Hallicrafters SX-28 issue
 

I have observed (while monitoring mixer output with an oscilloscope
during rf alignment) that the output of the mixer is, at zero beat,
about one half of what it is a few KC away from zero beat on all
bands. I have successfully performed IF and BFO alignment several
times (with the same result each time) and the natural crystal
frequency is 455.9 KC.

To me this suggests that the mixer is putting out a signal that is
different than the IF frequency, and thus I am losing some
sensitivity.

Can anyone suggest if and how I should adjust the local oscillator so
as to get maximum signal strength from the mixer at the IF frequency?

Hallicrafters SX-28 issue
 

"Hank" wrote in message
...
I have observed (while monitoring mixer output with an
oscilloscope
during rf alignment) that the output of the mixer is, at
zero beat,
about one half of what it is a few KC away from zero beat
on all
bands. I have successfully performed IF and BFO alignment
several
times (with the same result each time) and the natural
crystal
frequency is 455.9 KC.

To me this suggests that the mixer is putting out a signal
that is
different than the IF frequency, and thus I am losing some
sensitivity.

Can anyone suggest if and how I should adjust the local
oscillator so
as to get maximum signal strength from the mixer at the IF
frequency?

I'm not quite sure what you are looking at and what you
mean by zero beat. Normally, to set up an IF with a crystal
filter the test signal, whether at IF frequency or an RF
signal, is adjusted to be right on the crystal frequency.
This can be done using the BFO but not to get zero beat but
to get the maximum output at some convenient audio
frequency. Once this is established the IF transformers are
adjusted for maximum output. Most receivers are set up using
a modulated signal with an audio frequency well within the
pass band of the IF, say 400hz. The IF is adjusted with the
AVC off and the RF or IF level at a minimum so that the RF
gain can stay high. The reason for this is that some IF and
RF stages suffer from some detuning as the AVC voltage is
increased. You want maximum selectivity for weak signals so
they should be set up with a weak signal.
I prefer to set up IFs using an RF signal rather than
direct injection to the mixer because I've found that direct
injection can sometimes detune the stage a little.



--
Richard Knoppow
Los Angeles
WB6KBL

Hallicrafters SX-28 issue
 

On 3/19/2010 9:30 AM, Hank wrote:
I have observed (while monitoring mixer output with an oscilloscope
during rf alignment) that the output of the mixer is, at zero beat,
about one half of what it is a few KC away from zero beat on all
bands. I have successfully performed IF and BFO alignment several
times (with the same result each time) and the natural crystal
frequency is 455.9 KC. snip

Hi, Hank

I don't have an SX-28, but have done a similar alignment
with an HRO-50. One key thing is to find the "natural crystal
frequency" with the phasing capacitor set to half mesh.
You need to eyeball that capacitor, in case someone slipped
the phasing capacitor knob zero setting over the years.
If necessary, mechanically adjust the phasing control knob
to zero with phasing capacitor at half-mesh.
Then align the BFO coil for the same frequency, with the BFO
capacitor set to half mesh. If necessary,
mechanically adjust the BFO knob
so the BFO knob reads zero with that half-mesh
BFO capacitor setting.

Then use that frequency to align the IF transformers.
On my HRO-50, I wound up with a 457 KHz IF
center frequency.

73,
Ed Knobloch