Hi, Rick
My guess is that you are hearing power supply ripple. Put
together 8 penlite - c cells - flashlight batteries and try the
radio on that. One of the downsides of direct-conversion receivers
is that they are very susceptible to power supply hum, being that
all the signal gain is at audio. If you have a gel-cell I will
permit you to use that. I'm pretty sure you will find that the hum
is gone.
Another source of hum, again power supply related, is a ground
loop between the ground on the power supply and the ground for the
antenna. Since the power connection and the antenna connection are
at different points on the board, if there's any potential
difference between the 2 grounds it will show up as hum in the
audio.
Jim W4JED
Linux user since Debian Bo
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Good morning,
I received my 40m QCX last week and took a few days to build it.
What an excellent kit with an amazing manual.
After power on everything appears to be working, I was able to
align successfully and I'm able to hear stations clearly. I have
sidetone and I hooked up the RF output and on transmit with a 12v
power supply it shows 5w output (this seems high?).
I noticed a slight hum and that increased when adjusting the audio
gain. Waving my hand around IC9 increases the hum the most.
Touching resistors in that area make a pretty loud hum (R35. R34,
R33, R28 etc)
I took some voltage readings on the ICs in this area and came up
with the following:
IC8 Pins 1, 2, 3, 5, 6, 7 are all 2.64v (4 and 8 are as expected).
IC9
Pin 1 4.84
Pin 2 4.84
Pin 3 4.5
Pin 5 2.6
Pin 6 2.6
Pin 7 2.6
Pin 8 11.8
Looking through other posts here it seems R59, and IC8 could be a
problem areas.
Looking at R59 - ohms were 3.3k. Voltage on both sides and it
appears to be 2.5. I would expect one side to see a drop? Does
that indicate a short somewhere?
I've checked for continuity between all pins on IC8 and IC9 and
found none. Solder joints seem to be OK under a magnifying glass.
I've reflowed a few questionable pads but this has had no effect.
All I have is a multimeter to troubleshoot and not much knowledge
(although I've certainly learned a lot already!)
Any pointers in the right direction??
Thank you very much!
James Daldry W4JED