HX-20 Low Audio

[Al Waschka]

Fellow Heath'ens -

I am restoring an HX-20 to go with a Drake 2-B to replicate my first SSB
rig.  I got the HX-20 along with an HR-20 receiver and an HP-20 power
supply at the local hamfest for $50.  They are a little rough, but they
should clean up OK and after a little touch up paint on a few front
panel scratches and repainting the cases they should look like new.  The
receiver had a problem in the S-meter which I fixed and could use an
alignment.  The power supply was missing the indicator lens.  The
transmitter didn't seem to work at all.

I have been through the HX-20, found and fixed several problems (open
resistor, bad LSB crystal, cracked slug) and aligned it. Power output
seems low (40 watts on 80, 40, and 20, decreasing to about 10 watts on
10 meters.  The heterodyne oscillator voltages are low (about 60% of
what the manual says) but the tube checks good.  Except for the low
output it seems to be working on CW but not very well on SSB.  At the
moment I do not have an LSB crystal, so I have performed all steps with
the USB crystal in the LSB socket.  Since this oscillator is after the
crystal filter, I don't think it makes any difference.  After all
alignment I returned the crystal to the USB socket and operation in USB
is identical to that previously in LSB.

I have been through the entire alignment process with no abnormal
indications.  The "carrier null and sideband frequency adjustment"
process on page 82 where you set the carrier oscillator so that the
power output is 1/8 scale works.  The power output with the modulator
unbalanced and the frequency tuned off is about the same as with the
audio gain set at CW which bypasses the filter.  This all seems to
indicate that the filter and V4A are OK.  The carrier also nulls out
which seems to indicate that the balanced modulator is working OK.  In
SSB I can just barely see the output meter wiggle with the audio gain
wide open. The meter is full scale with the audio gain set at CW.  V1,
the audio amplifier and modulator driver checks OK on a tube tester and
by substitution. Two microphones work about the same.

Maybe a coupling cap bad or something.  Does anyone know what level
audio should be getting coupled into the balanced modulator?

Does anyone have any other ideas?

Thanks,

Al K5TAN

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