I've an old Tektronix 184 with an 10 MHz OCXO hooked up to a nuvistor
tetrode (valve). The crystal is connected between the cathode and the
first grid. The cathode end of the crystal connects to earth through a
variable 6 to 10 micro Henry inductor.

2nd grid should be at about 50V (I think) as there's a 75V zener between
there and the 125V anode supply, but in fact it measures at around 70V DC on
my DMM???

The anode (plate) is connected to a tank tuned to the 5th harmonic.

Without a 10x scope probe at the junction between the inductor, cathode and
crystal, the system produces an output, so the oscillator is clearly running
when I don't try to probe it.

If I put the scope probe very near the junction, it acts as a aerial, and
picks up the signal at a very low level. The moment I touch the connection
with the probe tip the oscillator stops (sometimes I see the correct output
of about 70V peak to peak as shown on the schematics for a fraction of a
second).

I suspect something is wrong with the circuit, but can't work out what it
might be ...

Why is this likely to be happening?
--
Dave Partridge (remove xwy from replyto email address)