[R-390] More PTO stuff

[Tim Shoppa]

N4BUQ wrote:

>  I was thinking of attempting to replace only the 
> 15pF coupling capacitor.  That one may not be as temperature sensitive
as 
> the others in the "tank".

Certainly it has a smaller total capacitance.

Regulars will recognize that I had a lot of fun screwing around with
Cosmos PTO innards several months back. Investigating what small
tweaks to endpoint inductors, cleaning up the grease inside a godawful
Raytheon rebuild, etc. If you want to replace PTO innards for fun
and educational value, I'm sure there's a good amount to be had. But
in terms of productivity, if all you want is a working aligned PTO in
the end, then this kind of horsing around is EXTREMELY time consuming
and probably not worth it because you'll end up with a non-alignable
PTO most likely.

Again, I don't want to discourage you doing experimentation and posting
the results. I myself am very interested in the intricancies of PTO
alignment
and calibration outside the goals of "just get it working right".

But if you've got no problem with PTO alignment and only
problems with output level, then randomly subbing out tank capacitors
will cause you more problems than you solve.

> HOWEVER, I'm not sure what effect connecting the signal generator
directly 
> to the grid does to the bias point.  This may account for the increase
in 
> signal more than anything.

Very general points:

A one-tube oscillator is by its nature a "self-limiting" oscillator. If
the gain
was less than one with low/zero amplitude, it wouldn't oscillate. As
oscillations
start to build up, eventually nonlinearities in the active element (in
this case,
it's supposed to be the tube) to limit the gain to one.

The nonlinearities that limit amplitude are VERY closely related to bias
points.

Indeed, I think you've nailed the problem at bias (including quiescent
plate
and cathode voltages/currents).

Squirrely non-ohmic resistors (especially in the plate and cathode) can
make
power-off ohmmeter readings not directly relevant. My one piece of
wisdom:
if a carbon comp resistor exhibits being more than 20% off or shows
evidence
of past charring/swelling/overheating, then assume the worst and just
sub
it out. ALL 2.2K plate/cathode resistors are suspect.

Tim.