[R-390] Tracking down low sensitivity 0 to 8 mhz problem

[Jacques Fortin]

Jordan,

Using a 10Meg input DVM is good, but if there is not an isolation resistor
direct in the probe, connecting the + wire of the DVM directly to E209 can
detune the circuit.
Remember how the old Heathkit VTVM probe is made ?
In DC mode there is a 1M ohms resistor directly inserted just after the tip
and this was basically to prevent RF sensitive circuits to turn crazy with
the direct connection of a wire of unknown impedance.
And also because with the VTVM probe, the wire going to the meter is a coax,
making the situation worse.
Too long to explain why, but I made a DVM probe with a 10Meg inserted in the
tip.
The displayed voltage is half of the real value, right, but the insertion of
the 10Meg isolation resistor in any sensitive circuit is inconsequential.
Meaning, I just have to double the value read on DVM, and this is the real
DC value present in the circuit.

Your 17MHz oscillator problem now:
The V207 stage behave as there is not enough gain in it.
That could came from many places, the most unusual being an open or
otherwise defective decoupling capacitor.
Usual suspects: C328, C326, C275 (which is supposed to be a .033µF, btw).
Is L201 of the correct value and have a proper DC continuity ?
Could it be that T207 is damaged in some way ? 
If there is a resistive leakage across either the primary or the secondary,
you cannot get the proper signal amplitude at the plate.

Replacing SM caps by ceramic ones in resonant circuits is OK, but only if
the ceramic caps are of the NPO variety.
Unknown ceramic grade (Z5U, for ex.) cap can be OK for decoupling but not
for a parallel resonant circuit, as the losses in the dielectric could be to
high.
Or, in some cases, the dielectric constant changes with the voltage
excursions, and this kills the Q factor of the resonant circuit.
Result: loss of gain...
And you mentioned that T207 "peaks smoothly".
Well...
As far as I remember, the T207 tuning I experienced with my R-390As was
quite sharp.

73, Jacques, VE2JFE in Montreal