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Re: SX-28A Hum

 

Ch2 and C-43 form a parallel resonant circuit, a tank, if you will.
It is in the plate lead of the pre amplifier half of the 6SC7. When the
switch is in the position marked IN on the diagram it is shorted out by
the switch. The plate load of the tube is then R-37. When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out. Note that C-44 along with R-48 is a bypass filter for the B+ going
to both 6SC7 plates. Since the plate load with the switch in the IN
position is resistive there should be no frequency discrimination. In
OUT the plate load is a resonant choke (about 1100 Hz). I was surprised
the frequency is not lower but calculated it a couple of times. I have
not looked up the plate resistance of the tube. R-36 and R-37 are 100K.
C-44 is 10uF and R-48 is also 100K so there should be no audio at that
point even down to quite low frequencies.
As far as fidelity the SX-28 was intended to be a good fidelity
receiver for AM broadcasts. Like the Super-Pro, it is designed to have a
wide IF and, for the time, a relatively high quality output amplifier.
While broadcast stations in the old days were required to have good
performance to about 10Khz (double the standard now) few receivers could
recover much beyond perhaps 4 or 5 Khz. Hallicrafters offered a "High
Fidelity" speaker for use with the SX-28, a bass-reflex made by Jensen.
The bass boost offered is, IMO extreme according to the response chart.
There is a crude high frequency control, all roll off, probably to
reduce the effect of static and other noise. A HF boost would have
required another stage of amplification. They knew how to do it, see the
old (third edition) of the RDH for some circuits.
I think the labeling of the BASS control in the handbook is an
error even though it got continued in the 28A. Someone with an actual
receiver can determine it for us, I am just guessing from what's on the
schematic.
I am strongly suspicious of the tube, easy to prove by substituting
it.


On 2/16/2025 11:11 PM, Jim Whartenby via groups.io wrote:
So the hum in question might just be 60 cycle leakage instead of the
originally stated 120 cycle?? I guess that this is possible but what is
the explanation for the loss of the hum when SW10 is in the Bass (boost)
IN position?

I believe that the SX-28 schematic is correct after all.? With SW10 in
the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38
make up the plate resistance.? In the Bass (boost) OUT condition, CH2
and C43 are in circuit but only R38 is used as the plate resistance.? A
lot of control is accomplished by a simple SPDT switch.

It seems to me that the curves in Figure 11 of the manual are believable
since the contribution at 1kc of CH2 and C43 are, as previously stated,
clearly evident in the Bass (boost) OUT curve.? What the contribution of
R35 for the midrange frequencies in either curve is not mentioned but
clearly, the lower the resistance of R35, the lower the available
midrange audio frequencies.

Perhaps part of the problem is that the SX-28 was made a decade before
the high fidelity craze of the 1950s.? Today, over all tone is
considered to be composed of bass, midrange and treble frequencies.
Treble frequencies for the most part are not available in the SX-28
since the high audio frequencies are limited to perhaps 3 to 4 kc.
Midrange frequencies are fixed by C40 in the phase splitter and by C42
and the R35 pot in the 1st audio triode section of the 6SC7.? Since the
SX-28 is, in the end, a communications receiver, the loss of treble
frequency control is not an issue.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy


--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
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