Fortunately it took little effort to tame the self-oscillation in the (PA?) 10-to-4 transverter. First troubleshooting action was decoupling the DC supply to the whole transverter board. I used a PI network composed of 2x100nF, 2x1nF and one VK200-like inductor.
The inductor goes in-line with the positive voltage. Then 1nF//100nF at each side of the RF impedance.
If you land on this page for a similar problem, don't go crazy looking for the VK200 choke. Try winding as many turns as they fit on an anonymous toroidal core recovered from some switching PSU, or a molded inductor or whatever will exhibit enough inductance and low enough DC resistance. I used disk ceramic capacitors.
But it is not the end of the story. [Note to the casual reader: what follows applies to this specific transverter board]
After correcting the self-oscillation, current drain was above 40 mA. Not bad, but I had measured 24 mA.
When I connected back the transverter PTT control to Arduino, the XV went into TX. This means that the 5V from Arduino is not high enough for the transverter circuitry. I added a BJT buffer and everything went back to normal. I just needed to update the sequencer firmware.
On-the-air test to follow!