[Elecraft] Samlex 1223 & 1235 power supplies

[Paul Christensen]

> It is important to understand that 
sitting on top of the transceivers
 is
> NOT the primary interference coupling mechanism for RFI. Rather, noise is
> coupled out of the noise source external wiring (that is, the AC power 
> cord
> and the DC power wiring), which then radiates it INTO YOUR RX ANTENNA.

Jim, that's probably the strongest coupling mechanism most of the time 
although some receivers have poor shielding from the immediate environment 
and noise coupling can occur from the SMPS to the receiver's front end.

An example of this is the IC-7800 and IC-7700.  There's a product review on 
the web of the IC-7800 that boasts how the reviewer can hear many stations 
without the need for an antenna!  It's written as if that was a positive 
attribute of the receiver.  In fact, it only demonstrates how poorly the 
front end is shielded between the preamp/mixer and the antenna ports.  When 
I disconnect an antenna from a port, I want the Rx dead quiet -- not a band 
loaded with signals.

Another disturbing attribute of the '7800 and '7700 is that harmonics from 
its internal SMPS can be heard when the receiver is tuned from ~ 80 kHz 
through 160m with decreasing amplitude as the receiver is tuned upwards. 
Oddly, the noise increases as the antenna ports are deselected.  Granted, 
the '7800/'7700 are not meant for listening down in the LW/MW bands, but 
it's an indicator to me as to how poorly the power supply is integrated with 
the rest of the transceiver.

Paul, W9AC