[HomeBrew] My progress on the Mitch Lee receiver

[Philip B Atchley]

Hi All,
Here is the update on the "Mitch Lee" receiver that I've been building
the past month.  This is the "CW only" receiver with a -3dB 11 Hz
Selectivity!    

I've been listening to longwave beacons the past few nights with this
thing and am very impressed with the results, though it still needs just
a little refining.  The following is what I posted to the beacon
reflector today.

********************************************

Today I did some more extensive work on this homebrewed receiver.  You
may recall I had problems with the first pre-amp not working, using the
same basic design I've used for a number of years (Lyle Koeler's
Universal Pre-amp with the addition of a 530KC LPF and tuned circuits
from about 9KC through 530 KC).  The difference was I'd used a number of
substituted parts, including the Varactors (tested good), and the FET. 
Looking back I suspect I "May" have made a wiring error that wasn't
apparent and probably hidden under a part somewhere.  Those small pads on
the R/S Universal PC boards get solder bridges pretty easily. 
 
PRE-AMP:
Anyway, first I stripped the FET & Bipolar transistor and their
associated bias/load resistors from the board, leaving all the LPF and
tuning coils intact.  THEN, I built a new pre-amp based on Roelof's
mini-whip design with Steve's modifications.  I used two of the same
Varactors as before, the MVAM2 back to back diode pair.  I then tested it
OUT OF THE RECEIVER to see how it worked.  It worked fine, but had
considerably less gain than I'm used to with the "Lyle Koeler" Universal
Pre-amp.  Something I feel is needed in the setup here (You can always
reduce or attenuate a signal, hard to get more once the design is
"soldered").
 
SO, I proceeded to make some changes.  Essentially I used Lyles Universal
Pre-amp design, substituting a J310 FET for the MPF-102 and an A733 for
the 2N2907.  I prefer the A-733 for it's slightly higher gain and the
lead arrangement lends itself well to circuit layout.  The higher gain of
Lyle's circuit arises from the fact that it's a common source FET driving
a Common emitter Bipolar instead of Roelof's Source Follower driving an
emitter follower.  
 
This worked out VERY well.  As it turned out, using Roelof's adjustable
bias circuit for the FET was overkill as the high gain direct coupled
circuit worked best with the FET at zero bias.  Yes, I know that probably
isn't going to give the best IMD figures, but with an efficient LPF
followed by a high Q tuned circuit IMD probably isn't much of a factor
anyway.  (I'D NEVER do this for a wideband Active whip though).  As it
turned out, I had to put a high value "Q killing" resistor (270 K) from
the tuned circuit to ground as I had oscillations at certain tuned
frequencies.  The optimum value seemed to change from band to band, but
270 K took care of everything.  Since I have an unused pole on the band
switch I MAY select a separate optimum resistor for each band at a later
date.
 
How does it work?  It seems to be working out very well.  It has slightly
less gain than I was getting out of the outboard pre-amp, but not by
much.  That is probably due to better coils and a "real" tuning capacitor
in the outboard unit. It "seems" overall to be a bit quieter than the
straight Lyle design using an MPF102.  I'll do some extensive listening
tonight and see how it works.

AUDIO and Product Detector:
I retuned the BFO some, up from a 300 Hz tone to 400 Hz.  I found that
300 Hz seemed to aggravate a resonance in the headset, sometimes making
copy of weaker signals difficult.  It also "seemed" to have somewhat less
ringing at 400 Hz, though why that would be I don't know as it wouldn't
effect the IF tuning any.  Doing this lowered the audio output from the
Product Detector a little, probably because it's moving the BFO Xtal
further away from it's most "active" frequency.
 
BANDWIDTH:
I did a little work on the "Wide" IF strip so that it's gain is nearly
equal to that of the narrow strip, making it more usable.  Also, the
higher BFO offset cleaned up the audio on this one some as it's
"centered" slightly below the narrow strip (the only way I could
"stretch" those Xtals that far).  So the tone was getting a little muddy.
   
 
OVERALL:
I'm quite impressed with this receiver and what a fairly simple design
can do.  Just how well it can really pull stuff out of the "mud".
 
NEXT ON THE AGENDA:
The next step will be to build the 1 Hz wide Audio Notch Filter module
and get it installed.  Since I decided to move the audio frequency up a
little I'm sure glad I hadn't already built it!  
 
73 de Phil,  KO6BB
DXing from the "Beaconeers Lair",  
where DX begins at the noise floor, and only 938 
different LF beacons have been heard since June/99.
Merced, Central California, 37.3N  120.48W  CM97sh

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