Modificarile aduse BitX20 !!!

Acest post este un copy paste.

I’ve put together some of the popular modifications and improvements that can be used on the
BITX Version 3 circuit.  These were found mostly through searches of the message archives in the
yahoo group site.  This list is by no means inclusive.  If I missed any that you feel are
important and current, or see any mistakes, please let me know and I will make changes as
appropriate.  I tried to put in the source of the mod, but it was hard to tell from the list
sometimes so it may not always be correct or present.


In group site “files/” folder:

q1_mod.GIF (prevent transmit clipping by Q1 – farhan)

In group site “files/modifications/” folder:

DC path for mixer diodes.pdf  (improve mixer performance – vdberghak)

clear LF.pdf (filter to prevent mixer RF from entering audio section – vdberghak)

Amplifier stages BITX17.pdf  (efficiency improvements for PA – vdberghak)

LM386-PTT-Pop-Mod.pdf (reduced speaker “Pop” on PTT – mark/n7eku)

In group site messages:

Q1 transistor type change.  (change RF amp transisor to high frequency/low noise cable TV type –
frohne KL7NA, allison KB1GMX, et al.)

Change RF commutation diodes for lower intermod distortion.  (change bi-amp switching diodes
from 1N4148 to BA243/4 or 1N4007 – romi/YO8CAN)

Lower 2nd IF amp gain.  (change R16 to 1k & R17 to 4.7k; maybe also change R14 from 4.7k to 1k –
untested – rahul)

Fix PCB error in mic amp.  (R50 should go from supply rail to base of Q10 as shown in schematic
– rahul)

Carrier balance pot.  (change R47 from 100ohm to 200ohm for better impedance match ala BITX 20A)

Crystal filter coupling.  (change C14,19,60,61 from 100nf to 100pf ala BITX 20A)

LM386 voltage.  (change R26 from 100ohm to 10ohm for more volume)

LM386 frequency response. (frequency response can be adjusted by an RC combo between pins 1 & 5:
pin1+15k+5nF+pin5 for high frequency roll off 6dB/octave after 1.5kHz, or pin1+10k+33nF+pin5 for
bass boost)

LM386 frequency response.  (output electrolytic capacitor + speaker impedance form a high pass
filter: 150uF + 8ohm = 132Hz cutoff, 220uF + 8ohm = 90Hz cutoff, 330uF + 8ohm = 60Hz cutoff)

LM386 s/n ratio.  (signal to noise ratio seems better with lowest gain: pin 1 & 8 open –

LM386 power supply noise reduction.  (according to the LM386 datasheet, pin 7 should be bypassed
to ground with a 1-10uF cap – mark/n7eku)

PA T2 ferrite.  (FT37 size is spec’d too small for this application and could easily overheat –
destroying its magnetic properties; use size FT50, FT68, or bigger)

VFO harmonic filter.  (install rc filter as shown in bitx20a schmatic)

From Leonard’s Website:

VFO voltage regulation.  (change from zener to 3-terminal voltage regulation for VFO frequency
stability – )

Electronic tuning.  (change VFO to voltage controlled varactor tuning – )


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