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My brand new oscilloscope

Discussion in 'Troubleshooting and Repair' started by (*steve*), Nov 20, 2018.

  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    As you all know, I'm a sucker for "new" test equipment.

    How's this:
    S54A_Scope (Medium).jpg

    It only has a few faults:
    1. The beam doesn't focus very well
    2. The focus changes over time
    3. Changing the X gain changes the X position slightly
    4. The cal signal doesn't seem to work
    5. The graticule illumination seems non-functional
    6. It's a bit grubby.
    Maybe I'll find more when I do some more tests :)

    The seller described it as "Does work, but a couple of controls are playing up, needs servicing, pots & switches probably need cleaning"

    The service manual is readily available.

    All the parts are well described -- there's only 1 tube (the display), everything else is solid state. The only odd parts I've identified so far are some selenium rectifiers and a couple of germanium diodes. There are quite a few electrolytic capacitors, and I'll probably just replace them, along with the selenium rectifiers in the voltage doubler.

    I'll probably leave the ceramic, polystyrene, polyester and polycarbonate capacitors alone for the moment.
     
    davenn and Arouse1973 like this.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    And some more information about it.
     
  3. WA2YNE

    WA2YNE

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    Nov 20, 2018
    I would say, replace all of the electrolytic capacitors.
    Also, replace the Seleniums with silcon diodes with dropping resistors.
    Seleniums are a hazard when the burn out. I read that they emit a toxic smoke, or whatever.
    you need dropping resistors with the sikicon diodes, because the seleniums had a far freater voltage drop than silicons. Other than what I have mentioned, I don't know wjat may be tyhe problem.
    Oh yes, it just dawned on me, the CRT may be gassy.
    I recall trying to adjust a color picture tube, but it was impossible to get the colors to converge because of the tube being gassy.
    YMMV
    Wayne WA2YNE
     
    mailmad likes this.
  4. kellys_eye

    kellys_eye

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    Classic stuff! I've had a few of those in my time - sold them on as my needs changed.
     
  5. davenn

    davenn Moderator

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    well it is "new" for you

    I expect to see it cleaned up and fully operational within 6 months ;)
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    And here it is inside.

    S54A (Medium).jpg

    I'm planning to replace the electrolytic caps and the selenium rectifiers.

    I'm also a bit concerned about those high voltage ceramic capacitors near the selenium rectifiers. they look OK from above, but take a look at the end of them:

    s54A-4kvcaps (Medium).jpg

    These are 0.01μF 4kV caps. They look a bit grotty. Could it be from the rectifier to the left of them?
     
  7. kellys_eye

    kellys_eye

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    Component sourcing in the original manufacturing years must have been a real fun time! There has to be a dozen different resistors types and heaven's alone how many different capacitors!!!

    Are you replacing these parts on a 'need' basis or preventative?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I'm leaning toward preventative replacement. Originally I was going to replace ALL the electrolytics (and selenium rectifiers), but I wonder if it's justified replacing the low value, low voltage caps.

    There is an oily deposit on the case near the mains transformer. A this stage I don't know where it came from. It's not near any capacitors. I'm going to have to check that transformer in case something bad has happened to it. Although given that the scope has basic functionality, there's a limit to what could have failed there.

    I have three pieces of equipment I'm looking at right now. One of them has a handful of tantalum capacitors. I've had trouble with tantalums going short circuit, and I wonder if I should replace all of them too? The failure mode leading to a short circuit failure seems to require a low impedance connection to something capable of out-pacing the "self-repair" within the capacitor (see page 3). Both recent cases I've come across have had these caps directly across the supply rails. My feeling is that if they aren't a short (and aren't across a supply rail) then I'll leave them alone.

    Back to the selenium rectifiers, they are in a voltage doubler and by my calculations the maximum output voltage would be 3.2kV. Given that they're rated for 3.4kV, the diodes are sailing quite close to the wind. However, the capacitors are rated for 4kV, so they are more conservative.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Various parts arrived today. Would you believe these are 4kV diodes? They are the same size as 1N4001's. The image shows the RS part number so you can look it up.

    PSX_20181204_134044 (Small).jpg

    I think I might just use two of these in series to replace the selenium diodes (that were rated for 0.5 mA).

    Now... A series resistor or not? This article suggests I should always use one. It recommends I calculate the resistance based on the expected voltage drop and the current -- with the aim of reducing peak current. Considering these diodes are connected to ceramic capacitors, the risk of peak currents damaging them should be fairly low, and I've calculated a similarly small risk for the lack of a voltage drop. But would ripple increase?

    Wikipedia says that the voltage drop is about 1V per 25V of PIV (so about 136V for the 3.4kV selenium rectifier).

    On this basis, my back of the envelope calculation suggests a series 270k resistor.

    One option I have is to take the rectifier out, and carefully measure the voltage drop at a number of small currents (say 0.1mA to 0.4mA) and calculate a best fit voltage drop and series resistance. An option is a combination of series resistance and a zener diode.

    Does anyone have any thoughts?
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Interestingly, this article suggests zener diodes on their own (ok, in series with a silicon diode) are an option.

    It also has quite a bit to say about longevity and reasons for selenium rectifier failure.

    Regardless, I think that characterizing the rectifier would be an interesting exercise.

    All I need is a 200V variable current source capable of 0 to 0.5 mA. That sounds doable! The power dissipation will be relatively tiny, and my 1000V variable power supply is well up to the task! I guess a 400V transistor will be sufficient as the pass element.
     
  11. kellys_eye

    kellys_eye

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    The HV diodes I have in my junk box tend to be three times the length of those! Such are the improvements in semiconductor manufacture I suppose?

    What you propose re series resistor (or even a zener - which sounds like a better solution to me) sounds perfectly acceptable. You're lucky to have such a HV PSU anyway - I don't like working around high voltages at the best of times so yours would scare me witless.....
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I am VERY careful around it. It doesn't apply a lot of current, but it's still sufficient to kill.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I did two things today.

    The first was construct a constant current circuit suitable for up to 0.5mA and a couple of hundred volts.

    Here it is:

    const i (Medium).jpg

    I'm testing it here with an 11V zener. And it works like a charm :) The two 2k2 resistors in parallel give a current very close to 0.5mA. I have a couple of 10k resistors I place in parallel to add a little over 50uA at a time.

    The next objective was to remove one of the selenium rectifiers. To do this I had to remove the PCB. It has 9 screws, is held on by a further 6 bolts (holding brackets for capacitors), and two pots on the front panel. To get to the second nut on one of the pots I needed to remove the front panel, which requires removing another 10 or so knobs (some stuck very well), 2 more screws, the bezel around the CRT, 4 more nuts, and probably something else I've forgotten. Then I had to pick up all the little pieces that fell out and place them with the screws in zip lock bags.

    Then success! Well, no. There's still something holding the centre of the board and I can't see it. There's a heap of cable ties on the other side, but it feels far more like an invisible screw.

    Oh well... The adventure will continue.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Oh it's a pig. I might have to remove the CRT and just try to get at the connections from the back of the board. Unfortunately there's a transformer partially in the way and all the stuff behind the various controls that I need to work around. And lots of wires too short to allow much movement. This scope was not intended to be serviced!

    But, having spent time glaring at this board for quite a while, I'm noticing a couple of caps that have partially vomited out their contents. And that's got to be a good thing.

    While I'm doing stuff, I think I'll replace the various lamps with LEDs. At least I won't have to replace them again!
     
    Last edited: Dec 10, 2018
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Some of these call capacitors are pretty old.

    This one will be having its 50th birthday next year.

    IMG_20181215_121124.jpg
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Here is the result of a test of the selenium rectifier:

    upload_2018-12-15_14-49-27.png

    The two diodes are a little different in their characteristics. I have no way of knowing if this is due to the normal spread of Vf or if one of the devices has a fault. I'm going with the top graph because if you're talking about several hundred volts, then the difference of 3V or 4V of Vf isn't going to be a huge factor.

    The Vf is about half of what was predicted, and the series resistance an order of magnitude off! As a consequence of the lower Vf, I didn't need to power up the high voltage power supply during testing either. :)

    My conclusion is that the equivalent circuit is a diode in series with a 33V zener diode and a 27k resistor. In practice, I'm going to place 2 silicon diodes in series because selenium tolerates overvoltage better than silicon, and the silicon diodes are really small -- so they'll fit. The resistor will have 13 volts across it, so no problems with exceeding the voltage rating. And given the very small capacitors, the overload during the initial charge is not likely to be too much of a problem (shunting the resistor with a 100V zener would solve that too).

    I'll make up a replacement and see how it performs.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Here is the graph of the replacement's Vf vs I curve:

    upload_2018-12-15_17-16-43.png

    The curve in yellow is the replacement. Unfortunately all I could find was a 27V zener, not a 35V zener :-(

    The curve seems similar enough. What if I put a 13V and a 20V zener in series?
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    33V zener (green) looks like a good match to the higher Vf selenium rectifier:

    upload_2018-12-15_17-37-7.png

    Practically speaking, the 27V zener (yellow) is probably the way I'll go.

    Any comments?
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Here's the recapping so far:

    recapped (Medium).jpg

    the red and the orange caps (just below and to the right of the switches at the top) are in a position that's really hard to get to. I'm contemplating leaving them for now.
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    And here are the replacements for the selenium rectifiers.

    1545880515226717.jpg

    I will be heatshrinking thrm, but they don't look so interesting after that :)
     
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