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Need help fixing IO-102 Oscilloscope

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  • glyff
    I bought a Heathkit IO-102 oscilloscope at a pawn shop for $15. Unfortunately, it doesn t seem to work right. Here s a link to the manual
    Message 1 of 64 , Mar 17, 2014
      I bought a Heathkit IO-102 oscilloscope at a pawn shop for $15. Unfortunately, it doesn't seem to work right.

      Here's a link to the manual

      https://archive.org/details/HeathkitIO102Manual

      I have followed the instructions in the "Tests and Adjustments" section starting on page 61. The 100V adjustments were extremely sensitive and I was only able to get them to around 90-110V.

      Using the "without signal generator" instructions on page 65, this is the closest I can get it to look like a square wave.

      http://imgur.com/a/hnvS1

      The display holds steady with what is shown in those pictures unless I fiddle with the controls.

      When I set the vertical control to either 10x or 100x, it pretty much just shows a flat line even when the vertical gain is all the way up. There's a slight "blip" but it doesn't look like a square wave, more like a triangle going up and then back to just slightly above the baseline. I couldn't get this captured well on camera.

      The trimmer capacitors don't seem to change the display at all.

      The 40uF capacitor (C7) on the vertical amplifier circuit board was puffy so I replaced it. That didn't make any difference though.

      I checked all the voltages on the voltage chart for the vertical amplifier circuit board on page 90 and they are all pretty close.

      Any ideas?
    • bobfburns2000
      Zener diodes look like a conventional diode in the forward direction and an insulator in the reverse direction until the applied voltage exceeds the zener
      Message 64 of 64 , Apr 6, 2014
        Zener diodes look like a conventional diode in the forward direction and an insulator in the reverse direction until the applied voltage exceeds the zener potential when they rapidly start conducting.
         
        You can test zeners using a DC supply greater than the zener potential and a series resistor to limit the maximum current to safe value.
         
        73

        Bob

        Bob F Burns G3OOU
        C Eng, FIET, MSE
        G-QRP 6907
        Crystal Palace Radio & Electronics Club: www.g3oou.co.uk
        Technical web site: www.qsl.net/g3oou
        @BobFBurns on Twitter: http://twitter.com/BobFBurns
         
        In a message dated 05/04/2014 00:44:42 GMT Standard Time, riley@... writes:
         

        So I finally got it working. I'm not sure what exactly fixed it though.

        I tried hooking the signal generator directly to the gate of Q1. When I did that square waves displayed correctly. When I hooked it back up to the regular input on the front though, it didn't work at all anymore. I realized I must have blown out something in front of Q1.

        So, I remove C1, R1, D1, D2, and C401, and tested them all. Both D1 and D2 tested bad so I replaced them with 1N5239B zener diodes. The other components all tested fine. After I soldered everything back in, it started working correctly.

        I had tested D1 and D2 before and they tested fine. I just tested that they were working as diodes. I don't know how to test the zener voltage. Is it possible that their zener voltages were off but they were still working as diodes? Would that have caused the problems I saw?

        Thanks everyone for all your help. I've learned a lot and I have a working oscil loscope now to show for it too.

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