more-knowledgable will chime in, if I steer you too wrongly, below.)

    Also, the HV usually just needs its DC level measured. And yours may not need to be measured at all, except maybe "just to know where it is". So it would probably be safer to construct an HV probe for a (very) cheap multimeter, instead of trying to rig one up using an expensive scope and its more-specialized probe. Then, just keep your scope probe away from the HV.

    It seems like 100X should be good-enough, although it depends, of course, on the max voltage to be measured and the multimeter's max allowable voltage. But even with 40kV (which I don't think your scope's HV would even be close to), the 100x would only give you 400v. So if your meter can handle 400v, 100x should be OK. My Tektronix DMM916 can handle 1000vdc and 750vac.

    But there might still be "difficulties". Most common 1/4W resistors can only handle about 250v ("WVDC"), IIRC. So even for just 10kV you'd need to use 40 of them (plus a few extras to get down to 100x). i.e. You should NOT just use something like 9 10Meg and a 9Meg and a 1Meg (or whatever it worked out to be, exactly) to get your 100x divider. But look at some resistors' datasheets, to make sure (through, e.g. mouser.com. Here are the datasheets for Xicon 1/4W 1% and 5% resistors:).

    http://www.mouser.com/catalog/specsheets/XC-600046.pdf

    http://www.mouser.com/catalog/specsheets/XC-600035.pdf

    It looks like you MIGHT be able to get away with 500v per 1/4W resistor, for a short time anyway, which would then require a minimum of only 20 resistors.

    1/2W resistors look like they're OK with 350v. So maybe you could use 33 of them, instead of 40 1/4W ones.

    If you have to order resistors, anyway, maybe you should consider using high-voltage resistors. For example, the Ohmite MC series (just the first ones I noticed, in the Mouser.com catalog) have ratings from 2kV to 10kV, with values listed from 100K to 100Meg (and "available" from 100 Ohms to 5000Meg). But they're $3.25 each. However, you might only need a couple of them, if you only wanted to be able to go up to 10 kV. That's just an example. The Ohmite "SLIM-MOX" series has ratings from 1kV to 20kV. Just another example. You'd just have to look at some datasheets, and check actual availability. Optimizing price and number of required reistors is "left as an exercise for the reader".

    Construction will be somewhat critical, too. You'd probably want to put the resistor assembly into a long piece of small-diameter plastic pipe, or tubing, or something like that (and then make SURE that you only touch the low-voltage end!!).

    You could use hardware-store thick-walled PVC (does it come in 1/2" or smaller diameter?) and use the glue-on end caps with holes drilled in them to hold things in place. If it's so long that it's too unwieldy, you could use slightly-fatter pipe if necessary and make a "90-degree zig-zag" with the resistors. But you'd probably have to use some sort of glue to hold them in place. Maybe you could fill the tube with some sort of epoxy or silicone, even, although you would need to be very sure it's extremely non-conductive.

    You should probably also do a google search for DIY HV probes. I have seen some on the web.

    Good luck.

    - Tom Gootee

    http://www.fullnet.com/~tomg/index.html

    -
     
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