[AMRadio] retro 75

Brett Gazdzinski Brett.Gazdzinski at verizon.net
Sun Apr 11 20:41:26 EDT 2010

I tried both 50K and 100K pots, with the same results.
I did not check all the parts, the detector fet might be bad, or something 
else wrong, but the lower resistor fixes it.

The mic I have has a battery (AA) that fits inside to provide power.
I have not had a chance to try other mics yet, I have a hand mic from a 2 
way radio that might work well, and a bunch of hifi balanced mics....

The weather has been too nice to be inside playing radio though...


----- Original Message ----- 
From: <k1swl at earthlink.net>
To: "Brett Gazdzinski" <Brett.gazdzinski at verizon.net>
Cc: <amradio at mailman.qth.net>
Sent: Sunday, April 11, 2010 8:10 AM
Subject: Re: [AMRadio] retro 75

> Brett-
> 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
> Hz.
> 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
> parts
>> 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
>> N2DTS
>> ----- 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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