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Maintenance on Tektronix SC504 Scope Plug-in

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  • Maintenance on Tektronix SC504 Scope Plug-in

    This month I've had some time available to tackle some bench instrument maintenance that I've put off for some time. Last time I was at my storage lockers, I had brought back a second Tektronix SC504 80MHz dual trace scope which operates in their TM-500 series Mainframe (a plug-in instrument). When my shop became inaccessible back in July 2020, with new space allocated for me as a temporary shop, I brought in a second bench set-up group of instruments. At home, I plugged in one of the two SC504 scope plug-ins, and the first one I powered up yielded a burnt smell, and didn't power up. I removed it, plugged in the other one, which came up fine. I fitted out a TM-515 Traveler Mainframe, along with a couple additional plug-ins as I'd need them, and did bring over my current probe system from my main shop.

    Having brought in a DM501A 4.5 digit DMM from home as well as a couple old Simpson 464 DMM's...all three not working, I also brought in this SC504 to see if I could find the failure.

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    After removing the side covers, along with the top and bottom covers, I first looked for the obvious signs....burnt parts. This instrument is full of dip Tantalum capacitors, which from years of equipment maintenance, I've found all too often, they fail short, taking the instrument down, sometimes with further damage. I found the first obvious culprit...a discolored YEL/VIO/GRN bodied part with a clear burn mark thru the side of the body. Everything in these plug-ins tend to be densly packed, so I had to move the harness aside, bend the body to reach in to clip the leads off, then unsoldered the leads. Getting the solder out of the holes was a PITA. I have the service manual for the instrument, so finally found the part's component designation number, then found it listed in their parts list, finding it was a 4.7uF/50V part. I didn't have a replacement in that value in my dip-tantalum stash, so ended up using a 10uF/100V Nichicon PW series 105 deg Al Elect part.

    Then, having found one, I spent more time looking thru the rest of the instrument visually, where on one side, it was chock full of 22uF/15V power supply bypass caps on the +/- 5V supplies.

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    I had found on the opposite side, where the power supply regulators reside, it seemed like I was measuring very low resistance on both the +5V and -5V supplies. Most of the dip tantalums on the right side are for th0se +/-5V rails. Getting one lead lifted on these caps was very tedious, so I didn't spend a lot of time trying that. I did remove the trigger switch PCB assembly which also had a pair on it, and, once removed, they measured fine, charging up with the DMM Ohmmeter. I did remove a couple parts successfully, only to find those were fine, while the solder pads revealed the same low resistance...less than 100 ohms. I stopped, and unplugged my other SC504 scope from the mainframe on the test bench, removed it's side covers to see what I measured on it's +/- 5V supply rails. Got the same readings. I put that scope back into the mainframe, and then finished replacing one other 22uF/15V cap with a 22uF/100V Nichicon PW series Al Elect part, and put the covers back on, plugged it into the TM503 mainframe I also had here.

    The scope powered up. Took a moment to get the traces onto the screen, got the beam intensity and focus set, then had to exercise Ch 2's Position control a while as the pot was behaving like a dirty pot. Finally got stable results with that, and appeared to have a working scope again.

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    I then found the Trigger Switches had some latching issues. The AC Coupling Trigger Switch wouldn't latch. After removing that board for inspection, it appears a tab on the actuator is broken, so it won't lock into AC Coupling. I had to check my images of what's in the lockers, recalling that I had a broken SC503 storage scope plug-in.

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    Now, hopefully, this SC503's Trigger Switch assembly works better. Plus, it looks like I also have some control knobs that are in better condition than those on this SC504 scope. So, a spare parts instrument can come to the rescue. Chances of finding replacement parts this many years out of production....good luck....only bone-yard instruments or those who have pulled those apart to sell off used assemblies.

    When I next visit the lockers, I'll also bring back the other two Tek DM501A 4.5 digit Multimeters. The one shown in the TM503 mainframe next to the now-working SC504 scope wasn't working.....turned out to be push-button interlocking switch contact issues. Same problem with the two Simpson 464 DMM's....one which was my first DMM bought back in 1976.
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    Logic is an organized way of going wrong with confidence

  • #2
    Shouldn't this really be in the "Tool and Test Gear" forum under Music Electronics ?
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      You're right! I hadn't even noticed that category was there. Now, how do I move this and some recent posts under the same topics?
      Logic is an organized way of going wrong with confidence

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