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Author Topic: Curing squegging / transmitter Parasitic oscillations?  (Read 3985 times)
KC9KEP
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« on: February 09, 2013, 04:48:07 AM »

Hello,

Thank you all for your help with my circa 1965 SSB transmitter problems (posted earlier)! 
I believe I’m much closer to correct operation.

In my last messages, I did a lot of work by-passing DC points, adding shielding, re-dressing leads, etc.

I also peaked the resonance of L3/L4 and L5 tank/driver circuits by using my spectrum analyzer and located aluminum shielding cans to replace the previous steel cans.  I checked through the audio pre-amps and they look good (on a scope) to me as well.  The neutralizing capacitor does null the output with plate voltage removed from the 6DQ5.

The transmitter will operate until I peak both the Plate/Output tank capacitor and the Grid Tank capacitors at which point, it breaks into parasitic oscillations.  De-tuning either tank will reduce output level and cause the transmitter to stabilize.

I do believe that I’m seeing squegging; perhaps oscillations at multiple frequencies.  It is very visible on the scope and spectrum analyzer.

I can further adjust the neutralizing capacitor to stop these oscillaions, only to end up “chasing” the squegging.  If I eliminate it for one setting of the plate capacitor, it will re-appear at another setting of the plate capacitor.

As N3QE describes it,
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KE3WD
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« Reply #1 on: February 09, 2013, 07:39:47 AM »

Sometimes adding a bit of capacitance in series with neutralizing cap using a suitable cap in the pF range can move the neutralizing point to better match some of the newer tubes. 

I recall one case where I had to put some few pF in parallel with the neutralizing variable cap in order to achieve the desired result, never figured out exactly what was going on with that one, but didn't spend a lot of time theorizing or agonizing over the why. 

Worth trying.


73
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G3RZP
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« Reply #2 on: February 09, 2013, 08:18:56 AM »

First question is what frequency are the oscillations? If they are at the operating frequency, then the neutralising needs fixing in some way. If they are VHF, then you have a problem with the parasitic suppressors. If they are LF, the usual cause is RF Chokes in grid and plate circuits.

A quick and dirty but effective method  is a small neon taped to wooden skewer. Start at the plate cap and run it around the circuit leads. A distinctly blue/purple glow that disappears as you get along to th tuning cap is an indication of VHF parasitics. A deep red glow that carries on along the tank coil suggests LF parasitics. For these tests, use no drive and reduce the PA bias until the plate current and volts gives about half the rated plate dssipation.
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WB6BYU
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« Reply #3 on: February 09, 2013, 09:14:35 AM »

What happens if you disconnect the audio input to the modulator:  do you  still
have the same problem?  If not, it might be RF getting into the audio stages
and causing squegging there, which then modulates the RF stages with garbage.
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KC9KEP
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« Reply #4 on: February 10, 2013, 02:39:54 PM »

Thanks guys :-)

GW3OQK suggested that I increase the value of the "300 pf at the bottom of the grid coil .. should be more like 1000 to 1500 pf."  Adding 680 pF to the 300 pF seemed to work well.

With the 6DQ5 plate voltage and cathode resistors disconnected, the neutralization capacitor seemed to work well.

I was looking at the 3 MHz signal at the transmitter output using a scope and adjusting the neutralizing capacitor for a null, which happens when the neutralizing capacitor is at about 75% meshed. 
After the 3 MHz signal was nulled, I swept the plate tank capacitor and there were no sudden peaks or jumps. 

There was another peak that appeared at the end of the plate capacitor's travel at 5 MHz, but this is the FT-243 crystal frequency (that gets hetrodyned with the 9 MHz "suppressed" carrier frequency). 
Since I wouldn't tune the plate tank to peak that frequency, I wouldn't see this as a stability issue.

But .. When I reconnect the 6DQ5 plate voltage and resistors, my problem comes back .. i.e., when peaking
both grid and plate tanks, the output pops up and oscillates.  The cathode current goes way up.  Numerous
harmonic lobes pop-up around the 3 MHz fundamental output frequency.

My spectrum analyzer shows these oscillations right on top of the main 3 MHz lobe vicinity.  There are
no oscillations higher up .. for example, nothing at 100 MHz on up.  I can audibly hear the amp "sizzle"
when this happens.

I did remove the audio pre-amp tube .. no difference.  This morning, I could agitate the oscillations by touching the frame of the FT-243 crystal.  Maybe RF is getting into the VXO chain?

Do you suppose the output section still needs some sort of neutralization?  It looked really good when
adjusting the neutralization capacitor with the 6DQ5 power disconnected.  Or am I barking up the wrong tree?

One other thing that I don't like.  The plate voltage is very high on the 6CL6 amplifier/driver.  The spec is
300VDC max but it measures at 355VDC.  (My first 6CL6 became intermittent last night and had to be replaced.  I
wonder if I should add some plate resistance.)

So, copious thanks again everyone.  (I'd hate to give up after all this effort!)

73

--KC9KEP
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KC2ZFA
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« Reply #5 on: February 10, 2013, 04:15:34 PM »

Is the 6cl6 operating straight through (80M in/80M out) ?

If yes, it may need to be
neutralized too with a few turns near L5 going to a few turns near L4 (you'll have to
experiment with the number of turns). The loops should be connected with one 180 degree twist in
the wires connecting them, or with one wire between them and the loops oriented in the opposite
sense, i.e., on the L4 loop ground the turn nearest L4 but on the L5 loop ground the turn farthest
from L5 and connect the loops with a single wire.

Also, it wouldn't hurt (I think) to put a parasitic suppressor on the 6cl6 plate.

Peter
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KC9KEP
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« Reply #6 on: February 10, 2013, 04:34:21 PM »

OK Peter .. Good ideas for me to try.  Thanks!

Yes, the 6CL6 is running "straight" .. same freq in and out.

As a matter of fact, it was one ill that I thought I had cured.  At first,
I could get the squegging with the 6DQ5 out of its socket.
The 6CL6 would break into oscillations by itself.

Shielding the 6CL6 wire leads seemed to help, but it could still be a trouble spot.

I know what you mean about a coil next to L4 & L5 .. sort of a
cancellation effect.

73

--KC9KEP
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