Saving A Scope From The Dumpster

If you read Hackaday, you probably get the title of [SunEstra’s] post: A Casual Date with the Dumpster. Many great hacking projects start with finding one man’s trash. This June, [SunEstra] rescued an old Tektronix 2465B oscilloscope, which appeared to be in good shape. Why we never find four-channel 400 MHz scopes in the dumpster is hard to explain, but we are still happy for him, if not a little jealous.

As you might expect, powering up the scope was a disappointment. Relays clicked. Lights flashed. But no display. Adjusting the grid bias on the CRT brought up the display, but it also brought up something else: an error message.

The scope was complaining of “test failure 05-40.” A look through the manual reveals that is “positive level too positive.” Huh. Too much of a good thing, we guess. The test checks the A5 board, so a visual inspection there was the first step.

Unsurprisingly, there were electrolytic capacitors leaking electrolyte. This is, apparently, a well-known problem with this scope. Replacing the electrolytics with some similar tantalum capacitors. In a few cases, the corrosion had eaten pads off the PCB, and some were damaged during the removal. It took a little ingenuity to connect the new parts on the board.

The result? A working scope. Maybe the scope will help repair the next thing that comes out of the dumpster. Sometimes, the best dumpster dives involve intercepting the gear before it hits the dumpster. We keep hoping to run into one of these on the curb (the linked post seems dead, but the video is still there).

9 thoughts on “Saving A Scope From The Dumpster

    1. Yup. I had a Tek TDS210 fail during calibration just last night actually.

      Turned out the juice was escaping the capacitors on the PSU PCB.
      A fistfull of United Chemicon low ESR caps had it back up and running and passing adjustment and verification with flying colours.

      Now to hack it to a TDS220 (send PASSWORD PITBULL then MODEL TDS220 via GPIB) and then a full recal at 100MHz to make it stick and it’ll be a nice scope to give to someone.

  1. I simply love the idea of repairing a piece of equipment that, itself, can be used to repair other equipment. There is poetry to that.

    Now that I think about it, maybe this is how the machines are low-key learning to self-replicate and this is how Skynet happens.
    I gotta think more on this.

    1. It has a fistful of them, but only one has a bad reputation.
      U800 tends to go bad (and you get a characteristic jittery display) but at this age, all the naughty ones have pretty much been found so as long as you don’t run the scope with the case off witout an extra fan blowing on the main board, they will _generally_ be fine.

      If you are going to junk one of these scopes (shame on you if it can be fixed!), be sure to keep U800 for someone who needs it! (Along with the rest of the scope…)
      It’s the large-ish DIP chip bolted down with two screws and nuts with the ‘TO220 style’ tab sticking out one end.

  2. In answer to the question that wasn’t asked — ‘”How do you recognize these components?” A typical electrolytic capacitor (other than SMD’s) is in the shape of a tiny soup can. It often has an “X” scored into the metal top. The side will have either a Positive or Negative (+ or -) marking and the replacement MUST be installed to match (there is often a + or – mark on the PC board). When they fail (and many do) the X will blow out slightly and issue some dark goo. The goo can damage the PC board. De-solder and replace the faulty component with one of a similar uF rating and at least as much as the voltage rating (a little higher voltage rating on the replacement is generally OK). Dell got into a firestorm of trouble when they refused to fix this component under warranty (they demanded $800 for a replacement motherboard). This part is cheap — obtain from or even eBay. Dell caused their own problem when they hired a legal firm to represent them, only to find the Dells used by the legal team soon had the same problem as the customers suing them.
    As electrolytics age, they may also dry out internally. This is what causes an old tube-style radio or amp to hum intolerably (AC voltage is getting into the DC voltage supply) and old TV’s to have wavy lines in the picture.
    There are a variety of different kinds of capacitors for different uses. It’s not a good idea to replace one with a different chemistry, but the rating (nanofarad, picofarad, microfarad, etc.) is mildly flexible. BTW, I’ve fixed a TV, a desktop PC and other devices by simply replacing a gooey electrolytic. The common sign that they’re about to fail is a gradual increase in intensity. IOW, It was bad yesterday and it’s worse today, and now it’s getting harder to get it to even start up, pop open the back and start looking for oozing caps. This is a simple, cheap fix, but requires a simple soldering pencil and electronic solder. Check YouTube for knowledge and practice a little bit.
    Finally, there are many articles about using capacitors in place of or alongside powerful batteries. While built very differently from batteries, huge capacitors measured in farads (impossible to imagine just a few decades ago) can be used to capture and release tremendous energy in a VERY short time. They don’t hold a stored charge as long, but have valid uses nonetheless.

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