[TMC] SBE-3 & SBE-6 on AM

[Richard Knoppow]

----- Original Message ----- 
From: "Duncan Brown" <duncanancy at earthlink.net>
To: "tmc" <tmc at mailman.qth.net>
Sent: Friday, February 28, 2014 7:10 PM
Subject: [TMC] SBE-3 & SBE-6 on AM

     Previous stuff snipped. I want only to address the hum. 
If this is 60 hz hum its almost certainly coming from the 
heaters, that is, unless this rig has a half-wave rectifier 
in it for something, such as bias.  The return of the 
individual heaters may be through the socket screws but 
there must be a central connection from the transformer to 
the chassis.  I wonder how that is arranged.  I had a 
problem with a Collins R-388 that had considerable 60 hz 
hum. It was very puzzling since it was there even with the 
rectifier tube pulled out.  I found a ground return through 
the frame of the headphone jack. The jack was not making 
good contact with the panel because of paint under. Scraping 
off the paint cured the hum (probaby a star washer would 
have done the job).  Because of the low impedance of the 
heater line its possible there is still some resistance 
there somewhere.  Of course hum can come from many other 
sources but some are pretty unlikely (like induced hum from 
a transformer).  It might also be from a tube with 
heater-to-cathode leakage. Some circuits are very sensitive 
to this.
    I am not familiar with the equipment so will make no 
suggestions about the AM problem except that a lot of 
primarily suppressed carrier transmitters do not do AM well.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com