[TenTec] LONG: Audio improvements on Argonaut VI

[Bill Cotter]

Carlos,

I recently repaired a Delta 580 that had no audio output. 
Inspection found that the LM-383 had a corner blown off. I ordered 
a couple of the LM-383 AF amps and replaced the defective one. The 
audio stage worked, but with a lot of hiss, AND it burned my finger 
to the touch.

I put a scope on the AF output and saw that it was oscillating 
around 100kHz. All the components surrounding the amp tested good 
(cap values OK, low ESR, no leakage), so I dug out the LM-383 data 
sheet. Right away it was clear that TenTec had deviated 
considerable from the recommended circuit.

The U1 feedback loop capacitor C5 installed was a 4.7uF and 
National recommended a 470uF, so I changed it to the recommended 
value of 470uF. Also, TenTec had only installed C4 a .22uF mylar, 
when national called for the output stabilizer pair of a 0.2uF and 
a 1-ohm resistor in series. I added the resistor. The 100kHz 
oscillation went away, and the amp was dead quiet with the AF gain 
all the way down. Unfortunately, with the gain up the OP amp noise 
from the preceding stages came into play.

Lesson learned, TenTec took a shortcut to save a few cents on 
components while deviating from the manufacturer's recommendations. 
I had a second Delta 580 on the bench for a PTO rebuild, and it did 
not have the AF problem as the first Delta. Go figure, sometime you 
get away with things, sometimes you don't. I have seen these AF 
output issues in the Corsair-I series as well.

My suggestion is to examine the Argonaut schematics for the audio 
chain, and look to the manufactures recommendations. Did TenTec 
follow these recommendations, or are there compromises that may 
come into play? Also, use a scope or spectrum analyzer to see the 
nature of the hiss - is it wideband or does it have specific 
spectral components?

73 Bill N4LG


At 03:24 AM 8/21/2018, you wrote:
>Hi,
>
>I think that most of you are not understanding the question.
>
>- I bought a (lovely) Argonaut VI.
>- The received audio sounds distorted, it improves with headphones.
>- To get a feeling of the noise generated (solely) by the audio 
>chain I
>minimize any noise generated by the RF/DSP chain by:
>
>   - disconnecting the antenna
>   - taking RF Gain to the minimum
>
>
>- I still hear a hiss, even worse than the Corsair II hiss in the same
>conditions, so:
>
>    - it is a known design feature and can be improved (like the 
> Corsair
>II), or
>    - I have something damaged.
>
>
>
>
>Any hints will be greatly appreciated.
>
>
>About the clicking noise when turning the dial: I'm just asking if 
>it can
>be improved.
>
>
>
>73,
>Carlos VK1EA
>
>
>
>
>
>
>On Monday, August 20, 2018, Carlos <carlos.peco at gmail.com> wrote:
> > Hi,
> >
> > I just bought an Argonaut VI and got disappointed with the 
> received audio
>(when comparing it with a Corsair II).
> >
> > When monitoring with headphones it improves a lot, but there is 
> some hiss
>in the background, even without antenna and the RF gain all the 
>way to the
>left.
> >
> > Do you guys think that I might have a damaged audio amplifier? 
> Is there
>an easy way to improve the received audio with the in-built speaker?
>PBT/BW/NR settings are OK, I even performed a factory reset.
> >
> > Using headphones, I can hear a weak pulsing noise when rotating the
>frequency encoder. Is there a way to eliminate that ?
> >
> > 73,
> > Carlos VK1EA
> >
> >
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