[R-390] tube based product detector.

[Jim Haynes]

On Sat, 17 Apr 2010, Cecil Acuff wrote:

>  Do we have a nice tube based product detector mod for the 390A documented
> somewhere.  Anyone have experience with one...as in having built one into
> their rebuild.  I am considering this approach on one of mine when I go
> through it .  Thought maybe something on the audio deck.
>
I'm hesitant to say anything due to my lack of expertise with this...
Believe there have been several things in Electric Radio and Hollow
State News, but will have to look them up.

Some people take the position of keeping the R-390A "pure" and build an
outboard product detector or sync detector that hooks to the IF output
and either has its own audio output or feeds back into the receiver's
audio stages.

Besides the product detector you need to tackle the AGC problem.
There are a number of mods that involve using diodes in the AGC
circuits to get fast-attack, slow-release action.

It's harder to put a product detector into an R-390A than it is with
some less-modular receivers, because you have the limited number of
connector pins on the IF module.  Typically the product detector mod
requires putting in a relay to switch the audio between product and
diode detectors, operating the relay from the BFO power.

You can learn a lot by googling for Dallas Lankford, and in particular
www.kongsfjord.no and clock on "The Dallas Files"

Once you get the AGC modified you can do pretty well without a product
detector by increasing the BFO injection.  The reason a diode detector
doesn't work well with SSB signals is that the BFO level is insufficient.

Jim W6JVE