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Simpson 438 Colorscope Oscilloscope

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  • Simpson 438 Colorscope Oscilloscope

    A friend gave me this, said his dad used it a long time ago but he didn't know anything about the condition. It's super-clean inside but I'm sure after sitting for at least 30 years the cap cans need replacing before I fire it up. I could probably do it on the variac I suppose.

    Does anyone know anything about this particular unit? I know it's not actually a color screen but it was free and I'll use it if it's any good.
    --Jim


    He's like a new set of strings... he just needs to be stretched a bit.

  • #2
    https://www.radiomuseum.org/r/simpso...lorscope.html#
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Thanks, I found that. I was just wondering if anyone here had experience with it.
      --Jim


      He's like a new set of strings... he just needs to be stretched a bit.

      Comment


      • #4
        It's similar to my first scope, my Dad's Eico 460. I used it for my organ/synth experiments for a few years but it had serious
        limitations like:

        AC coupling only - can't show DC levels
        Lots of drift - constantly having to adjust horizontal and vertical position
        Little to no calibration - hard to make measurements of time or amplitude
        No triggered sweep - hard to get a stable display
        Low bandwidth - 300 KHz is barely enough even for audio

        My second scope was an HP 130B. Still all tube, built like a tank, but still only 300 KHz bandwidth
        My third scope I bought new. A Telequipment D31A 10Mhz dual trace. That was a great scope. I miss it (snifff),
        but I eventually needed more bandwidth for digital circuits.

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        • #5
          It is like a cheap guitar. It works and you can play it, just not as good a thing as a more serious guitar. dmeek's story sums it up.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            Well that's a bummer. Thanks for the info.
            --Jim


            He's like a new set of strings... he just needs to be stretched a bit.

            Comment


            • #7
              You can do a ton with it. Make it work, learn about scopes. Like a early guitar, playing it for a year teaches you what you might want to look for in a better guitar. I like most expect a triggered scope for its stable screen, but if you have a sine wave up there, and it slowly drifts across the screen because it is not triggered, you still can easily see it is a sine wave and whether it has clipped peaks or whatever. There is nothing wrong with something being basic, just realize that it is and as a wise man said, "You've got to know (its) limitations."
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                I agree It's still useful. I would prefer it over one of those mini lcd digital "oscilloscopes" you see on ebay

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                • #9
                  Funny you should mention that. I got one of those and it is cool for about five minutes. I'll bring this up slow on the variac and see if it works.
                  --Jim


                  He's like a new set of strings... he just needs to be stretched a bit.

                  Comment


                  • #10
                    After a while on the variac, bringing up voltage slowly, it appears to be alive. Now I need to get a probe and go through the manual service instructions to see if it will cooperate.
                    Attached Files
                    --Jim


                    He's like a new set of strings... he just needs to be stretched a bit.

                    Comment


                    • #11
                      Well that fizzled. I turned it on today, again on the variac, let it run for a while and noticed the screen had gone dark. Something shut down thermally inside because I couldn't hear the transformer humming either but the fuse is still good. So I shut it off, turned it back on and the image started to come up. Then it went dark again and I could smell something hot. Shut it down, pulled the cover and I don't see anything physically burned but something was obviously on its way to doing so. I plugged into the LBL and both the indicator light and the 100w lightbulb came on dim. After a few minutes I shut down, plugged it into the non-limited outlet and turned it on. After about a minute it started smelling hot again. I don't know if this is worth putting any diagnostic time into or not.
                      --Jim


                      He's like a new set of strings... he just needs to be stretched a bit.

                      Comment


                      • #12
                        A simple test you could try is to turn it on with the 2 5Y3s removed. If something is still burning it's probably the transformer and not worth fixing. If it seems OK then it could be the filter caps C35 2X .5/1500V or C32 3X 10/450.
                        I like working on test equipment. It's usually an educational experience if nothing else.
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                        • #13
                          It almost looks like the top rectifier is going to the CRT section and the other is a standard circuit. This CRT stuff is not something I'm familiar with. I'm not totally sure what's going on here to be honest. I pulled the 5Y3s out and it isn't heating up now. But that .5/1500 cap can is a monster. Does it need to be that big physically or can I use alternatives there? Again, it appears to be on the CRT side. They're over $60 at the parts houses.
                          Attached Files
                          --Jim


                          He's like a new set of strings... he just needs to be stretched a bit.

                          Comment


                          • #14
                            You can use 2 of these, about $5 for both. https://www.digikey.com/en/products/...hwBUcBpEAXQF8g
                            If you can power up with V9 installed and only V8 removed. then it is likely those caps. Be careful, there is 1200 or more Volts in that area.

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                            • #15
                              I let it run for about an hour without V8 in and nothing got hot so I know it's on that side of the circuit at least. I'll get a couple of those caps, I don't have any rated that high voltage-wise.

                              That is a strange circuit. I know it's not an amp but it looks like the rectifier is wired backward.
                              --Jim


                              He's like a new set of strings... he just needs to be stretched a bit.

                              Comment

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