[AMRadio] DX-100B on Phone - bad audio distortion / oscillation

[Bernie Doran]

learn something new every day, 1625s in a KWM2, perhaps they control the 
dynamoter?
----- Original Message ----- 
From: <ne1s at securespeed.us>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
Sent: Saturday, December 03, 2011 2:56 PM
Subject: Re: [AMRadio] DX-100B on Phone - bad audio distortion / oscillation


> Rick and list, see comments below.
>
>> Connected up my off-center fed dipole for 80 and 40, which I have
>> used for a couple of years on a few rice boxes and a Collins KWM-2A
>> and it always worked acceptably if not spectacularly.  Tuned it up on
>> 3885 and listened for a while, putting in an occasional call.  Seemed
>> to work OK for a while but then the output on the scope just took off
>> into some kind of oscillation.  I backed off the audio gain and the
>> drive to zero and no joy, had to switch from Phone to CW to make the
>> oscillation go away.
>>
>> Since then, it has been kind of random... sometimes it works fine and
>> gets good audio reports on 3885, other times it takes off into
>> self-oscillation.
>>
>> The transmitter is rock-stable in CW, this only happens when I switch
>> to Phone, and then not all the time.
>>
>
> These are strange symptoms. I would otherwise think it is RF rectification
> into the audio, except  you say you can turn the AF gain all the way down
> and the oscillation persists. It's even weirder that you can turn the
> drive pot all the way down and the oscillation persists.
>
> Does the RF output frequency change when it goes into oscillation, and you
> kill the RF drive? I'm thinking perhaps the finals are taking off on their
> own, since they appear to be generating RF with no drive from the 5763. If
> so, you may need to add neutralization. But I can't explain why this would
> happen in AM and not CW.
>
> Another possibility is an ultrasonic oscillation in the modulators, which
> may be because you've got some really "hot" 1625s. But this wouldn't
> explain why reducing drive to zilch doesn't kill the RF output.
>
> Sorry, so far I can't come up with anything that fits all the symptoms you
> describe. Maybe I'm not interpreting something correctly. If I think of
> something later, I'll post it.
>
> Good Luck, Rick!
>
> 73,
> -Larry/NE1S
>
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