[Central-Electronics] 20A Sideband Rejection Problem FIXED

[Jim candela]

Update,

	Getting rid of the wire wound carrier null pots DID NOT fix it! This is
bizarre, but a prior owner moved the GREEN RF coil in the balanced modulator
about 3/8" further away from the RED coil. It was directly under the diode
module instead of alongside like the red coil. The wire wound pots were set
pretty far from center, and that was a clue. A quick look at the Peak RF
voltages coming off the RED and GREEN coils showed that the GREEN coil
output was about 20% that of the RED.  I put everything back to stock, and
balanced the coils out for a good null with the pots near center. The Peak
RF voltages coming off the coils are nearly equal now. BODA-BING-BODA-BOO
the problem is fixed! I guess the null circuit as it was putting stress on
the diodes, and maybe they were leaking from too high a back bias when the
RF power (SSB) was too high. The diodes may have been the non-linear
element.

	Now I get a nice pure CW on SSB with 1Khz audio input up until about 95%
maximum power. In that last 5% I see distortion from the audio driver stages
starting to show itself. Even so this is pretty good. I hope to soon close
this chapter on the saga of QRO 20A #3. I have been avoiding the chapter of
how to get the EL-34 working correctly on 40, and 20 meters..

Hmmmm maybe I can use my nice Bournes 1K "Knop-Pots" in the 20A balanced
modulator. These are pricey 10 turn pots with a built in dial that has two
hands like a tiny clock (sell for about $70.00 each).

Thanks for reading, and to those that replied outside the group,
Jim
WD5JKO



-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of Jim candela
Sent: Thursday, June 17, 2004 11:16 PM
To: central-electronics at mailman.qth.net
Cc: jrsa55 at yahoo.com
Subject: [Central-Electronics] 20A Sideband Rejection Problem Found


        Hi All,

        I know I've been boring you guys with my QRO escapades, but this
last rig
has really been kicking my rear end. I don't want to admit how many hours
I've put into this one. I think I found the culprit tonight. The problem was
basically that as the RF power goes up using a single tone input (SSB Mode)
the carrier output from the tone starts showing modulation (distortion) that
gets progressively worse with increasing power. This 20A with a single EL-34
with 600 volts on the plate puts out 50+ watts carrier on 160 meters, and it
runs 12.5 watts AM 100% modulated, and the waveform is picture perfect.
Switch to SSB, null the carrier, and insert a clean 1 Khz sine wave into the
high level audio input, and advance the gain. The LSB carrier waveform is
pure DC up to 10 watts out, and then it goes south with increasing level.

        Looking at the audio going into the balanced modulator circuit does
show
SOME distortion. Sure both the 6U8 triode, and the 12AT7 drivers all
contribute some distortion, but nothing like what I see at the RF output of
the rig (only on SSB). I went as far as adding a multi-pole negative
feedback circuit around the 6U8 triode to cut distortion, and Taylor the
frequency response some. Total waste of time. I started looking at the
balanced modulator diodes, and even swapped them with a known good set. No
change. Using a scope looking at the waveforms around the diodes (yes I know
the scope is detuning the circuits) showed some really ugly stuff that only
appears when the RF output shows distortion, and on AM it all looks good.

        Then it occurred to me. Many years ago a former owner of this rig
replaced
the balanced modulator pots with 10 turn wire wound pots. They seem to work
great for nulling the carrier, so I never gave it a second thought until
today. The pots are bypassed end to end like stock with a .005uf ceramic
disc capacitor. As a test I bypassed one side of one pot end to wiper with a
,005 uf capacitor. Low and behold the distortion was reduced considerably,
but the null was upset a little. Still I could get to about 30 watts before
the distortion was nasty. It looks like it's the dam WW pots!

        So here is where I need help from the group. On the 20A the balanced
modulator runs at 9 Mhz. I suppose the inductance of a 1K wire wound pot is
going to behave differently than a 1K carbon pot. Can I just put two .005 uf
caps on each pot (wiper to each end), or is it going to require me going to
carbon pots? My head hurts trying to visualize the goings on with a phasing
balanced modulator circuit, but on AM only one side (2 diodes) gets audio,
and on SSB both sides get audio 90 degrees (or 270) to null the unwanted
sideband. If anybody knows the nuts & bolts theory of this circuit, please
tell me whether I can make the nice 10 turn wire wound pots work, or do I
need to go back to a single turn 1K carbon pot?

Regards,
Jim
WD5JKO

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