Group,
Today was a interesting day in 20a land. A friend, Yogie, (kc5mip)
brought
over his ailing 20a. It had a few surprises. First the QT-1 was home-made,
and there were various pots & switches on the front & back that CE didn't
install. A little more looking, and the high dollar relay (the one with
10,000 turns of #99 wire, or so it seems) was replaced with a hermetically
sealed unit. The best was noticed last, the 6AG7's had been replaced with a
pair of 6DQ6 horizontal sweep tubes. The 5U4 had been replaced with a wimpy
5Y3.
The effort to get this thing on the air was significant, and hour by
hour
progress was made. After about 6 hours the rig relented, and things were
working fairly decent. It is interesting that the sweep tubes offered no
more than stock 20A power on 80 meters. I could get 13 watts with a 5U4
rectifier, and 19 watts with a solid state 5U4 replacement.
The 6DQ6 sweep tubes need lower screen voltage than the 6AG7's, and the
original modifier simply used a screen dropping resistor. This is OK for
class C finals, but for AB1 linear amps this is taboo. Making a good stiff
150 volt screen supply takes effort, and more time than I had. I tapped into
the voltage divider for the 6BA7 screen, and bypassed the junction with a 22
uf @ 450 wvdc capacitor. The screen voltage starts out at about 150 volts,
and sags to about 125 volts at max CW output. This is limiting the maximum
power. With a 6 hour time window, what other option was there?
The grid bias (G1) was still stock (-10 volts). Key the rig, and the 6U5
practically goes out(very dim). The B+ dropped from 380V to 240V from PA
idle current. I paralleled a 100 K resistor in the bias network with 180 K
to bump the bias to -15 volts. Now the key down B+ is a more reasonable 300
volts with 5U4 rectifier. Maybe the idle current was still too high, but no
easy way to measure it.
We could now make decent AM at 1 Khz audio, but it sounded terrible
(tinny)
on my receiver. More modifications to weed through. The 12AT7 two stage mic
preamp had coupling capacitors reduced from .005 Uf to 720 pf. After putting
that back to stock, it started sounding decent. Still at 2 watts out, and
sine wave 1 Khz modulation input, the + mod % was ~ 50% at the same time as
100% negative. All stock 20A's do this more or less on AM. My fix here is a
little negative cycle loading. This loads the audio feeding the balanced
modulator during the 180 degrees that the 1n34A balanced modulator diodes
are back biased by audio. Adding this network provides a more uniform load
on the audio amp over the full audio cycle. A 130 ohm resistor, and 1n34A
diode in series, and connected to an RCA plug (cathode to pin) and then plug
this into the rear RCA audio out (used for scope XY pattern). Must take out
for SSB. Now at 2 watts, the 20A would modulate to about 80% (+) @ 100% (-).
Drop to 1.5 watts, and it is symmetrical. This is a little worse than a
stock 20A with the negative cycle loading plug, and much better than stock
without the negative cycle loading RCA plug.
Time to try SSB. Very poor SSB rejection. Shucks. Looked at the audio
phase
shifts at red & green 9 mhz RF coils. I could vary amplitude, but not phase.
Getting late so tried another PS-1 from another 20A awaiting
restoration/modification. Nice. I got a complete null of the unwanted
sideband just from the two audio pots. Switch from SB1 to SB2, and still
perfect! No way, this can't be. I have spent days trying to get it this good
before, and settled for less! Better lucky than good I guess. Never had to
adjust any of the coils..
Look for Yogie on 3880-3890 from San Antonio for his "one of a kind"
20a.
Now I need to fix that original PS-1 that someone "adjusted"....
Regards,
Jim Candela
WD5JKO
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