[Hallicrafters] Re: HT-32 audio

[Mark Shaum]

Quote from Jon:
--
 better than communications grade audio.   Are there a well known set of
  tricks for the HT-32 for getting the most out of it as there are for
  say the Johnson Valiant?    Changing the first audio grid resistor,
  increasing this cap, taking out that cap, that sort of thing?
--
As Al mentions, his HT-32 sounds very nice in stock form.  There are no
published modifications that I'm aware of to address any issues with the
audio chain.  There just isn't much there to fiddle with, mic preamp
stages and the balanced modulator, period.  If a cathode bypass cap or
such went south in the mic amp, that would certainly clobber the lower
frequencies.  Has your buddy checked resistors and replaced any leaky or
off value caps in the audio stages?  Has he substituted a known good
tube for the balanced modulator (7360 I think?)

Just recalled, in some QST (I think) there was a small article or even
just a "Hint and Kinks" note about adding ALC to the HT-32 series, using
the same general circuitry as used (again, I think) in the HT-44.  I
recall a single tube being invovled, prolly sensing any smidgen of grid
current flow in the 6146's then providing feedback to reduce the gain of
an intermediate stage.

A random thought, you could partially bypass the crystal filter with
some sort of resistive pad or parallel tuned circuit that would widen
out the skirts of the bandpass.  This may not be desirable as it would
also allow more of what would be a not-so-suppressed opposite sideband
through.

I'd suggest sweeping the HT-32 with a fixed level audio oscillator and
measuring the result on a wide bandwith SSB receiver with a volt/db
meter, AGC off, with gain set below any overload threshold.    You
should see a fairly flat response for that 2400 or so hz of bandwidth.
If there are too many lows and not enough highs, or the opposite, you
could vary the carrier osc frequency to shift things a couple hundred hz
or so.

A two tone IMD test wouldn't hurt either.  There could be something
weird going on like RF feedback getting back into early stages of the
rig and distorting the audio.

I'm fresh out of ideas at this point!

73! - Mark