How long can it take to tighten half a dozen screws, wire a dozen wires?
More time than you expect.
Based on previous experience with the documentation, I expected to have to
trace out where to bring out off-board connections. However, there is one
page in the documentation supplied with the kit that has a diagram of all
the chassis wiring and it is 99% correct. The other 1% is reversed wiring
for the narrow/wide filter switch.
I added a 10 uF electrolytic cap between the regulated power feeding the
AGC circuit and ground. I hoped that this would help the AGC oscillation,
especially since there was no large cap on the output of the regulator IC
anywhere. It certainly couldn't hurt. How did I pick 10 uF? There was
one lying on the bench. (Plus experience says that 10 uF is a reasonable
-- if slightly small -- value to put near a suspect low frequency circuit.)
I had to reform the PA transistor leads to get it to line up with the
mounting hole when the board spacers were installed. That was a little
painful but do-able. The TO-220 transistor mounts beneath the board,
bolted to the bottom of the chassis -- not the most convenient place to
work. The kit instructions mention insulating washers and hardware for the
transistor, but I found none in the kit. Fortunately, I had some Kapton
film and nylon hardware in the junk box.
Got everything wired and mounted in the box. Phew! Glad that's done.
This, of course, is the time when the most frustrating smoke often slips
out, so I gingerly turned on the power supply. No smoke. No audio on the
scope. Oh yeah, gotta tune it. Can't find the signal! Turn up the
generator level. Tune again. Turn up the scope sensitivity. There it is.
Tunes OK, but wasn't I getting a lot more signal out of the receiver
before? Check the notes. (This is why you keep notes during the debugging
sessions.) Yes, much more. OK, check power supply current. OK with no
signal. Three TIMES normal with signal. Hmmm. Start backing through the
circuit with the scope. On the output pin of the LM386 audio amp, I see
lots of signal, just as before. Between the IC output and the phone jack
is a 20 uF Electrolytic coupling cap and the volume control pot. No, no,
no, I'm not that stupid (this time)! Volume control pot is full on, and
the signal is controlled properly by it. Check the cap. How do you check
an electrolytic cap soldered into a board that is mounted securely into a
box? Usually, the PVC cover on a mini electrolytic does not cover the top
of the cap. If you put a scope on the metal top, you usually see a signal
which is roughly center tap on the cap. (Amazing, isn't it?) Put the
scope on the top of the electrolytic cap and I see a half-amplitude signal.
That can only mean that the headphones side of the cap is being pulled
pretty close to ground. It makes sense that the power consumption would go
up when trying to drive a lot of current to ground. Power off. Ohmeter
out. Sure enough. One of the standoffs used to mount the board to the
chassis is shorting to a pad on the board. Fix that and we have full
audio.
OK, a quick test of the receiver confirms that it is working. It tunes
from just below 7.000 MHz to somewhere in the novice band. Nice work
Larry!
The AGC is not oscillating. That is good news. I'm not sure that I'm
going to like the fairly long recovery time constant in on-air operation,
though.
Key the transmitter and we have a good output.
I notice that the transmitter has a loud sidetone which triggers the AGC.
This is a drawback of the design, which keeps the receiver open to hear the
transmitter directly to produce the sidetone. Not sure how this will fly
in Peoria.
Check the RIT. Works OK.
Check the audio filter again. (Forgot to tell you that I checked this out
before putting it in the box.) Works but the front panel switch is
backwards. This switches the audio filter in or out of the circuit.
Unbolt the mounting hardware and rotate 180 degrees. OK.
Check the transmit offset. Within 100 Hz of where it should be.
The front panel power LED is dead in both directions. Need to dig it out
of the glue and put a new one in.
Check the medium term VFO stability: I put the signal generator on
7074.600 and tuned the receiver for max audio level which was at 700 Hz
tone as measured with the scope. Put the scope's frequency cursors on the
audio tone peaks and then just let the receiver run for an hour and a half.
Then I checked the audio tone frequency and found that it was 670 Hz -- 30
Hz drift in 1.5 hours.
Had to wire up a couple of adapter cables. Who on earth would use RCA
phono jacks for antenna and DC power?
After I fix the LED and put the cover on, it will be time for the ultimate
test. Details at 11:00 UGT (Unified Gwangahala Time).
Mike K1MG
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