Regardless of where I set the AGC threshold voltage via R1, the K2 always has more gain with the AGC off than with the AGC on. The lower the AGC threshold voltage the less the difference between AGC off and on.
In accordance with the MC1350 datasheet, to find the maximum gain of the MC1350, I grounded pin 5 of the MC1350 via R4 on the Control Board. I lifted the end of R4 on the U2 side and attached a ground. The gain increased slightly. With the AGC on the AF output averaged 12.9 mV about 1 mV higher (0.7 dB) than with the normal AGC circuit. What was interesting was that with the AGC off, the AF output averaged 13.9 mV, about 1 mV higher (0.6 dB) than with the AGC on. With the AGC off the gain of the K2 was about 0.3 dB higher. With the output of U2A pin 1 disconnected at R4, I would have expected no difference with the AGC switched on or off. My guess is that the AGC mixer U1 (SA612) is loading down the IF OUT lead connected to pin 8 of the MC1350 which is reducing the level of the IF signal sent to the product detector. What else could be causing this? If indeed the AGC mixer is loading down the IF signals when the AGC is on, would this not change the impedance at the input to the 2nd crystal filter and affect the filter shape? Darrell VA7TO K2#5093 -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
