crystal osc problem

[KC2ZFA]

Any kind soul out there to do some diagnosing-at-a-distance ?

I built an exciter using the hand-drawn circuit shown below in the first pic.

The oscillator is from the Orr book (see relevant circuit and text
in 2nd pic below). I added the 200 Ohm cathode resistor to key with the 807
cathode. The oscillator is tri-tet on 20/15/10 with 40 xtal, and tuned-plate
on 80/40 with 80/40 fundamental xtals.

As built the cathode coil/cap grid-dips at 10 Mc...that accords with what
Vernon Chambers wrote long ago, i.e., the cathode LC should resonate about
midpoint between the fundamental and first harmonic so with a 7 Mc crystal
I do have a tri-tet on 20/15/10.

The (semi)good news: it "works" with 40M crystals, and I get harmonic output
too. The problem is the 80/40 coil dips in the range 3.2-6.9 Mc with the
max/min capacitance available. So, varying the oscillator plate cap near
min capacitance I get a slight dip but no oscillation, and then it goes
into oscillation with the cap at any value past near min up to max.
I'm guessing here that once I fix the plate coil to cover 3.6-7.3 Mc
I should get a dip when the thing oscillates. Would this be the
correct expectation ?

The bad news: I get no oscillation with 80M crystals. I do get a deep
well defined dip in the oscillator plate current (from 50 mA to 20 mA)
but always with the plate cap near minimum whatever the crystal freq in
the band. Can anyone divine what may be wrong ? Mind you, the plate coil
tunes the whole of the 80M band as I determined with the grid-dipper so
the plate circuit is correct.

In all cases I get 10 mA to the oscillator during key-up and between
20 and 40-50 mA during key-down. Thus far all this testing is with no
hv and no bias voltage to the 807. I did dip the output JEL coils and
they are in band. I include a shot of the innards around the 6ag7 socket.
I have not yet bypassed the filaments.

Hoping for Santa to fix me up !

[KA0HCP]

I'm a bit fuzzy tonight, but I'll take a stab at it.

Problem: not oscillating at 80m

Cause:  insufficient feedback

Solution:  Try the recommended fix for problems with 160m not oscillating:  Place 5pF cap between Plate and Control Grid.

If I got the wrong end of the stick on this, I apologize.    b.

[W7TFO]

Screen/plate E ratios are important with ECO oscillators, maybe play with that screen pot some, and watch the I.

73DG

[KB2WIG]

Sluggish Xtal maybe?


klc

[KC2ZFA]

thanks guys...but it must be the cathode resistor that I put on the 6ag7...
I just got bored reading and trying to think what may be wrong on 80M
(I have 6 80M crystals, they couldn't all be bad) so I bypassed the 200
Ohms.

Now 5 out of my 6 80M crystals work perfect. Oscillation on 80 starts
as soon as I give it a bit of screen voltage, and with the pot half way up I get
a nice dip at resonance to 30mA total for the oscillator.

And now that it works properly on 80M, I get what's expected with a 40M
crystal: no variation of osc current as I rotate the osc cap, as it should be
given that the coil doesn't get into 40M  

I guess the ghost of Orr is now smug that I couldn't "improve" on his circuit.

Please criticize my wiring...and the corrected circuit. Let me have it !

73 de Peter

[k4kyv]

Try a small "gimmick" capacitor of about 1 or 2 pf from grid to plate.  That schematic looks to me like a basic pentode xtal oscillator on 80m. The problem is with the 6AG7. Unlike broadcast receiver audio output tubes like the 47, 59, 6F6, 42, etc, the screen grid of the 6AG7 provides excellent rf shielding between control grid and plate, so there is no feedback path between the input and output of the tube.  BCL audio tubes are  poorly shielded for rf because they were never designed for rf service, so they make excellent xtal oscillator tubes, by depending on the internal capacitance due to incomplete internal shielding to complete the feedback loop.

I presently use a type 59 xtal oscillator tube in my HF-300 rig.  I selected that type because it has a separate lead out the base for the suppressor grid, rather than internal connection to the cathode, allowing me to switch it over to grounded grid service as a buffer stage for the outboard VFO. 

I once built up an exciter using a 6V6 as the first rf stage fed by an external vfo, and the thing was so  prone to self-oscillation that it actually generated a beautiful stable but annoying carrier right in the middle of 160m with the plate supply turned completely off!  That one had me scratching my head for a while, until I discovered that the hot filament of the 5R4 rectifier generated about a +15 volt charge on the plate, enough to set the 6V6 into oscillation. Apparently some stray electrons were energetic enough to strike the plate and produce that potential.  I cured the problem by rewiring the socket and replacing the 6V6 with a 6AG7.