[Central-Electronics] 20A ???

[Jim Candela]

Paul,

That RCA jack with a red wire does go to the balanced modulator. On AM we use just one audio channel, and that channel is sampled by that RCA jack. This signal is used for monitoring purposes. I use it as a sync signal for my scope, going to channel 1, selecting channel 1 as the sync source for scope sweep. Then I look at sampled RF on channel 2. My scope has a X:Y function as well, and here the signal from that RCA jack is "horizontal", and the RF is "Vertical". Then when using the 20A on AM, or DSB, you get a single trapezoid (AM), or double trapezoid (DSB). This is very useful for monitoring modulation percentage, and linearity (straight walls on the trapezoid).

   As an example of using that RCA jack, look at my post here (3rd one):
http://amfone.net/Amforum/index.php?topic=35581.0


   To get a clean audio output from that jack, you must make some sort of low pass filter on it as there is a lot of 9 Mhz energy on that signal. Inside the 20A, I add a series resistor at the RCA jack, and a capacitor to ground. I don't recall the exact values, but 1K, and .001 uf would be a good place to start. You don't want to add much phase shift at audio frequencies.

  If you have a solid sate audio amp with a low impedance output, then that jack could be an external audio input for use on AM or DSB, any resistor added would need to be removed.

Did I answer your question? :->

Jim
Wd5JKO 


________________________________
 From: Paul Kraemer <elespe at lisco.com>
To: central-electronics at mailman.qth.net 
Sent: Tuesday, January 21, 2014 5:02 PM
Subject: [Central-Electronics] 20A ???
 

Did CE have a later version 20A with possibly a phone patch or line input on
an RCA jack on back panel?
I have one here that has an RCA jack located below the terminal strip
It is horizontally located below terminal 9
Goes to a red wire in the harness to vicinity of modulation switch
My other 20A does not have this.
The hardware used is exact same as the rest of the hardware, the wire does
not look added and the holes are clean cut (unlike many diy adds).  Also,
jack is vintage style. Any ideas?

Another question.  Anyone have a source for the front panel crystal socket
and the rear panel 120v receptacle?
Thanks and may spring be on the way!
Paul K0UYA


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