There was a bad batch of parts in the finals that a group of us
bought. That left me with an almost finished radio - no good! I have
an SW-80+ kit that I put together a long time ago but hardly use.
I stole the final from it for use in this radio.
After finishing the radio, I was looking forward to putting it on
the air. I had visions of hooking it up to a battery and my headphones
would be filled with signals... it didn't happen like that... at all.
First of all, I heard nothing. A little pop when the rig came on, but
that was it. I took the radio back to the bench and traced the audio
amp. I noticed that I had left off R10. After installing R10 it still
didn't work.
I hooked up an RF generator and started tracing the signal backwards
with my oscope. When I got around Q3, the keying switch, my probe
slipped and I shorted something to ground. A pop was heard and I let
the smoke out of Q3 (actually it shot its plastic guts straight down
on the board). Q3 is a 2N3906 and I didn't have a spare in my
junkbox. Some research on the net and viewing of some spec sheets
showed me that a 2N4126 is identical except it handles less
Voltage. It was still within spec for what I needed and I happened to
have a few of them so I used one. I confirmed on QRP-L that is in fact
a common replacement.
Still nothing in the amp though. Then I discovered that some of the IC
sockets had been pulled off their pads. I reheated them and tested them
with a continuity tester. OK, good to go... nope, still nothing.
I then rechecked my RF Gain and Tuning pots and realized I had them both
wired in wrong.
Finally, signals in the phones!
I then touched the keying wire to ground in hopes of hearing a signal.
Nope. Back to the scope.
I was getting some AC voltage off T2 and into T3 but it died just after
Q4. I then realized I had R24, the 500ohm variable resistor wired in
wrong. After fixing that I started to get RF out!
I peaked T2 and T3 with my scope and adjusted the aforementioned R24
to maximum gain while keeping an eye on my waveform. The waveform distorts
with too much current let loose from R24. I'm now getting about 2 watts
out and it looks clean.
While keying the radio, I was noticing a loud click on key up. I
posted to QRP-L about and Jim Kortge replied with this:
Maybe, but most likely not. More probably a
wrong value of capacitor on the keyline, or
gate of the mute FET, or perhaps a bit of unwanted
coupling between the keyline and the audio, or
maybe excessive voltage drop in the power
leads/supply to the rig. Can you tell if the
click is on key down or key up. It might help
us to figure out where the problem is.
Jim was correct. The problem turned out to be a bad connection from
the Gate of Q1 to C24. After reheating the solder joint the clicking
stopped. The rig now works great! I should also add here that Jim
has been instrumental throughout this build. He has answered many
questions and designed in some mods that I plan on trying.
I think I've skipped over some other problems I had. I spent a good six, if
not eight hours going over the rig with my scope. I traced the rcvr all
the way though and the xmit all the way back. It has been a VERY good
learning experience so far.
My next update will be after I finish an enclosure for the radio.
I will also post some oscope shots that show how to tune up the radio.
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