Folks,
I got my 38S built to breadboard/smoke test stage last
weekend (along with several TenTec SW kits for the Boy
Scouts, but that's another story...). It powered up
and ran fine, with some minor tweaking, except for one
problem...
There was a nearly continuous moderate level crackling
noise (like chicken frying) in the receiver. This was
independent of RF gain, power supply, or rig location.
After much effort I was able to track it down.
I built the 38S with C26 = 330uF, 3 extra wraps around
L1 for tuning range, and an audio taper pot for P2 (but
unfortunately, reverse range, so I tune low-hi CCW!).
Otherwise stock at this point. I do have the "thump".
In tracking down the "fried chicken", I revisited all
the solder joints, cleaned up the flux some, and reseated
all the IC's (all socketed, yes, tin sockets), and even
swapped NE602's. No effect.
I tapped around (non-conducting tapper, of course) to
find anything loose, and found C18 very sensitive. (This
sensitivity to physical vibration is called microphoning.)
I replaced C18 with a junk-box ceramic .1uF, and it got
worse! I then put in a relatively HUGE molded higher
voltage cap, and the "microphoning" went away. The fried
chicken did not.
I hooked up my rather mediocre oscilloscope and tracked
the fried chicken back through the audio circuit through
C15. Square wave-like thrashing between 2 voltage levels,
~4 mV. I swapped C34, C14, C15, put .1uF between pins 4&8
of U4, again to no effect. I then noticed it on the key
jack, and then chased it to the +8V supply!
I then took out the IC's one by one and powered up, and
STILL had the fried chicken. Well, I happened to have a
7808 in my junk box, so I replaced VR1 (the *V regulator,
and YIPEEE!!! Fried chicken all gone! Buuurrrp. Phew.
Now I can go back to adding mods, stopping the Thump,
putting the rig in a box, and getting on the air! The
fried chicken chase ate almost a week of spare time...
If you hear me out there, QRS a bit, my CW is pretty
rusty, but it'll pick up after a few rag chews...
72's, (and Norcal, THX!, the 38S is finger-lickin' good!)
--Mark
-- Mark Gaponoff (gaponoff@macconnect.com) 73's de KJ7EM.
"Life has meaning, but a poor signal-to-noise ratio."
-- Ann Gaponoff (gaponoff@macconnect.com)
"Si hoc legere scis, nimium eruditionis habes."
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