Wednesday, February 01, 2012

It's a BITX...

...Not to take literally....

Some time ago I used parts of the "bitx" transceiver for a small receiver, mainly the first mixer and the crystal filter, for the SSB demodulator and audio I used the NE602/LM741/LM386 combination, it worked good but never finished it (at least give it a decent box).
Now decided to use some this parts (all the RF receiving strip) for a NAVTEX receiver since I found in the crystal box one that oscilates around 9.510 Mhz... so with more 490Khz (one of NAVTEX frequencies) and it sits in the middle of "bitx" IF (10Mhz). Since I was building basically all the receiver part why not make a full version, so here part of the build process...

First tests using the ADE-1 mixer:
Now fast forward and some soldering....bellow image from top to bottom upright pcb's the 9.5Mhz oscillator the 4Mhz vfo (horizontally laid) and the 10Mhz BFO (with LSB/USB, two white variable caps)


I add already build last time the first mixer with diodes but now decided to use an ADE-1 mixer, also for the SSB demodulator one was used.

The first problem was in the audio stage... supposedly the easiest part, I was getting smoke in one of the limiting resistors, luck (and experience) avoid a total burn out, after some tests and some bad words I found the issue, I was looking at an LM380 schematic and not the LM386 used on the transceiver (remember to always have only one schematic in front of you).

Second issue that was driving me nuts was the lack of gain, I had gain in the amplifier after the crystal filter so I tough that there was some issue with the crystals, but they had been tested in "real live".... I ended up making the post crystal amplifier of "bitx" version 3, no luck... more bad words and another day to think about the issue, I decided to replace the ADE-1 (was suspecting of it) in the SSB demodulator and made the original bitx mixer, it worked first time with a lot of gain...more bad words thinking I had ruined one ADE-1 mixer (their expensive you know?). Decided to check the ADE-1 small adapter PCB that I made, yups, that was the issue, a small short from a tiny little piece of cooper... oh well, now it works again with the ADE-1 just because I have more mixers than binocular cores...

Can you spot the multi-turn carrier null pot of the original "bitx" schematic?

Now again with the ADE-1 in the SSB section (lower right part of the image)

I still need to finish the VFO (done but not tuned) because I was driving the first mixer with the Si570 kit (it's easier when doing tests). The idea is to put a switch between VFO (4-4.350 Mhz) and Fixed channel (9.510) for NAVTEX. obliviously a switched band pass filter for 14Mhz and 500Khz...

Have fun!

2 comments:

Eric Söderman said...

Thanks! Intresting post. I have started to try to build my first superhet rx/tx. It will probably be the bitx20. Atleast I have started to build the VFO and I was very happy when I saw that it was working at first power up yesterday. But it was a little too low in fq so next step is to double check the L and C values.

Eric SA5BKE

Ricardo - CT2GQV said...

Hello Eric,
With VFO's we never get right on the first time! I normaly do an estimated guess on the number of turns and then start removing. For any TX or RX strip the best way is always start from the audio part, it's easier to troubleshoot latter on. The bitx it's easy and rewarding.