Posted by
Matt Osborn on
Sep 05, 2005; 4:50am
URL: http://elecraft.85.s1.nabble.com/K2-AGC-problem-tp381868.html
Someone, Don I think, posted a message this week suggesting that the
background noise with no antenna connected should be the same with AGC
either on or off. I checked mine out of curiosity and discovered that
mine was quieter with AGC on.
Before making adjustments to R1, I thought I'd run through the AGC
voltage checks in the manual to see what they looked like.
With RF gain at maximum, pin 5 of U2 = 3.8vdc
I turned to Troubleshooting/Signal Tracing (Appendix E, page11) and
made the following measurements on the control board:
Using the XG2 signal generator, 7040khz @ 50uv
Test 1, XG2 off, RF Gain MAX
U2 pin 1 = 3.41vdc - expected 3.6
Test 2, XG2 off, RF Gain MIN
U2 pin 1 = 4.50vdc - expected 4.6
Test 3, XG2 on, RF Gain MIN
U2 pin 1 = 4.50vdc - expected 6.9 (VFO adjusted for max)
U2 pin 5 = 4.89vdc - expected 5.0
In test 3, U2 pin 1 is 2.5vdc too low.
Just for grins, I compared the voltages on U2 pins 1 & 3 at various RF
Gain settings and I found the same voltages on both pins; pin 1 seems
to be tracking pin 3 accurately; I think this is OK.
Using an RF probe, I measured the anode of D1 and found 65mv with XG2
off and 110mv with XG2 on, a 45mv swing. Using an DMM, I measured the
cathode of D1 and found 4.73vdc with XG2 off and 4.86vdc with XG2 on.
Q12 measures:
C = 7.9vdc
E = 4.5vdc
B = 4.5vdc
I'm inclined to believe Q12 may be defective. Before I order parts,
though, thought I'd check with you folks and see if I'm even in the
ball park.
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