R&S EB200 Miniport Receiver Dial Knob

I was lucky enough to get a Rohde & Schwarz EB200 Miniport receiver in working but less than mint condition. Apart from cleaning and fixing some bumps on the covers, the dial knob was very broken. This knob is a custom part designed by R&S that proved impossible to get in 2023.

Repairing the original (broken) knob

Not having any other option, I decided that fixing the old knob was the way to go. It was missing the flat part, and the bearings could be seen from the front. I used the dremel and some fiberglass PCB material to replace the missing plastic. After gluing it in place, I was not sure how it would finally turn out:

But after some sanding and a few coats of paint, it looked fine and it was comfortable to use. By looking at it, or using it, there was no way to tell how bad it was and the repair it had inside:

A better knob

One thing that was missing was the recess for the finger, so you can turn it quickly. Also, I was worried about the paint showing signs of use for being touched with the fingers. While looking for a better option online I found this very cheap knob, that had similar dimensions and the recess for the finger:

The problem was that the back side was completely different from what was needed. The original knob is just an empty shell with a threaded bottom, that once installed holds a brass knob with bearings inside. The new knob is designed to receive the typical 6mm shaft, something the EB200 does not have. The shaft on the EB200 does not rotate, it holds two bearings that let the assembly rotate. The rotating assembly includes a disk encoded with magnets that flies over hall effect sensors on the front panel to sense the rotation. So I removed the knob assembly from the receiver and carefully measured it:

I then decided what features were required in the new knob to correctly fit just like the original one:

To hold the unmodified new knob on the lathe jaw, I machined a brass holder and used some thin kapton tape to avoid scratching the anodized finish of the surface:

After the features were machines, including the 1mm-pitch threads, I could check that the brass body that goes inside the knob did fit correctly:

After deburring and last touches, I removed the holder and the knob and then had to face face 1.8mm off the brass body so that the new knob sat very close to the front panel once installed. Otherwise it would stay too far from the faceplace, leaving a 2mm gap:

And yes, once installed looks great, works well and should last for a long time without worries: