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

[Duncan Brown]

Richard,

Yes, I'm pretty sure the 60 hz hum is from heater current traveling 
through the chassis and the use of nut plates with ground lugs to mount 
the tube sockets (and no star washers).  This unit has a separate supply 
chassis, so no transformer on the SBE-6 main chassis and I checked for 
heater to cathode shorts.  I will try running separate ground return 
lines for the heaters of the more sensitive tubes.

Thanks for your comments,

Duncan
K2OEQ

On 28-Feb-14 22:32, Richard Knoppow wrote:
>
> ----- 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
>


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