Modification of Libra 40m QRP TRX
Some time ago, I bought from polish friend pcb of QRP transceiver Libra 40. It was almost complete, only coils and some components were missing. One hour of work Libra 40 revive. After tune input circuit and setup BFO output level into detector I could hear many CW stations, but their signals were very distorted. Author probably wanted continue in successful constructions with MC3362, in my humble opinion, he did not succeed. This TRX is built on MC3361 chip, which is primary intended for FM. IF of this chip is constructed from differencial amplifiers and limit already by low levels of input signals from the mixer. There is good sensitivity, but AF output signal is very distorted.
I uploaded video of Libra first reception on the Youtube and Patrick TK5EP registred this negative attribute too. We exchanged several emails, where he noted me about strong distortion of AF signals by CW stations. Patrick contacted also author and he recommended reconnect XF output from pin 5 direct into pin 8, to the detector of IC 3361. I tried modification immediately and Libra became non-sensitive “potato”. I added next IF stage with 2SK241, but sensitive did not improve.
One option, how improve this TRX and how remove distortion is use new IF with gain, at least 50db and make own detector. I choose classical IC MC1350, where I can easily change the gain and passive product detector with diodes.
Schematic
After soldering additional PCB with new IF on the top of XTAL filter and conecting signal wires, came moment of surprise. TRX Libra plays CW marks like full-value ICOM rig, without any distortion. My son and me, were very surprised of good quality of receiving signals.
However, Libra has still one more bad attribute. Higher noise, which is produced by first mixer in MC3361, AF filter and amplifier. I tried decrease gain of IF, but it caused only receiver lower sensitivity.
This innovation made TRX Libra little bit better, than original one. I share photos and schematic to everyone, who is intrested in innovation too.
Original Libra reception
Libra reception after IF modification
Comments
Hello! It is a great chance
Hello!
It is a great chance to improve Libra with Your modification :-)
Can You be so kind and help me - what kind of filter (in a metal cube) did you use?
Is it 455khz If filter like Toko?
And where to precicely connect the new module that You bild into Libra?
Maybe You could send me schematics of libra with the changes i should make.
Tom SQ3LVD
My qth is Poland , Poznań
Hi Tom, metal box is coil TR1
Hi Tom, metal box is coil TR1 with extra 6 secondary winding, schematic of mod board is in just this blog post. In simple wording, you just bypass 1/2 of chip which works well only on FM. On mod board is IF amp + demodulator. As it is marked, source is output from Xtal filter(5). BFO(8) is connected to demodulator and audio is pushed back(9), where 1k2 resitor is removed from board.
Good luck and clean signal wish,
Tomas, ok4bx