[VHFcontesting] CATV RFI from 2m SSB (UPDATE)

w3sz w3sz at comcast.net
Wed Feb 16 06:52:22 EST 2005


I am not an expert, but I quote the ARRL RFI Book:

In bold print on page 10.8 there is a heading "When Not to Bypass".

The first sentence, also in bold, and the only sentence in bold font under  
that heading, says:

"Do not apply bypass capacitors to the output of transistor amplifiers.   
See Figure 10.9".

The text then continues on to say, "Contrary to previous literature, it is  
not a good idea to bypass speaker leads with a capacitor.  This works just  
fine for tube amplifiers, but solid-state amplifiers are apt to break into  
full-power oscillation with capacitive loads.  Damage to the amplifier is  
the probable result".

While some amplifiers may do fine with capacitors, should your neighbor's  
amplifier be damaged, so would your relationship with him and likely other  
neighbors as well [bad news travels fast].  Even if the problem mentioned  
in the ARRL RFI book were a very low probability event, in my mind it  
would not be worth the risk.  I would avoid the bypass capacitors across  
the speakers even if I got a slew of comments back saying that it was OK.   
For even if it WERE OK, with that text out there, if something else caused  
his amp to go up in smoke in the future, you'd be in a potentially bad  
situation.

Hope that helps,

Roger Rehr
W3SZ

On Wed, 16 Feb 2005 02:17:04 -0500, John Kemker <john at kemker.org> wrote:

> Caps across the speaker terminals at the back of the speakers.   
> Non-invasive.  1pF should be fine.  Use a ceramic cap.
>
> Don't even have to solder, if the speakers are attached using binding  
> posts.  Just bundle the cap leads in with the speaker wires.
>
> You might also help him trim the antenna^H^H^H^H^H^H^Hspeaker wires  
> evenly, so they won't act as antennae.
>
> --JohnK.
> 73 de W5NNH
>
> DUGAS, JASON A. (JSC-EP) (NASA) wrote:
>
>> *****UPDATE*****
>>
>> Well this afternoon, I did some more investigating.  Here are the  
>> results...
>>
>> I called my neighbor to verify exactly what type of interference he was
>> getting.  He said that ONLY THE AUDIO was being affected.  Said he had a
>> perfect video signal.  However, in the process of investigating  
>> further, on the way home from work,
>> I tuned my FT-100D in the truck to 145.25MHz and as soon as I got with  
>> in
>> spitting distance of my neighborhood, I heard the carrier on FM along  
>> with a
>> nice little hum.   Tune up to 145.35MHz or so and the noise is all but  
>> gone.
>>
>>
>> Got home and powered up my weak signal setup and the TS-850 w/ DEMI
>> transverter says the noise (on SSB) is right around 145.249Mhz.  I  
>> didn't bother to ask whether his audio was being fed into a stereo  
>> system
>> (which might have some nice long speaker wires with which to receive my
>> signals on!) or if the TV itself is picking it up in its audio circuits.
>>
>> I'm certainly not going to open up that television of his.  If there  
>> isn't a
>> stereo involved and its just the t.v. what are my options?  I'm going  
>> to call him again tomorrow and verify how he's getting his audio--
>> through the t.v. or through a stereo receiver.  Sure would be nice if I
>> could go over there and put a couple torroids on his speakers'  
>> transmission
>> line and be done with this mess, but I suspect he's just using a t.v.
>>
>> More comments invited!
>>
>> Jason
>> KB5URQ _______________________________________________
>> VHFcontesting mailing list
>> VHFcontesting at contesting.com
>> http://lists.contesting.com/mailman/listinfo/vhfcontesting
>>
>
>
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-- 
Roger Rehr
W3SZ
http://www.qsl.net/w3sz


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