Fixing my Weller WESD51 soldering iron - quality not found

I went to the only local electronic part store eons ago to buy replacement part for my older soldering iron. It was a decent iron that lasted such a long time that the store no longer carry the replacement parts.  I bought the WESD51 as it was on a back to school sale and Wellers parts and accessories was easier to find. It was a lemon. Ironically the serial number happens to be 0404 (Quality not found!?)

4 hidden screws under the rubber feet

Right from the start I had a lot of issues with the iron not reading its temperatures correctly. I thought it was a connector issue, but later I found that it was a manufacturing quality problem.  They change a PTC fuse into a larger package (green 2X2 at top of PCB) but didn't do a good job of touching up the soldering on the mismatched footprint.  So I had to fix it up in the first few months.

Control PCB

A trimpot has limited cycles, so personally I wouldn't use it for a temperature control knob. It did last a very long time, but obviously not long enough vs the rest of the iron.  A few months ago,  the temperature setting started jumping beyond my set point due to wiper contact issues. I knew I had to repair it.

PCB back side

As I was opening it up, I found out that they didn't do a proper crimping job on the AC leads of the power transformer as they tried to crimp a tinned wire instead of bare wires!?  Solder is soft, so eventually the wire came loose.

Poor crimp job

Here is the cheese knob for the trimpot.  It was held together with a self tapping screw. I had to put in some hot glue to hold the screw.

Cheap plastic knob for the temperature control

 I ordered the smallest 10K linear potentiometer I can find and they arrived today.  I couldn't find any 2K pots.  Since it is to provide a 0V to Vcc to the ADC. The increase in source impedance could affect the ADC result. One way to fix it it to wire a 0.1uF cap from wiper to Gnd, but that is not needed here.

They are not the greatest quality, but I got a bag of 5 for cheap. They came with washer and a nut.  I use a pencil to trace the outline of the washer on the PCB and milled out a notch for the locating pin.

Modification to the PCB

I mounted the pot onto the PCB and soldered the pins to the existing pads with wires.

Pot connections

I had to remove a bit of the material in front case to clear the mounting hardware for the pot.

Making room for the mounting hardware

I use a reamer to enlarge the hole, so that the PCB can be mounted flush.

Old Radio Shack reamer

It ain't pretty as it is soft plastic. The hole is hidden away by the knob.  The shaft is barely long enough.

The big hole I made

I broke off the cheesy plastic off the knob.  I got some cheap plastic knob from China that is small enough to fit inside the cavity.

Temperature control knob

It wasn't tall enough nor wide enough, so I had to improvise.  I melted about 1/2" of hot glue stick with my hot air tool set at 120C in the cavity.

Hot glue to the rescue

I pushed the plastic knob into the molten hot glue and wait for it to set.  I filed down the knob so that it is flush.  I added some extra hot glue for support on the sides as well.

Improvised knob

One of the things I did to prevent the plastic mounting post from splitting was to use a piece of cable around it.  I wish they would use threaded inserts.