[Hallicrafters] Re: HT-32 audio

[Al Parker]

Hi Guys,
    The ALC article is in Feb. 1965 CQ mag, by W8PZX.  I got a cc of it a
few wks ago from Bill, WD0X, (I think), and it looks like a good mod.  I
plan on trying it the next time I am forced to get into my HT-32, as that
and the absence of a plate current meter are the only small dislikes I
have.  (I don't work inside anything that heavy unless forced.)
    It also covers, in an old fashioned way, going to SS in the p.s.
That's definitely recommended.  I don't agree with it where it doesn't
fully remove the 5v filament winding from the ckt, and nowadays you sure
don't need 5 diodes in series in each leg.  It does recommend 150 ohm
dropping resistors in each LV leg, to reduce the voltage.
    One thing to try with your on-the-air checks would be to switch
sidebands and see if the audio quality changes.  If so, then look at
carrier osc. freq. and sideband switching osc. freqs.  An aside, I've seen
ref. to the 7360 as bal. modulator in the HT-37, and maybe HT-32, but all
that I've seen, and all schematics I've seen don't use the 7360.  I think
it came along very shortly after the HT-32 was introduced, I remember
thinking in the late 50's early 60's that I oughta buiild a prod. det.
using one, but never did.  Since then it's become hard to find, and not a
very easy tube to use thruout it's (short) lifetime.
    I've always wanted to do an audio input sweep as Mark suggests, maybe
someday I'll get a round tuit.
73,
Al, W8UT

----- Original Message ----- 
From: "Mark Shaum" <k9tr at dtnspeed.net>
To: <hallicrafters at mailman.qth.net>
Sent: Thursday, May 05, 2005 12:51 PM
Subject: Re: [Hallicrafters] Re: HT-32 audio


>
> 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
>
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