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LF BITX
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Arv Evans
Paul
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On my BITX20, and the BITX40, I used an AF design that is different from the LM386 that Farhan specified. There are some schematics of this in the BITX20 Group files section under K7HKL. My AF section uses two NPN's (2N2222)as AF amps, driving an NPN/PNP pair of power transistors for about 4 watts peak AF output. With this the AF level is more than adequate. There have been cases of low receive volume in BITX20 units being traced to RF transistors that showed less than optimum gain at 10 & 14 MHz. That was the case on my first BITX20 unit. I found that supposedly good 2N2222's from a surplus source were much inferior to some 2N3904's that I used to replace them. In another BITX20, VFO instability was traced to use of a transistor having too low an hfe factor. Work on my BITX-LF is going slowly, due to other more pressing work on (1) a 74HC4046 based H&P circuit, (2) installing and configuring DSL for Internet access, and (3) it is now spring in Idaho and outdoors activities are competing with ham radio activities! So far, my BITX-LF has the receiving AF, BFO (3.579 MHz), BFO-mixer, transmitter AF, and Solid-state T/R switching sections completed. I have a basic 3.579 MHz IF filter soldered together but not yet tested for shape factor. The above-mentioned 74HC4046-based H&P VCO will become the BITX-LF VFO if all works as anticipated. It will be interesting to see how your LF version of the BITX evolves. One major problem I have encountered thus far is finding suitable miniature inductors for 160 KHz work. Arv K7HKL _._ On Sat, 19 Mar 2005 16:44:20 -0500, paul daulton <k5wms@...> wrote:
A couple of years ago I built a transmitter strip using two ne602 chips --
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See also in the Files directory 'modifications', 'clear LF' to
improve the loop gain of the LM386. The capacitor should be made 10uF. My BITX17 is not generating a loud noise but when a station is heard it is a lot of volume. Chris. at lowI have pc board from far circuits for bitx. Have rec working but I haventvolume, Had to repair signal gen also had much time to evaluate attenuation |
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Rahul Srivastava
Dear Arv and friends,
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Hi!
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Here in keeping with common availablity I went ahead with BITX20 version using off shelf components. I used BC547s available, probably Chinese varients. The performance was not upto the mark. Only with beam antennas could we hear something. Hoping this as one off case? made 3 more units and all gave similar results, Now it is confirmed the stock components need some careful selection. All on BITX V2 PCB and one on copper clad as per Farhans by VU2DAD Dev an?80 yr OM.?
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Ultimately I made few changes cause 3 other units were made by few of my friends least not their efforts go waste. I realised that frontend gain was optimum to overcome losses so I worked on adding some gain in IF stage.
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Q3 instead of single? device?added one more and?made it a shunt feedback type amp achieving slightly higher gain and still keeping with RC amp topology. Surprisingly similar approach has been followed in Norcal 20 design this makes things nice and good. A 220E resistor in series at input helps in maintaining the required impedance by brute force method.
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?Replacing the Rf preamp transistor by 2SC2570A a cheap cable TV amp transistor improved the noise figure. BFR91-96 could be slightly expensive substitute.
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?The coils are commercial 10.7Mhz?FM?IF?tuned to 14 using 33-39pf caps (plus strays),
?original at 2.7uH they requred abt 68pf for 10.7mhz and 47pf for 14 as per calculations.
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In audio stage I have seen many homebrew design using LM386 chip but of all the ones I made?,?were running at full gain / open Vol most of the time (probably at max distortion level). I also feel the OP coupling capacitor is bit low at 50uF a 220 or 470 would be?slightly better choice.
?Picking up from another design and incorporated into one txcvr that I am making I find much better performance if we follow the PD into 3 pole RC network using 1k and .01 caps (omit preamp transistor).
?For preamp now we use the LM386 as used in Elecraft K1 with the anti hiss mod of ,01 and 10K resistor in series from pin?5 to 8.? I follow it up with a TDA2003 amp chip and?now whole? rig?rocks.?
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I am working on new board my BITXV3 PCB this also has link coupled 10MM coils and provision for doubly balanced mixer ie another balun added.
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2N3904 I feel would be better choice for transistors mainly there are less chances off getting it sub standard. Secondly it has a much lower capacitance in comparision to BC547. All in all this has been a good learning experience and some brain?excersicing project rather than just component stuffing kit projects off late.
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Thanks to all for support.
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73
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Rahul VU3WJM
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Arv Evans wrote: Paul
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