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Author Topic: Valiant Sidebands  (Read 1862 times)
KA4KOE
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« on: June 19, 2013, 11:00:17 AM »

I've already posted a query on AMfone about this issuep; no clear answers. Essentially, my Valiant has assymetrical sidebands on the 40m band. The upper sideband has less energy than the lower sideband. You can't notice much difference unless you're looking at the waveform with a panadapter.

I've been told this is some sort of FM distortion. The regulator and oscillator tubes in the VFO have been replaced, as has the "Chernobyl" resistor.

I've done W8JI's chirp mods which eliminate circulating currents, particularly on 40m.

Is there a fix or is this just a quirk of this transmitter that I shouldn't worry about.

A link to the Flex screen capture is here

http://amfone.net/Amforum/index.php?topic=34279.msg265544#new

Thanks.

Philip KA4KOE
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G3RZP
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« Reply #1 on: June 19, 2013, 12:16:51 PM »

Any time you AM and FM at the same time, you will get assymetric sidebands. Question is , How Bad? Does it really matter?
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W4OP
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« Reply #2 on: June 19, 2013, 12:43:00 PM »

have you listened to the carrier while under modulation on a receiver? How much shift are you seeing

Dale W4OP
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KA4KOE
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« Reply #3 on: June 19, 2013, 01:26:24 PM »

Not noticing any carrier shift that is significant. Looking at the waveform, it appears the double peak is around 200-300 Hz offset from carrier either direction, then things level out. My hunch is to leave it alone.
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KE3WD
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« Reply #4 on: June 19, 2013, 03:16:59 PM »

Power Supply filtering or regulation probs can be the source of FM'ing in a lot of the older rigs. 


73
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KA4KOE
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« Reply #5 on: June 19, 2013, 08:38:36 PM »

On one band only? It only does this on 40m.
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G3RZP
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« Reply #6 on: June 20, 2013, 03:03:26 AM »

When on 40, is the VFO on 40?
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KE3WD
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« Reply #7 on: June 20, 2013, 07:31:20 AM »

On one band only? It only does this on 40m.

Depends on the design of the particular rig.  Those that have different power switching schemes for various functions, often via the bandswitching, may indeed exhibit the problem on only one band or more.  

Studying the schematic and tracing the thing down can find that possibility or rule it out.  

Keyword is "possibility" - troubleshooting is detective work, to my way of thinking, each possibility must be investigated but it has also been my experience that I get into trouble when blanket ruling something out altogether before investigating.  Then there's old man Occam to consider as well. 

Tracing backwards with the 'scope may pinpoint the problem a lot quicker.  


73
« Last Edit: June 20, 2013, 07:34:35 AM by KE3WD » Logged
WX7G
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« Reply #8 on: June 21, 2013, 06:37:24 AM »

Can you test this again with a pure tone rather than speaking into the mic?
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