[AMRadio] retro 75

Brett Gazdzinski Brett.Gazdzinski at verizon.net
Sat Apr 10 23:21:52 EDT 2010

As a big home brewer, I tend to pick things apart...
Besides the lack of agc, the receiver really works well for such a low parts 

I received my modulator chip today, thanks, and installed it.
I got about 30% modulation with the led flashing, using an old electret 
condenser microphone.

I changed some values around, and tried a small pot to set the modulation 
levels, but am still working on it.
With the feedback loop open, its possible to get over 100% modulation, a 
good sign!

The microphone sounds semi nasty, but very punchy, which is what you want 
with low power.

Thanks for the fun kit!


----- Original Message ----- 
From: <k1swl at earthlink.net>
To: <Brett.Gazdzinski at verizon.net>
Cc: <amradio at mailman.qth.net>
Sent: Saturday, April 10, 2010 8:07 AM
Subject: RE: [AMRadio] retro 75

> Brett-
> Glad you found a solution. So far, I haven't heard from anyone else
> experiencing this issue.
> I set the audio volume on the Retro-75 up for 'room-filling' levels. The
> ALC control engages only at high signal levels and keeps the LM386 audio
> final from clipping.  With the 3" speaker I'm using, I can fill the entire
> house with audio before this happens. It's admittedly not much of an AGC
> function per se.
> It wouldn't be too difficult to improve on the receive AGC action, but it
> would take more complexity. In particular, an additional audio amp stage
> would provide the levels needed for a lower AGC threshold. The AF gain
> control would also need to be relocated outside the control loop.
> Otherwise, the gain control does nothing once the threshold has been
> reached. I solved this on the old WM-75 with a 100-ohm pot/rheostat right
> at the audio output.
> 73- Dave Benson, K1SWL

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