Tektronix PWS4721 power supply teardown / diagnostics / repair

I broke my PWS4721 power supply by connecting it to a battery and driving current back into it. (I don’t remember if I reversed the polarity, or just had the output voltage lower than the battery voltage.) The end result was that the power supply had it’s output shorted internally, so that the output voltage was always near zero, and the current was right at the maximum current limit but no power was coming out of the front (or back) connections.

The broken component turned out to be Diode D408 on the main circuit board right in front of the power output header. It appears to be a reverse polarity protection diode, so it’s likely that I accidentally reversed the leads when connecting to the battery (all I remember is the spark). This diode is a 1N5408 (general purpose 1000 volt 3 amp diode) which I was able to replace for $0.40 (Although my total cost was closer to $10, as I ended up buying 10 of them just to have a few more sitting around if needed, and shipping cost me $4.33 from Digikey.) The only specialized tool I needed was a Trox-10 (t-10) screwdriver to remove the security (star) screws from the factory maintenance port so I could remove the back panel.

Of course, I had to disassemble the entire unit to get to the bottom of this circuit board to make de-soldering and diagnosis easier.  The output power rails were shorted before I removed the diode, and were NOT shorted after I removed it, and the diode had failed shorted, conducting in both directions.

In retrospect, I could have probably desoldered the diode in place from the top of the circuit board (the long lead would have been easy, and since the diode was already ruined, I could have heated up the body and pulled the whole thing out from the top and then replaced it without removing the entire circuit board. But, at the time, I wanted access to the rest of the circuit board just in case the diode wasn’t the (only) problem.

You know it’s a high quality piece of equipment because in addition to checking that it worked, they let it burn in to find out if any parts were going to fail quickly, and then calibrated it!

If you want to see how to tear down a PWS4721 and what is inside, here is the video:

 

And a few photos of the main board with heatsync and top logic board:

 

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