The Guts Of Switched Mode Power Supplies, Brought To You By Oscilloscope Repair

The Tektronix 2000 series of oscilloscopes are a mainstay for any electronics lab. They work, they’re relatively cheap, they’re good, and they’re available in just about any surplus electronics store. [Mr.RC-Cam] has been hoarding one of these for twenty years, and like any classic piece of equipment, it needs a little refurbishment every now and again. Now, it’s time. Here’s how you repair one of the best values in analog oscilloscopes.

This repair adventure began when the scope died. There were no lights, no screen trace, and a brief hiss sound when it was powered on. (Ten points if you can guess what that hiss sound was!) Armed with a schematic, [Mr.RC-Cam] dove in and pulled the power supply, being careful to discharge the CRT beforehand.

There were no bulging capacitors, no obviously overheated components, and just a little bit of dust. The only solution was to look at the parts with a meter one at a time. Removing the big caps provided access to a row of diodes, which revealed the culprit: a single shorted diode. This part was ordered, and a few other housekeeping tasks were taken care of. The lithium battery on the processor board responsible for storing the calibration constants was replaced, and the new, smaller, caps got lovely 3D printed mounting flange adapters. Now, this old ‘scope works, and we’ve got a lovely story to tell around the electronic campfire.

22 thoughts on “The Guts Of Switched Mode Power Supplies, Brought To You By Oscilloscope Repair

  1. “hoarding one of these for twenty years,”

    I don’t think cherishing one piece of equipment for twenty years would be considered hoarding. Now, if he owned fifty of them, that could be considered hoarding.

    1. Depends on whether you’ve been using it for the 20 years, or just keeping it for the off-chance.

      I’ve got a portable scope from 1989. Changed out the battery cells and fixed the 1x-10x probe with a new 50 Ohm 1% resistor because someone had managed to blow it out. Using it every day.

  2. >Ten points if you can guess what that hiss sound was!

    Anything making a sound implies mechanical action, which can be produced by a capacitor or a coil, or by an electric arc. Making a “hiss” implies it has the character of noise rather than a periodic signal, which would point to an electric arc from some contact problem or faulty insulator. Knowing it has a CRT, high voltages are involved, which also supports the idea of an arc shooting across somewhere.

    Guess: shorted trace, or a bug crawled in an unfortunate place.

    Now let’s see what the answer is.

    A: shorted diode causing the PSU fail to boot up. Hissing sound caused by a “chirp mode” during the boot-up process.

    1. Not a 10/10 guess, but I did make it before reading the rest of the article and the source.

      Usually when something is making noise, that means significant energies are involved, which means significant currents are flowing somewhere (or high voltages arcing across), which points to a short circuit somewhere.

    1. Yes, and you don’t have to do any labor, and it’s guaranteed to work, and has a warranty. And it’s a DSO with a color display, USB, and waveform storage.
      If you like repairing old scopes as a hobby, that’s great, but if you just want a scope that works, that’s not a good way to do it.

    2. * Analog scope vs digital scope. Digital scope can produce artifacts under some situations if you don’t know what you are doing.
      * Tek analog frontend vs some Chinese designs. Enough said for those who cares.

      If you can have only one scope, I would say go for the flashy digital ones. It is an option if you get one very cheaply.

  3. Interesting.
    I have that ‘scope’s great grandparent here. It is a 2213 model. I have had it since 1983, when my father presented it to me claiming that, “The fellow who was supposed to do repair work on the computers who ran his phototypsetting business abandoned it.” As for why that happened, we spent an interesting year trying to figure out why. Now 35 years later, a company who specializes in new and older equipment sales and leasing wants to me swap it for a DSO along the lines of any both Tek and Keysight make. Since I use the latter’s software, VEE for managing my gadgets, and that company is considering ending it I have turned them down. Oh and Brian the three tribbles say “Hello!” and want to know when you’ll next be visiting the NYNJ group.

  4. Your story brings back fond memories of the same problem I had about a year ago.

    I have a few 2465’s of the bare version and “A”, which appear to use the same power supply. All were purchased “parts/repair” on Ebay, and all but one (an “A”) needed repair, and except for a recant purchase of a TDS784C, the 2465A is my primary analog scope. One was water damaged, and became a parts unit.

    One scope had the same power supply problem that you described here, and basically did the same as you did with U2201 with the milled socket. I didn’t replace it with a known good one though. I used a simple curve/signature tracer to look at all pins relative to the power and ground pins to eliminate it as a suspect. Basically ohmed it out, but the curve tracer puts an alternating (aprox. 1KHz) +/- signal on the inputs and outputs with a connection to the VCC and GND pins, and displays the waveform on a scope. Check out Mr. Carlson’s Lab on Patrion for the circuit. It is GREAT! I too found the problem to be a rectifier diode, and replaced all electrolytic’s due to their age.

    The battery issue reminded me that I need to replace that as well. Thanks!

      1. Fortunately, I have spares from the water damaged unit that I tested and are in good condition. I do have another 2465 that has a vertical jitter, that I expect will need one of those chips.

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