Jerry KE7ER notes that the schematics on the hfsignals.com website for the Bitx40 are final but not necessarily correct. He then goes on to talk about some of the variances and essential hacks.
This website is dedicated to the uBITx. However, so that everybody who also has a BITx40 doesn’t miss out entirely on the occasional “gold nugget”, some interesting articles about the BITx40 may appear from time to time on this website. Jerry’s comments on the variation between the published circuit diagram and the actual board currently in production meets the “gold nugget” standard.
Differences between the BITx40 Board as manufactured and the manufacturer’s Circuit Diagram
- The two modulator diodes D15,D16 plus balancing pot R106 got replaced by a BAT54s dual schottky: https://groups.io/g/BITX20/message/33385
- All those BC849’s? Have always been MMBT3904’s.
- L5 and C103 of the BFO are missing, the BFO is at just about the right frequency if the crystals are matched without any adjustment. Usually.
- L4 in the VFO is not stuffed on the board, meaning the analog VFO doesn’t oscillate. The VFO comes instead from the si5351 PLL chip on the Raduino through the Bitx40 connector labeled “DDS1” in the schematic.
- T6-4 is tied to C142 and R144, U3-3 is tied only to the 12v rail labeled “TX”, schematic does not show that clearly.
- RV1-2 is not connected to U3, the schematic drawing is just a little tight there.Otherwise, Jerry thinks that the schematics on HF Signals are correct, reflecting what is currently being shipped. Nothing has changed in Bitx40v3 boards shipped since Dec of 2016.
Changes that should ideally be made to the BITx40 Board
He then goes on to note that he thinks there are a few things that perhaps should change:
- D7 and C130 should be across the coil of relay K1, D8 and C164 are correctly across the coil of K2
- R141 is currently a 1/4 Watt 10 ohm resistor, should be 1/2 Watt as have been reports of it burning out.
- The surface mount MMBT3904 at Q13 could be replaced with a through-hole version of the same part, the 2n3904, as it must dissipate a fair bit of power during transmit. Even with the radio off but hooked up to an antenna, a nearby QRO transmitter or electrical storm can blow Q13.
And finally Jerry suggests the following as essential mods for all purchasers:
- Best fix for the Bitx40 is to add a couple back-to-back 1n4148 diodes:
The back-to-back diode mod comes from Raj: https://groups.io/g/BITX20/message/19105
Note that he suggested adding an incandescent lamp, which can be quite effective:
- Here’s a few more notes about what can blow in that transmitter:
- Second harmonic suppression at 14mhz is close to good enough but marginally legal, best to add a 100pf cap across L7.http://bitxhacks.blogspot.com/2017/01/nd6ts-suppression-of-pesky-2nd-harmonic.html
- Once you get all of that sorted out, you can search the forum for cures to the PTT pops and carrier bursts and lack of AGC, and sometimes insufficient mike gain.
- Once you have proven the radio to work, you should download Allard’s improved Bitx40 code: