[Heathkit] Audio gain pot and FAST/SLOW AGC

[Kenneth G. Gordon]

On 7 Mar 2007 at 23:54, Chris Kepus wrote:

> Hi Ken,
> If anyone has a source for these and replies off list, would you be
> kind enough to share that info with me?  

You bet! See below.

>Your idea to use an existing
> "pot hole" (our Transportation District folk would enjoy that!)

Hee hee! So do I! :-)

> to be
> able to add a switching function while retaining the original control
> is very clever. 73, Chris W7JPG

Gee! Thanks for the compliment Chris, but it wasn't originally my idea. I 
cannot now remember where I first heard it, but the time frame was 
back in the late 1960s. It may even have been in a Hints and Kinks.

Here is what Tom N0JMY sent to me:

http://www.vibroworld.com/parts/tech14.html

as a possible source, which I will check out this morning.

The way you implement the FAST/SLOW AGC is fairly easy. 

First, install the 1 meg pot with a push-pull SPST switch on it in place of 
the original AF gain control.

Then, find the AGC timing capacitor. I can't remember the proper 
number right now, but it is easy to find, and is a fairly large value. 
Looking in the Circuit Description section of your manual for the AGC 
system will show it pretty quickly.

Then, lift the ground end of that capacitor, and connect it to one side of 
the switch. Connect the other side of the switch to the nearest handy 
ground point.

Then, pulling the switch gives you FAST AGC, and pushing it in gives 
you SLOW or NORMAL AGC, or vice-versa depending on how the 
switch is set up.

In any case, since you are using the audio gain control for this function, 
it is very innocuous.

The FAST position makes a big difference on CW and certain digital 
modes like Pactor or AMTOR.

My next project is to make the necessary modifications to the MODE 
switching so that both the offset and the CW sidetone are about 750 Hz 
instead of that terribly annoying 1Khz canary tweet.

Ken Gordon W7EKB