[R-390] FW: Re: R-390 RF Gain Control

[jwkoest]

Your thoughts on this?Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: Jacques Fortin <jacques.f at videotron.ca> Date: 9/14/21  13:43  (GMT-06:00) To: 'R-390 Forum' <r-390 at mailman.qth.net> Subject: Re: [R-390] R-390 RF Gain Control Hello to all,Despite that it can be done to modify the TRANSFER FUNCTION (Vout vs Vin) ofa linear potentiometer with the use of a fixed value resistor, nothing canbe done to change the behavior of a linear RHEOSTAT (variable resistor with2 connections) by using any arrangement of fixed-value resistors connectedto it.The fundamental reason why a REVERSE-LOG rheostat is needed in a R-390/390Ais the fact that the RF gain control is connected in the ground return lineof the RF and IF stages, varying their bias to the control the stages gain,and in AGC mode this runs in parallel with the AGC within the receiver.Because that the relationship between the RF-IF stages gain and their commonbias line is non-linear, this is why a REVERSE-LOG control is needed.This is the same in many other receivers: the HRO series are anotherexemple.In MGC mode, the AGC line is grounded (or fixed in voltage in somereceivers) and the RF gain do it's job alone.Why this "compound" gain control scheme was chosen for the R-390/390A haveto do with the SQUELCH function that is present in the R-390 (and can be"activated" in the R-390A) for which the RF gain act as the SQUELCH TRESHOLDcontrol.Some other receiver architectures uses a LINEAR potentiometer and a negativeDC source to take control of the AGC line in "manual gain" mode.I have already suggested that if no REVERSE-LOG, 2W rating  potentiometercan be found, (one day, the Fair Radio stock will be exhausted..) that a 5Kohms, 2W  multi-turn can be used.Another trick (please don't kill the messenger) is to use a 5K ohmsLOGARITHMIC potentiometer but, in this case, the RF gain will work inreverse from the original design intent, meaning maximum gain a Zero,minimum gain at "10".Search for KA5021S28-ND on Digi-Key for ex.: 106 parts available vs zero forthe KB5021S28-ND, which is the "true" replacement.However, in both cases, the control range will be wider than the oneobtainable with a one-turn LINEAR potentiometer.73, Jacques, VE2JFE______________________________________________________________R-390 mailing listHome: http://mailman.qth.net/mailman/listinfo/r-390Help: http://mailman.qth.net/mmfaq.htmPost: mailto:R-390 at mailman.qth.netThis list hosted by: http://www.qsl.netPlease help support this email list: http://www.qsl.net/donate.html