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microFM - armstrong based
01-02-2018, 07:10 AM
Post: #1
microFM - armstrong based
here is armstrong based microFM
inspired by sinclair microFM
some minor changes ...by Aurel


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01-08-2018, 02:22 AM (This post was last modified: 01-14-2018 05:23 PM by Aurel.)
Post: #2
RE: microFM - armstrong based
And here is a prototype circuit...
I decide to build simplified version of Armstrong design.
My version don't have voltage regulator and electronic switch so
i have 5 transitors.
In first place because i have lot of parts and i cannot wait to see
how this design work...
Important part of this receiver is 70kHz cut off low pass filter created with
resistor 6k8 and capacitor 330pF which disable double tuning of stations
as we know FM bandpass is +-75kHz each station ,so we have here little
bit under that ,and on this way reception is more clean.

parts:
i replace 1n5 caps with 1n capacitors
T1 - base cap replaced with 120pF
T3,T4 collector caps replaced with 180pF
for diodes i use small glass diodes with lightblue line
which are 1SS.... type
transistors BC549 replaced with BC183
for T1 i try to use BF199 but not work with 1,5V
and require circa 2.7V to work, also i try
BFR 91A and work but i must change emitter resistor to 560 ohm
Anyway , it looks that work best with BF494 Smile
Also i dont use trimer capacitor (green), i have one but it
looks that work without .

For PCB i use my own drilled PCB with raster 14x4 soldering places.
I have one AM/FM plastic variable capacitor so i use FM side
and connect main pin with 5p6 cap,27pF cap is connected to
collector of T1(BF 494).
Coil - i made it with 0.9mm Cu wire so i wound 6 turns over
6.5 mm drill, then i strech coil to cca 40mm.

testing:
First time i connect battery(1.5V) to phone jack(5mm) then plug
ordinary 32ohm phones and i dont heare nothing...
I try to set working point of T1 with trim-pot (100k) but still
nothing then i look again into excellent John site (http://members.iinet.net.au/~cool386/sspcrx/sspcrx.html )
and i see that he redraw schematic and i figured that his limiter diode is polarized in a different way then is on original schematic.
Cathode of this diode must be on transitor T4 base NOT on GND(-).
So i do that and i get small noise in my phones which was good sign!
Then i again set trim-pot very slowly and i finally get typical noise.
I move volume potentiometer to maximum and and move
variable cap and i receive local FM station very very loud and clear
and i was become surprised about sound quality.
Receiver is very sensitive to direction because there is no antenna
- antenna is in fact built in coil L1.
Just for testing i add small capacitor of 2pF and connect 0.5m insulated
wire and i can say that receiver can pick up more FM stations on that way.
John stated that sound is very deep but i cannot say that maybe
because i use 1nF instead of 1n5 or becase i use BC183 , i am not
sure .

As conclusion I must say that i am surprised how receiver work very well
with very good sound quality.
I think that sound is better than with TDA7000 based chips.


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01-15-2018, 04:15 AM
Post: #3
RE: microFM - armstrong based
Here is PCB layout for microFM2
created on soldering point raster 14x4.


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