[ARC5] Parasitic suppressors on ARC-5 VFO?

[Ian Wilson]

Having a tough time getting a 7MHz VFO to be stable. The effect is a
random 'warble' of a few 100Hz, with occasional 'pops' of a few 100Hz.
The VFO is fed from a stabilized supply.

I believe the problem to be a parasitic VHF oscillation (see later for
stuff that I've already tried). If I insert a 1k grid stopper resistor
the problem goes away (but chirp and drift are made much worse). Is
there arcane wisdom on parasitic stoppers for plate & grid of the 1626
for this type of problem?

Thanks for any tips, ian K3IMW

Things that I've tried already:
- removing the 1625's
- using a different power supply
- replacing all the decoupling capacitors
- replacing grid leak capacitor
- replacing the dogbone ceramic cap in the tank enclosure
- replacing the resistors around the 1626
- cleaning the VFO variable capacitor
- cleaning between the tube socket pins
- disconnecting/replacing the 1629 (really desparate)