Hi Frank
Thanks for the note, and while searching for my hum I got to know the 2C
a lot better!
Turns out, my hum was caused by the VARIAC, and once I was convinced to
turn the variac all the way up, the big hum disappeared! There is the
residual hum, and the series resistor will help reduce the hum at lower
volume levels. As Garey kindly pointed out, the PA runs wide open all
the time, so at lower volume levels the signal to noise ratio is not
good. The resistor helps at the expense of having a higher AF gain
setting for phone use. The trick would be, I guess, to find a resistor
value that gives the same comfortable listening level for both headphone
and speaker use.
So, for testing my newly acquired boat anchors, I will revert to using
the LIGHT BULB in series with the AC line to test new stuff, which would
remain brightly lit of there was a short in the radio, and fade to dim
if all is well, and avoid the potential for 'false alarms' when using a
variac. Some rigs need full AC to even work (those with tube
rectifiers, for example) so the light bulb technique is better, and much
in keeping with the era of radios we work on!
All is well now, the radio is very sensitive and after using DeOxIt to
cleanup the switch contacts etc. she's playing great!
Now, to do the same thing with the 2NT!
Best 73
Kevin K7ZS
iz oos wrote:
Kevin
I remember I had put another 1000microfarad in parallel, I believe,
with C48 and that solved my hum. Another time I got some strange
audio, there were some bad solder joints at the output headphone jack.
My advice would be to check thoroughly all the joints at the audio
section.
Consider that if you use low impedance headphones you will probably
hear anyway some hum (placing a 600ohm resistor in series would
greatly reduce it)
Maybe it might be the 50Khz oscillator (bfo) the became offset for
some reason or the 405/505Khz oscillator. In this case, I believe that
you would hear "hum" just in USB/LSB mode, but not in AM mode.
One method I use to check which stage might have some hum problem is
to see on a computer the spectral analysis. I would check which audio
frequency you actually hear that is annoying. Then I would check it at
the IF (point 4 of T8). At point 4 of T8 the signal is at 50khz, I
convert it (trhough a mixer and a signal generator) to between 0 and
20Khz and I see the output on the screen. If there is still hum I
check previous stages at 455Khz output and so on.
73
Frank IZ2OOS
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