[Hallicrafters] SX-28 Bass boost issue

[Art Lebermann]

Russ....

The SX-28 schematic is correct.  Part of the confusion lies in the fact
that the SX-28 manual identifies CH2 as the "bass boost choke".  This is
not actually true.  With the Bass switch "IN", the coupling to V14 (6L6) is
a conventional RC circuit.  R37 is the plate load for the first section of
V12 (6SC7).  CH2 and C42 are not in the circuit.  Frequency response is
determined (in part) by the tone control, with rapid roll-off at higher
frequencies.  This gives more apparent bass.  With the Bass switch "OUT",
R37 is removed from the circuit, and the parallel combination of CH2 and
C43 becomes the plate load.  CH2 and C42 are resonant at about 1100 Hz, and
have the effect of "peaking" the response at this frequency.  This "EQ"
produces a relatively flat response from 30 Hz to around 1200 Hz
(Selectivity - "Crystal Sharp").  For reference see the "Audio Filter
Curves" in the SX-28 manual (Fig. 11, in the SX-28A manual).  The "Audio
Fidelity Curves" show the combined effects of Selectivity, Tone, and Bass
controls.

So - put the circuit back the way the schematic is drawn, and all should be
well!

73,
Art Lebermann
W6REQ



> [Original Message]
> From: Russell Barber <rbarber at pacbell.net>
> To: <Hallicrafters at mailman.qth.net>
> Date: 5/27/2007 5:34:59 PM
> Subject: [Hallicrafters] SX-28 Bass boost issue
>
> Dear group,
>
> I am doing a complete down-to-the chassis rebuild of a SX-28.  I started
with the power supply and now and am now finishing the  rebuilding the
audio amp and phase splitter.  The schematic shows the bass boost circuit
CH2 and C43 being shorted by the bass boost switch SW1D when the bass boost
is engaged.  When not engaged, the schematic  shows the plate resistor R37
is shorted out.  This looks like a error to me so I built the circuit
differently, I wired it so the CH2 and C43 are engaged with the bass boost
switch open.  When the bass switch is shorted, it shorts out CH2 and C43.
Also the plate resiter is always inline.
>    The issue I now have is that I only have 2 db of bass at 950 hz.  The
audio chain is flat out to 15 KHZ and has less than 1% distortion at 1 watt
across 5K.  All appears to be normal exept the rather weak and too high in
freqeuncy bass boost.  
>    Has anyone seen this issue and could shed some light on what is going
on?
>
> Best regards
> Russ Barber
> N3HPX/6
> ______________________________________________________________
> Hallicrafters mailing list
> Home: http://mailman.qth.net/mailman/listinfo/hallicrafters
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:Hallicrafters at mailman.qth.net
> ----
> List Administrator: Duane Fischer, W8DBF **for assistance**
> dfischer at usol.com
> ----
> Hallicrafters Collectors International: http://www.w9wze.org