[AMRadio] Distorted Valiant Audio

[Paul Baldock]

I pulled the rig apart today.

I was very surprised to find that the waveform at the output of the 
mod transformer (HV to Plate and Screen) is a nice undistorted 
waveform. I really thought there would be some problem in the audio 
chain but obviously not.

The waveform on the PA screens is ugly. It's more pointed than 
sinusoidal and has some wierd bumps in it.  I don 't remember that I 
ever checked a modulated screen waveform before, maybe this is 
normal. There's only the dropping resistor between the output of the 
mod trany and the screen, and then a 1000pF from the screen to 
ground, so nothing really to go wrong. The DC voltage on the screen 
is correct at 150V, and the audio drives it from 0V to 300V p/p.

There is a clamp tube on the screen so I pulled that and left it out 
with no effect.

The Valiant has 3 x 6146'in the final. I changed them 1 at a time 
with no effect.

And of course the modulated rf as stated in the original problem is 
much more pointed than the modulating sine wave.

A mystery

- Paul  KW7Y


At 07:59 PM 2/3/2013, Bill Guyger wrote:
>Paul
>
>I know you've gotten a couple of answers already, but I've been busy 
>all weekend trying to keep other people's radios playing. It sure 
>sounds especially with your observation of the sine wave distortion 
>that caps in the audio chain are dried up. That's a +/- 3dB SWAG, 
>but it sure fits what I've seen before in audio gear.
>
>Bill AD5OL
>
>From: Paul Baldock <paul at paulbaldock.com>
>To: AMRadio at mailman.qth.net
>Sent: Saturday, February 2, 2013 7:25 PM
>Subject: [AMRadio] Distorted Valiant Audio
>
>I turned my Valiant on today after many months of not using it, to 
>hear distorted audio.
>
>When I run a sine wave in to it,  the demodulated audio wave is not 
>smooth like a sine wave but looks more like a triangle, that is to 
>say much more pointed than a sine wave. This seems relatively 
>independent of level and frequency.
>
>Any suggestions?
>
>Thanks
>
>- Paul  KW7Y
>
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