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New project - 1967 oscilloscope restoration

Discussion in 'Troubleshooting and Repair' started by (*steve*), Nov 30, 2019.

  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I picked up this from a for sale group (these were the photos on the site)

    ad_0_1574851649274_compress93.jpg ad_2_1574851649285_compress76.jpg ad_1574851654890_compress59.jpg

    I was immediately attracted to the cut off twin flex! :)

    The guy who sold it to me told me it has been sitting in a shelf for years since his sister bought it from a school sale. So no real idea of the history of the scope, but at least it hasn't been "tested".

    The first thing I did was test for continuity through the transformer and shorts to the case. There were no obvious problems, so that's a good start.

    Google couldn't tell me anything about the Maxtern model 539 oscilloscope (if anyone does, then please pass it on). It looks like a fairly simple "budget" scope.

    Opening it is simply a case of removing 2 screws, in on the underside, and another on the back.

    IMG_20191130_095236_compress6.jpg IMG_20191130_094925_compress62.jpg

    After that the whole case slides off.

    But what is that control between the two trimmers? It doesn't want to move. That's of because all the other controls seem really smooth.

    IMG_20191130_095605_compress95.jpg

    Aside from some dust, it's in pretty good condition inside

    IMG_20191130_095713_compress12.jpg IMG_20191130_100007_compress17.jpg

    Now I can see that control at the back is a switch, and with enough force it turns (I thought it was a pot). But something is weird. There's wires hanging there. Are they cut?

    IMG_20191130_100152_compress40.jpg

    There's not a lot of room to see what's in there

    IMG_20191130_100301_compress41.jpg

    It seems to be a puzzle to solve later...

    The tubes are all Toshiba brand and the Caps also appear to be "good" makes. I removed all the valves and got some compressed air to clean out the dust. Much better!

    IMG_20191130_105302_compress29.jpg

    Oh, I forgot to show you underneath

    IMG_20191130_100820_compress17.jpg

    This was before cleaning. Very little dust.

    And the high voltage filter caps give us a date

    IMG_20191130_110903_compress79.jpg

    The transformer has the windings marked and seems to be in great condition

    IMG_20191130_110158_compress42.jpg

    There's 6 valves in this scope.
    1. 6X4 full wave rectifier
    2. 1X2B 1/2 wave EHT rectifier
    3. 6AQ8 double triode
    4. 6AU6 pentode
    5. 6DJ8 dual high transconductance triode
    6. 12AU7 dual triode
    And only 5 electrolytic caps.

    And 2 suspect looking paper caps, a 0.2uF 400V oil/paper caps, and a 0.2uf? 600V "oil" cap.

    I think the dual 0.2uF 1500V cap looks like a film capacitor.
     
    Last edited: Nov 30, 2019
    Harald Kapp likes this.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The electrolytic capacitors all read well over their marked value, that's a pretty good indication they're leaky.

    If anyone can confirm whether that dual 0.2uF 1500V cap is likely to be electrolytic I'd be very appreciative.
     
  3. bushtech

    bushtech

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    Love the wind tunnel tuned transformer:eek:
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sorry, I don't get you?
     
  5. bushtech

    bushtech

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    Transformer set at an angle
     
  6. BobK

    BobK

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    Very minimalist.

    Bob
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I'm not sure if that was too save space or to minimize noise. I kind of suspect the latter, but it seems a lot of effort for what isn't a particularly high end piece of equipment.

    Indeed, but quite cute in its own way.
     
  8. Martaine2005

    Martaine2005

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    It must be, 1967 was a great year! That scope must be 52 years old..:D
     
  9. bertus

    bertus

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    Nov 8, 2019
    Hello,

    Perhaps that the transformer was mounted under an angle to have less influence of the magnetic stray field.

    Bertus
     
    Last edited: Dec 1, 2019
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yep, I suspect that's the case.
     
  11. 73's de Edd

    73's de Edd

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    Sir Steve . . . . .


    From the faceplate markings and knobs and general frontal treatment . . . what does that unit think it is ? . . . . a Tektronix model 539 . . . wannabe?

    If anyone can confirm whether that dual 0.2uF 1500V cap is likely to be electrolytic I'd be very appreciative.

    Nope, I would expect the dual 0.02 / 1500V to be an oil filled paper capacitor, being at that date timeframe origin, and is being the HV filter cap for the negative supply from your 1X2B used in the unit, with a filter resistor seen between between its 2 sections.
    Paper dielectric mediums absorb oil, plastics-polys don't.

    I take it that the unit is having a 3 in oscilloscope display tube . . . . . maybe it's being a 3KP1 ?

    In looking at your loose, BLUE floating wires, seems to me that they are going up and connecting to CRT pins 5 and 6 thru isolative 100K resistors. (Those resistor types / construction don't mate up with all of the other resistors found in the unit.) Then the BLUE leads ran outside the case rear holes, to make external connections.

    If that happens to be a " 11 pin" CRT socket that accomodates a 3KP1, that connectivity would be accessing the vertical deflection plates.
    Possibly some add on wiring by the user for higher frequency, direct entry to those plates. ( AM RF modulation monitor use.)
    But as foolish in using bare wire as one would be in using bare " hook up " wire to carry a very low level audio audio signal, instead of shielded audio cable.
    In the case of the scope vertical drive direct to the plates, one would have to watch the effective added capacitance of using COAX cable in doing that.

    Thaaaaaaaassssssit . . . . .

    73's de Edd . . . . .

    Build something so simple, that even a fool can use, and then only a fool will want to use it.




    .
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Seems to be a reasonable deduction.

    IMG_20191202_123958_compress19.jpg

    Certainly 11 pin.

    Heater on pins 1&11, 5&6, and 8&9 appear to be deflection plates.

    There seems to be a separate heater winding for the crt. I guess this is due to the cathode being highly negative compared to the other cathodes.

    IMG_20191202_131115_compress70.jpg

    The 2 lugs are the mains winding. The orange pair heading up and to the right go to the heater. That other red/brown pair in a sleeve go to the heater/cathode of the 1X2B.
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    First replacements are the 20uF 350V and 20uF 450V. Both replaced with 22uF 450V.

    The negative side of the original caps was too the chassis. There middle lug of the tag strip was unused (and connected to the chassis) so I used that as a common point. I added a wire from the chassis ground point to the center lug because I can't be sure how good the contact is.

    IMG_20191202_152341_compress79.jpg
     
  14. bertus

    bertus

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    Nov 8, 2019

    Attached Files:

    Last edited: Dec 2, 2019
    (*steve*) likes this.
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Wow, 5Hz to 400kHz response (3db).

    No mention of that third control on the back on a quick reading of the manual...

    Thanks @bertus
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    As the new capacitors arrive one by one...

    0.1uF 100V replaced with 0.1uF 400V. This one was probably good, but it got replaced anyway.

    And the 50uF 15V cap replaced with a 47uF 63V cap I had in my junkbox. That one measured 107uF out of circuit, so it's definitely leaky.

    IMG_20191203_130555_compress63.jpg IMG_20191203_142205_compress91.jpg
     
  17. bertus

    bertus

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    Nov 8, 2019
    Hello,

    I also have found a manual for the 539A.
    That is a bit more extended as the other.

    Bertus
     

    Attached Files:

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  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    There is a different model described on this page.

    It's hard to say whether this is an older or newer model. On the one hand it contains some solid state devices, and newer looking resistors, but it has an earlier (lower) model number and is estimated to be from c1959. On the same site there is information on a model 537, looking much more modern and being (other than the crt) completely sold state (and dated 1970)
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    My next day deliveries are taking longer than usual. I wonder if they've been souvenired by some passer by?

    Anyway, the 10uF 150V cap is now replaced with a 10uF 450V cap.

    The original was an actual cap connected from somewhere to ground.

    IMG_20191207_103201_compress70.jpg

    The replacement radial capacitor is connected to ground at a different point.

    IMG_20191207_105404_compress8.jpg
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Let's take a look at that capacitor on the input terminal.

    From the bottom

    IMG_20191207_115733_compress47.jpg

    And from the side

    IMG_20191207_115840_compress40.jpg

    And when removed...

    IMG_20191207_120356_compress48.jpg

    Like most of the others it reads about 50% above it's nominal value, so it's probably leaky.

    A shame I left the replacement at home...
     
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