[AMRadio] retro 75

k1swl at earthlink.net k1swl at earthlink.net
Sun Apr 11 08:10:38 EDT 2010


If you're using an electret mike, did you add a bias pull-up resistor for
it?   My other thought is that the fairly low impedance looking into the
mic input is affecting signal level and adding emphasis to the audio
response. For an electret mic, you may want a JFET buffer there.

Oh yes- modulation in excess of 100% is easy with the control loop open.
The TDA2003V is a 10W amplifier. 'Overkill' for this application, but I had
nothing but trouble with the TDA7056 (3W) I'd tried earlier. I never found
the combination of layout and decoupling that kept that device stable, and
I was seeing a  stubborn 5-6 dB bulge in the audio response at around 500

I thought about your earlier comment about AF distortion and the 100K
resistor fix.  I wonder if you used a gain pot with a value higher than the
suggested 50-100K. The gain pot, rather than the 1M resistor at the JFET
emitter, should be dominating the time constant on the detector circuit.

  73- Dave Benson, K1SWL 

> [Original Message]
> From: Brett Gazdzinski <Brett.Gazdzinski at verizon.net>
> To: <k1swl at earthlink.net>
> Cc: <amradio at mailman.qth.net>
> Date: 4/10/2010 11:21:53 PM
> Subject: Re: [AMRadio] retro 75
> Dave,
> 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
> count.
> 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!
> Brett
> ----- 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
> > 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
> > 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
> > at the audio output.
> >
> > 73- Dave Benson, K1SWL
> >

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