Finished up KX1 #277 yesterday (YAY!) just as soon as the mailman
delivered the two missing .01uF caps (Thanks, Scott, for the prompt
mailing!), and started into the final alignment and testing ... when I
hit a snag: Zero output into the 50-ohm dummy load.
So, I break out the trouble-shooting guide and my trusty DMM and find
...
#1 - Q1 (JFET '309 buffer) has 0V (!) on Source, instead of 1.77 V --
I'm thinking this is a dead '309 and likely the cause for "nuttin' out."
Anyone care to confirm/kibitz?
#2 - U1 (MCU) pin 6 (/MUTE, !MUTE, MUTE-not, MUTE-bar, mute-on-low,
whatever you want to call it) is 1.05 V instead of the 3.4-ish V in the
T.Guide.
Now for the really strange thing: this morning I started judiciously
poking about with the oscilloscope probe to watch certain lines. This
time I had the KX1 hooked up to some powered speakers and noticed that
if I use AC-coupled scope-input on pin-6, the audio mutes for a bit (< 1
sec.), then comes back on. If I use DC-input on the 'scope, the audio
*stays* muted. I verified that the DMM causes the same phenomenon.
It seems to me that my U1-p6 is floating, and that touching it with even
a high-impedance 'scope probe (directly on chip) is enough to pull it
low, and kick the KX1 into MUTE mode. Does anyone know if this is
expected/standard? I'm suspecting there is *some* kind of problem, as I
only have 1/3 the normal voltage on this pin.
#3 - U2 (DMM) pin 19 (IOUT) had the incorrect DC voltage;
Troubleshooting says 0.32 V, I'm reading nearly twice that, 0.61 V.
Using the 'scope shows this is the HF going to the Buffer Q1 above.
Just for grins I tested this output on 40m and found max-out and min-out
(near extreme tuning edges on 40m mode), and got vastly different Vp (AC
peak) readings:
5.340 MHz -> 150 mVp at U2-p19; 70 mVp at Q1-Gate
(rolled back down a little at 5.0 MHz)
7.100 MHz -> 90 mVp on p19; 25 mVp at Q1-G
9.500 MHz -> 21 mVp on p19; 16 mVp at Q1-G
(9.5 MHz also had 7 mVp-p ripple at p19, and 1 mVp-p at Q1-G)
I realized there will be attenuation at higher frequencies, but I wasn't
expecting quite this much. Any comments on this one?
Any light anyone cares to shed on any of these would be greatly
appreciated.
73 de Trent - N7GMT
--
Trent W Snyder, N7GMT
Pleasant Grove, UT (DN40ci)
K2#3927, KX1#277 (almost), FP#-776, ARCI#11761
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