A very nice looking build in a metal case

Joel, N6ALT, has declared that he “is done with his µBITx”.   And we can see why!   This is  a very nice build in a very classy case.

After three attempts at different enclosures, he settled on this one from China. https://www.ebay.com/itm/123008133070?ViewItem=&item=123008133070

The PTT switch is mounted in the pen housing on a small piece of perf board.

It is built very well and super light. Only 4 screws hold the whole thing together yet it is very strong and rigid. Very ingenious.

The radio weighs 2 pounds. Joel did not leave the display mounted to the main board as it would make the display not centred and too low on the front panel. He built a short flat extension cable out of a female and male header.

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Metal case from Sunil VU3SUA (InKits India)

Sunil VU3SUA who is an active BITX20 list constructor has a business (Inkits) selling enclosures and electronic components for the Indian domestic and international market.  Sunil is now shipping his µBITx case.  This is a high quality metal case that is available in several colours: Maroon, Black, DA Grey, Siemens Grey, and Blue.

As you can see this is a very professional looking case.  The case will be  in high demand initially and is, therefore, only available on pre order.

Sunil’s website says that the first batch of pre order bookings has now ended and shipping has started (15th March 2018).   

We should see reviews of this product shortly on the BITX20 list.

Bookings for the Second Batch of pre order booking open now.  The shipping of second batch will start by end of March or Starting from 1st April 2018.

The universal case for the µBITx costs  US$34.99 with worldwide shipping via India Post costing US$20.    For a DHL  courier service there is an additional charge of  US$15.

Orders will be shipped in order of bookings.

For international orders from https://amateurradiokits.in/

For Indian domestic orders, go to http://inkits.in

Power curve flattening in the driver and pre-driver stage

Glenn VK3PE is attempting to flatten the power curve from the µBITx by playing with the driver and pre-driver stages. 

He has completed his mockup of the PA driver stages and

  • used MPSH10’s in all stages.
  • Used FT37-43 toroids all wound with 10 turns 0.25mm wire either tri- or bi-filar according to original Sch.
  • Bypassed all emitter resistors with a 220pF cap.
  • swept from C80 input to T10 secondary (as wound)

The result is a very flat response: better than 1db from 3Mhz to 54MHz.

Glenn plans to mock up the finals now to see what the net impact is on the power response curve.  If it is nice and flat he will replace the parts in the µBITx and we will have a solution that everybody who wants to flatten the power curve can use.

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Replacement to TDA2822 coming soon!

Sajid, VA3QY asked Ashhar Farhan where he could source a new and reliable TDA2822 audio chip.

Ashhar Farhan has replied, “I am working on an audio amp.  I am testing it live. I always personally use circuits on the air before recommending them. I hope that by Tuesday I will have something you can use.   About 100 out of 4000 boards have had this problem. and less than 20 of them have reported the blow out.”

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Audio Pop mod from QCX submitted by Greg N3NW

This audio pop mod from Greg N3NW appeared on the BITX20 lO Group list before this website went live in January.   The implementation may not look that elegant (sorry Greg), but this was about an experiment to reduce pops not an entry in a beauty contest!

Greg “borrowed” a rather elegant circuit snippet from the QCX transceiver that takes care of the pop completely.  This is effectively the same mod as that submitted byJoe VE1BWV (and no doubt taken from the same source) who confirmed that it works.  The mod has now been confirmed independently by both Pete W8KS  and John AD0RW.    So this mod, is known to work, however, it may come with one downside, for which John AD0RW has a work around.

The circuit diagram for the popular QCX transceiver (developed by QRP Labs) can be found near the end of the very well documented assembly guide for this single-band CW rig.  An excerpt that contains the TX Mute circuit installed by Greg can be seen below:

The TX mute switch in the QCX involves just 4 parts – a BS170 (Q7 above), a 0.1µF capacitor (C52 above), a 1N4148 diode (D5 above), and a 120K resistor (R60 above).

Greg used a 300K resistor for R60,  and a 440nf capacitor for C52 to adjust the timing, which keeps BS170 open just long enough when switched back to RX to remove the pop.  The CW sidetone gets weaker with this, however in my uBITX board 10K R253 was replaced by a 1K resistor to make the CW tone just loud enough with this mod.

Pete W8KS has, however, found that the CW sidetone (even after replacement of R253 with a 1K resistor) to be insufficient and asked for suggestions to address this mod’s shortcoming.

John AD0RW found a much better place to connect the drain of the MOSFET. First he replaced R70 (in µBITx circuit diagram (see circuit excerpt above) with a 510 ohm resistor.  Then he connected the MOSFET drain to the relay side of that resistor, i.e. to the point M2 on the schematic above.

This setup still shorts the incoming audio to ground and kills the pops, but leaves a voltage divider of R253 and R70 connecting the sidetone to the volume control, increasing the amount of signal injected.  Depending on your value of R253, this may be perfect as is. The value of R253 varies (some early production kits had a 1K resistor and more recent kits get a 220K resistor).  John’s was 220 k and he needed more volume, so he put a 50 k trimpot across R253. It is just about right at maximum resistance, but can be adjusted downwards if more volume is required.  If your R253 is a smaller value and the sidetone is too loud, just replace it with something of higher resistance.  The volume control now has final control over the sidetone level and can be tweaked in operation if needed.

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Mic Compression and Noise gate with SSM2167 module

John, VK2ETA, has  used the small circuit board “SSM2167 Microphone Preamplifier Board Preamp COMP Compression Module DC 3V-5V”available on eBay or Aliexpress as a compression and mic pre-amplifier.

He simply connected the input to the mic, added a 4.7K ohm resistor between the mic input and the 5VDC (taken from the Raduino) for biasing the electret and put a 10K ohms potentiometer in the output to adjust the power level to the mic preamp stage.

He didn’t modify his uBitx board,  but simply inserted the board prior to the mic input.  The gain of 20dB is reduced back with the output potentiometer. John removed the “R1” resistor and replaced it with a 51K Ohms resistor to get a 4:1 compression factor, up from the 2:1 as delivered, but this change has yet to be tested “on air”.

John hasn’t received any negative feedback about the compressor except when I pushed the output potentiometer too high.

Reference

 

Simon VK3ELH used the same board and a similar scheme for powering the module from the regulated 5v line on the Raduino.  It is also installed separate to the main board and inline with the mic input.

Simon used a 75k ohm resistor for compression and 1k ohm resistor for the noise gate and a 100k pot on output. At full output, his audio was readable but distorted based on an audio check QSO, so the output has been turned down.

He put a larger heatsink on the IRF510 to cater for the higher average output, as the stock one was getting warm!

A side effect of the mic being on all the time is that there is leakage through to the speaker and it causes some feedback if the mic is within 2 inches or so of the speaker.

PC Switch Box Case

KP1JBW has a great memory.  He remembered a post for the BITx40 that covered using cheap surplus PC switch boxes for cases.

He says “this one was perfect and cost only $4 from an electronic surplus store. My radio literally just arrived this afternoon so I quickly ran out to find a case.”

KP1JBW will add a plate on the back to cover the cutouts for centronics connectors that came with the case.

The original center switch hole is where the display naturally sits when plugged directly into the board. I will end up popping the front plate onto the mill to get a clean and square hole. This case is steel so I might end up going for something down the road that’s lighter. For the price along with a massive selector switch, wire, and a decent set of knurled machine screws, I cannot complain. I bought a larger case for $6 when I need to start looking at upgrades.

Joe KD2NFC made a smart comment about the challenges of wiring up all those cables to the Arduino Nano – there simply won’t be enough ports!

John AD5YE commented that these boxes were either serial or parallel distribution switches.  They take a serial port computer output (6 to 20 wires), or a parallel port (usually 20 wires) and transfer this to 4 different receiving ports. The most common use was to connect one computer to several different printers. They were quite common in computers before the advent of USB and (lately) wifi.

He suggests finding one nowadays at a decent price is a rarity. A few years back, they were very common and very cheap.  However, they make excellent radio cases though one usually has to redo the front and back parts.  John has several with double-sided PCB added as faceplates. In addition to the cases, you usually ends up with a 12P4T switch (which is fine for switching filter banks), and lots of #28 wire.

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VU2ZAP experiments with TDA2822 Audio Chip

Raj VU2ZAP has done some experimenting with TDA2822 chip failures.

FCI chips were sourced locally and plugged in. No failure was observed when using the chip with normal volume.  However, when he raised the volume to a high level the chip failed immediately!

Solutions

1: Add a resistor in series with the 12V line maybe 10-15 ohms to drop the voltage.

2. Simple solution is to use only a 16 Ohm speaker or add a series 8-10 Ohm resistor with the 8 ohm speakers.

3. Cut the power track to the TDA and insert a 78L08 or 78L09 to reduce the voltage. There is a convenient track which can be cut.  Solder either an SMD or DIL regulator onto the board at this point. This track leads directly to the chip and filter cap (see photo below).

The center lead looks soldered, but is in fact floating.  However, the tab is soldered to the copper groundplane below. The white wire is part of Raj’s fix to prevent the filter relays from clicking during PTT.  Raj suggests the board as illustrated now needs a brush cleaning!

Raj observes that with this mod, audio is fine but distorts at high volume. The 9V voltage out does not drop at maximum volume.

Feedback on the solution from Jerry KE7ER

Jerry KE7ER says “This looks like a fine solution!   The fact that the 78L09 inherently limits current to something reasonable is a bonus.  And it fits in there perfectly!

Jerry suggest that it might be a little bit easier to just rip that trace out entirely. You could place the part up against the through-hole on one side and run a short wire to the other through-hole.

Gain should remain as it was at 12v.   Just that when you turn the volume up it distorts earlier.

If what you have on hand is the TO220 LM7809 (or any voltage from 5v to 9v), it will fit in there nicely as well.  Glue it face down to the board with pins hovering over  the trace that Raj has cut.   Or, if you don’t like glue, solder a short wire from tab to ground plane.

ST makes the 78L** parts in a SOT-89 package.   This is the same package as U2, the 78L05 for the IRF510 gate bias.

TI and Fairchild LM78L** parts only come in the SOIC-8 and TO-90 (also some dinky BGA that we don’t want to talk about).  Digikey and Mouser don’t sell ST’s SOT-89, so a bit harder to obtain. The SOIC-8 should do fine, solder the ground pins to the ground plane for heat transfer.   The TO92 could work, although it would likely heat up faster because the heat has nowhere to go.

The maximum available current draw from a 78L09 would limit the maximum audio level.    Apparently this provides enough audio for Raj, and limiting that current does protect the TDA2822.   If it gets to be too much current for the 78L09, it will shut down till it cools off.   Just what we want, if it’s enough audio power for your situation.

If you want more power and less protection, use the 1 Amp TO220 LM7809 (or 08 or 06 or 05) face down, flat on the board.   ST sells the 1 Amp L78** in the Dpak/TO252 package, about the same as the TO220.  TI sells the 500ma LM78M** in the Dpak/TO252 package, might be an ideal solution for moderate levels of current.

Any of the above could be made to work, and in fact any regulator providing between 5 and 9 volts.   This assumes you don’t have one of the clones made from factory floor sweepings that smokes at 7v.

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Ashhar Farhan tells it as it is … the problem of the TDA2822

Ashhar Farhan VU2ESE, designer of the µBITx tells us the sorry saga of the TDA2822:

“Here is the story. This is going to cause a lot of heartaches. I chose TDA2822 after listening to a lot of bad press about the LM386. Upon looking at its harmonic distortions et al, it was found to be a reasonably good device. In, it went.

“Now, unfortunately, the TDA2822 production has entirely stopped. None of the standard suppliers, including mouser, newark, element14 have no stocks left. We tried to find other source of remaining stock from our reliable supplier who has been supplying to us in the past as well.

“He turned up with a batch. We tried the ICs in burn tests for. Ten of them on ten boards at full volume for a whole day. In retrospect, we should have tested every individual IC. That batch had these WX ICs as well. About 100 to 150 of these must have shipped, there is no way to know. The way it works is like this : Not all the ICs turned out by a factory are good. So work very well, some not so well, some don’t work at all. These wafers make it to the silicon industry underground where they are cut out from the wafers and packaged and sold as low grade versions of the same parts. We got a a hundred or so of these lemons.

“We finally located a source of brand new, high quality TDA2822s that are currently being shipped. They cost almost five times as much as we were paying while this was in production. Well, such is life.

“In the meantime, we have to look for alternatives to the TDA2822. I am highly inclined towards making a discrete device amplifier. We will never run out of discrete devices, the audio would be so much better. Does the gang have any ideas?”

Initial feedback suggests that there will not be a consensus reached on a replacement.  Some want to stick with the TDA2822 (assuming a reliable source of affordable devices can be found), some would prefer the ability to add in a module of the builder’s choice, and others would like to see the device replaced with discrete components.  David N8DAH suggests the drop in replacement –  NJM2073D-ND

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