[AMRadio] retro 75

Brett Gazdzinski Brett.Gazdzinski at verizon.net
Sun Apr 11 21:54:55 EDT 2010

Its a great fun kit.
I have not really got into it yet, today was yard work and motorcycle riding 
since it was so nice out.

I may have a dead detector fet, I had a lot of distortion on strong signals, 
and changed a resistor (R7) from 1 meg to 470k and it cleared up, but that 
is likely a byproduct of something else wrong.
Even so, it has plenty of gain and sounds great the way it is.
Its kind of nice, no signals, its quiet, really quiet. Even weak signals 
come through well though.
Last night was a zoo, but I had no problems coping n8ecr, even with the mess 
down frequency.

I am looking into adding a small pot to set the modulation level, and maybe 
play with the cap values in the compressor circuit.
I tried putting a pot in place of R27, but it does not work right, r22 might 
be a better choice.
I have some REALLY small pc mount pots...

I also need to study the design and see if it can be changed to just limit 
the neg modulation in some way, that might be interesting and help a lot on 
a low power rig.

The kit is so simple, its easy to play with things...

I am trying to talk Dave into making a deluxe rig for 40 meters.
You would likely have had a solid qso on 40 meters...


----- Original Message ----- 
From: <johndtate at post.com>
To: <Brett.gazdzinski at verizon.net>; <amradio at mailman.qth.net>
Sent: Saturday, April 10, 2010 11:53 PM
Subject: Re: [AMRadio] retro 75

> Hi Brett and the group.  You guys are having way too much fun with these 
> kits!! Just had a QSO with a few people and among them was Jack, W9GT and 
> although he was very hard to copy here in Louisiana, I was amazed that I 
> did catch a few words here and there despite an S9 noise level.  Had it 
> been December or January with no noise, the 2.5 watts would have easily 
> made the trip here.  Anyway, I just put in my order to Dave and Brett 
> maybe you could make a document with the mods you applied for everyones 
> benefit.  I'm looking forward to building this kit and putting it on the 
> air.
> John KX5JT
> -----Original Message-----
> From: Brett Gazdzinski <Brett.Gazdzinski at verizon.net>
> To: k1swl at earthlink.net
> Cc: amradio at mailman.qth.net
> Sent: Sat, Apr 10, 2010 10:21 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
> ----- 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
>> 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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