HP E3631A Power Supply

A 30 year old, high quality triple output power supply, great for all sorts of projects requiring multiple different voltage inputs including a negative rail.

Located above the soldering station.

Known Issue

"Bad" Rotary Encoder knob The issue can manifest in a couple ways 1. Spinning knob causes no response in voltage/current adjustment 2. Spinning knob leads to inconsistent voltage/current adjustment 3. Spinning knob leads to opposite than expected voltage/current adjustment

The Repair

Replacing the rotary encoder will fix the issue. It is a straightforward task that can be completed in 1 hour depending on your soldering experience level.

Tools:

Phillips screwdriver
Pry tool (small flathead screwdriver)
Solder sucker or solder wick
Soldering iron

Part:

PEC16-4015F-N0024 (~$2.50)
I think you could also use this part: PEC16-4215F-N0024 its the same except it has detent or 'clicks'. I chose the smooth wheel to preserve the original feel of the instrument.

Note: I've heard that you can clean the rotary encoder to get it working again but the part is cheap enough and soldering it is straightforward and should lead to a longer lasting fix.

Unplug the power supply. Safety best practice is to wait ~30minutes to ensure all high voltage capacitors are discharged. I'm not sure you are exposed to any high voltage traces when just removing the front panel, so proceed at your own risk.
Remove the plastic knob. Pull straight out from the front panel, evenly on both sides
Remove the nut and washer from the rotary encoder. It fits an 11mm socket if you have access, but can be done with pliers.
Remove rubber bumper from the front face of the power supply by stretching and pulling
Remove 4x Phillips head screws from the sides of the front panel
Gently slide the front panel away from the rest of the power supply body. Careful not to yank as there are many things still attached to the front panel.
With a small pry tool/flathead screw driver, gently pry up the green tab on the PCB and slide the PCB so the 3 white plastic tabs can slide through the PCB. Only pry it just enough to get over the white tab, as this can easily snap. If you are having trouble here, first check you removed the nut and washer from the knob on the front. Second, there is a cutout that you can fit your finger in to push the PCB laterally.
Separate the PCB from the front panel, it will still be attached to the main body by a while cable harness. Unplug the cable. Pull straight, as pulling crooked will bend the pins. I gently press with my fingernail to slowly work it off.
Desolder the old rotary encoder. Before doing this, do a quick visual fit check of your new rotary encoder to ensure it fits. I found a solder sucker works great for the 3 signal pins. For the 2 large 'clamping' pins, I find solder wick and then slowly wiggling them out, using a hot iron the whole time. You want to be very careful not to damage the traces/PCB. If you are having trouble, you can always cut the pins off of the rotary encoder so they are individual parts.
Solder the new rotary encoder on. Clean up all the solder out of the holes first so the new encoder slips in easily with some adjusting of the clamping pins. Make sure you solder the new encoder flush down to the PCB. Try not to wick too much solder to the topside of the PCB, to make the lives of future repairers a little easier. Clean up your flux!
Reconnect the white cable to the PCB with the correct orientation
Make sure you did the previous step, then place the PCB back in the front panel, sliding it until the PCB tab clicks. If it doesn't seem to be going, make sure the rubber buttons on the front panel didn't fall out.
Slide the front panel back onto the body. Make sure the power button slots in its cutout and no wires are getting pinched.
Reinstall 4 Phillips screws
Squeeze rubber bumper back on
Plug in and test! (You can test at an earlier step at your own risk)