[Milsurplus] Crystal Oscillator Vexation/TDQ

[David Stinson]

----- Original Message ----- 
From: "Rob Flory" <robandpj at earthlink.net>
To: <milsurplus at mailman.qth.net>
Sent: Wednesday, December 26, 2007 12:26 PM
Subject: [Milsurplus] Crystal Oscillator Vexation/TDQ


> Hi All,
> I have never had such a frustrating experience as I am having with my TDQ 
> transmitter. The oscillator was hammed up a bit, so I restored the circuit 
> to its original configuration, but I am not sure of the values of the 
> components.
>
> The oscillator won't drive the first tripler.  DC values look good, with 
> 200V on the screen and 250+ on the plate of the 807 oscillator, which runs 
> a 3rd overtone crystal at 16Mc(5.3 crystal frequency).  I've tried 50k, 
> 100k, and 150k for crystal load resistance.  20 and 100pF for feedback 
> cap.  Looking at the plate circuit with a scope, I can see the 5.3Mc and 
> 10.6Mc overtones but not 16mc.  Tuning the oscillator plate tuning results 
> in a plate current peak at 10.6Mc.  Nothing at 16.  When I put a 7Mc. 
> crystal in, I can get an Ip peak at 14Mc, and no joy at the tripler 
> either.
>
> The grid circuit of the 1st tripler seems alright.  I've tried replacing 
> the 807 oscillator and the 829 tripler.
--------------

Help me out here, guys- I ain't the brightest candle on the cake.
Most of you are smarter than me in this stuff.

Since you have 60 volts across the cathode resistor,
the mulitplier stage must be drawing current.
A frequency multiplier either has to run in Class C to generate harmonics
or run with low plate potential so it saturates quickly and produces them by 
"flattopping."
If we assume you're using high plate voltage (most likely),  you must have
grid drive or positive bias of some kind in order for the tube to draw 
current.
Otherwise, the cathode resistor would drop no voltage and it would read 0.
So either you have so much drive the stage is being driven linear
up toward class B or even A, thus limiting harmonic generation,
or you have positive leakage through the grid coupling cap from
the preceeding stage.


Pull the tube from the stage before and check your cathode resistor again.
If it has voltage, the tube is not biased to cut-off  (Class-C)
and it's not going to generate enough harmonics to work properly.
If this is the case, than something is making the grid of
the stage positive besides a drive signal.
The most usual culprit is leakage in the grid coupling cap,
which may not show up until it's got high voltage to block.
Even a small leakage will drive the stage into DC conduction,
spoiling harmonic generation.

GL OM ES 73 DE Dave AB5S