I see. I noticed that others have voiced a similar complaint.
The SA612, which is mis-identified on the schematic as 602, is spec'd as having a 17db conversion gain and capable of a -119dbm for 12db s/n/ at 45 MHz when driving the SA614 (not used here of course.) That's approximately 0.4 microvolt sensitivity which by itself would virtually always be well below the practical noise floor on 3.5 MHz.
Thus I am thinking there needs to be more gain in the IF section. An RF preamp will increase total system gain but at the cost of overloading the SA612 at some point, which will not be remedied by AVC control of the preamp. I wonder why that wasn't noticed at the design stage, although I guess maybe for a simple little product I am expecting too much from it. If you could hear as well as a $ 1000 new ham rig, maybe you would be frustrated with how few people could hear your 2 Watts?
How can one get more gain out of the IF of the Retro75. I don't believe it's possible since the output of the SA612 goes right into the detector.
There is a 2200pF NPO (C7) on pin 7 of the SA612. It's reactance is about 20ohms at 3.800MHz. I wonder if changing the value so reactance is greater than 20 ohms would change the AC gain of the stage within the SA612 and increase the signal to the detector? ? ? ?
http://www.stanford.edu/class/ee133/datasheets/SA612A.pdf See page 4 for internal SA612 schematic.
http://smallwonderlabs.com/Retro-75_Instr.pdf See page 9 for schematic of Retro75.
I suppose you're right about people being frustrated trying to hear my 2 watts. But don't you think it would be prudent to have receive sensitivity so one could here what others are hearing so as to not interfere with the others? I am surrounded by a couple of big guns and would hate to interfere with their communications because I could not here someone they are working. My 2 watts would certainly cause a problem with these few big guns if they are working someone weak and I couldn't hear the weak station.
The RF preamp does cause some overload if run wide open but I have an attenuator on the front so the sensitivity can be adjusted. It works just dandy. Controlling the gain of the add-on preamp can be done using AVC/AGC derived from the SA612 IF.
