[HBR] Product Detector

[Dan Merz]

Hi DW in nearby Utah, in commenting I may be mentioning things you already
know but I just want to clarify how I think about it.  With the 390a,
there's quite good selectivity coming out the i.f. BNC connector so you
don't have to improve on that with an external i.f. unless you're talking
about selectivity below the 1 khz mechanical filer, say for cw,  where
perhaps you might use a narrow crystal lattice filter  The 390a already has
the mechanical filters operating for the signal coming out the i.f.
connector, and a single xtal filter for single-signal cw.   All you need is
the product detector/bfo,  the agc circuit and perhaps an audio stage to
adjust the level before feeding it back into the diode load connection on
the back of the receiver.  I built all this for a 390a/390 with solid state
components but the same could be done with tubes,  probably the HBR circuits
with 6BH6 bfo/6BY6 PD  could be used (change freq to 455 khz) with a small
power supply to accompany.   I think these schematics are on the HBR site.
But maybe the most serious consideration is the agc circuit itself which has
to put out a voltage to be fed to the agc bus of the 390a, on the back also.
The solid state version I built (not my design) does this and improves on
the built-in agc of the 390 for ssb.  It needs to provide a negative voltage
up to about -20 volts or so for strong signals  I don't know that I've seen
a tube design for that in an external unit,  but maybe my memory is not that
good. I'll take another look through the early 390a mod articles that I have
and might find something.  Some builders tend to ignore the agc aspect,  or
try to fix the 390 internal agc.   best regards,  Dan (WA)   

-----Original Message-----
From: hbr-bounces at mailman.qth.net [mailto:hbr-bounces at mailman.qth.net] On
Behalf Of DW Holtman
Sent: Saturday, January 21, 2006 8:36 PM
To: hbr at mailman.qth.net
Subject: [HBR] Product Detector

Hello,

I have a question that I hope that someone can help me with.  I'm looking to
build a Product Detector for a R-390A receiver. The R-390A has a BNC
connector on the back with the If of 455 KHZ coming out. What I want to do
is build a quality detector in a chassis that will mount in a standard 19
inch rack. The mechanics I can handle. What I'm looking for specifically is

1.  Tube gear.
2. Built in AGC circuit, with at least 2 time constants, ie; fast and slow
as well as MGC using manual RF attenuation (pot).
3. At least 2 maybe as much as 4 If stages for better selectivity.
4. Don't know yet is it is better to use cans or crystal/mechanical filters.
5. Don't need a high power audio amp, will feed audio back onto the R-390A
and use the onboard audio amp.

What I'm looking for is a construction project from an old ARRL handbook or
QST article for a quality receiver. I will only use the If strip and the
detector and low power audio amp. If anyone has any suggestions on what year
handbook/'QST might have something promising, please let me know.

Thank you in advance for any information.

73's
DW Holtman
WB7SSN




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