Hints for removing blown TDA2822

Many constructors have suffered the failing TDA2822.   Today, few will have this experience as the v4 board doesn’t use the TDA2822 at all, but discrete transistors in the audio amplifier.

If you have a blown TDA2822 then it helps to know how to remove the blown chip.   Advice from other constructors is:

  1. Cut off the legs of the chip as close as possible to the chip itself
  2. Hold each leg tightly with pliers, while heating the soldered end on the board.  Wiggle the leg until it comes out.
  3. Clean up the holes using a solder sucker.
  4. Install an 8-pin DIL socket to hold a new TDA2822 or one of the potential replacement chips.

An add on PCB proposed by Howard WB2VXW

Howard WB2VXW has posted a PDF (download here)  of circuit diagrams that would be incorporated on a single add-on PCB as an addition to the uBitx board.

Please respond to Howard if you see anything to improve or other features to add in. You may also want to respond if you are interested in a PCB or a kit has Howard may offer it as a PCB or kit with sufficient interest.

Page 1
U4 is an audio amp to replace the problematic TDA2822. U3A and Q1 etc. form the AGC circuit, not original. U3B is a log amp the drives an analog S meter (the digital one is better) that is in my enclosure. U1 is a switch to put the CW filter in or out of the circuit. (Note there is a drawing error +VM shouldn’t connect to U3.)

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The Cheybchev 750 Hz LPF 0.1 db ripple

Page 3
I/O JP2 and JP3 plug into connectors soldered on the underside of the uBitx. The odd pins connect to the Raduino and audio connectors. The even pins are hardwired to the uBitx.

Page 4
Chebychev Low pass filters, 0.5 dB ripple for the external linear amp I posted previously.

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Relays for the above filters. Copied from and driven by the uBitx

Page 6
New driver for IRF510s. connects between VR1 and the output of T10 which is removed.


Ashok VU2KY has designed a PCB to apply some modifications to his ver 3 uBITx.   These include mods incorporated in the ubitX v4  board.

In addition he has included an S-meter sensor based on the LM386 chip used by Ian KD8CEC in his design.   For more details visit Ashok’s blog


Improving the CW wave shape on TX

Allan VK2GR was concerned to reduce key clicks and improve the shape of CW from the µBITx.

Being mainly a CW operator, he was concerned about the V3 board uBitx transmit CW wave shape being very hard – almost a square waveshape. As expected, key clicks could be heard on either side of the signal. A look at the circuit diagram and a little tinkering has vastly improved the situation.

The photo above shows where a short wire was soldered to the hot side of C1 for the tests. In reality the wave shape is now a little soft on the trailing edge, so 0.047 or 0.68uf may be sufficient for some people. More work could possibly be done with the CW keying RC network to the 1st balanced mixer, however this one component simple fix will suit my needs.

All that was required was to increase the value of C1 from 0.1uf to 1uf. Below are some oscilloscope photos showing the end result of improvements to the leading and trailing edge of the transmitter output using a 1uf connected across C1.


Microphone idea

Dave KD0UYH has made a microphone from an old desk lamp purchased from a “resell it” shop.  He says, “Not much to look at, but it serves my purpose. Used some vacuum tubing to attach the mic element and hot glued a handful of nuts into the base to give it some weight.”

Conclusions on how to eliminate spurs

Alison KB1GMX has advice to constructors on the spurs on the higher bands:

For bands below 20Mhz spurs are NOT an issue as the low pass filters catch it.

Spurs are only a problem for SSB and frequencies greater than 20mhz.

NOTE: due to the way the uBTX does CW it is never an issue on any band.

The short form is when you mix two frequencies you get a third, in a perfect world.

The diode mixers used are handy but they can present conundrums.  If any of the three ports (IF, LO, and RF) are mismatched (think SWR) The signal can be reflected back in.  Since DBMs are omnivorous in that any port can be input or output and if mismatched both!  This does not include effects of distortion in the source signals.But in the real world things like this exist.

Double balanced mixers also suffer from overload, too many and too strong and you get a plethora of signals.   What that means for lots of simple and complex reasons you can get “spurs” or spurious  outputs that are undesired.

Basic math, addition and subtraction:

If you mix 45Mhz with 73mhz you get 28mhz.  We want that  and the radio needs that.   However if any of the 28 gets reflected back into the DBM where it originated it mixes with 45mhz and we get 17mhz.With those four signals you get mixtures of those like:

  • 73-17=56
  • 28-17=11

Those are “first order” as they do not involve harmonics.  They will be the strongest, but not always equal strength.

Both inputs can have harmonics like 90mhz and 146mhz and the 34 and 56 coming out can have harmonics too.  If you add and subtract all the possibles you get an increasing sea of signals some weak some stronger.  We will not cover the possibles as the first order ones are the most troublesome.

The solution traditionally applied is band pass filters or if it isn’t between 28 and 29.9999 the filter strongly attenuates it.  But you need a band pass filter for most every band… uBitx takes the path of below a certain frequency you only need low pass filters and fewer of them.  And it generally works well especially for 80 though 17M…

But at 20mhz and up the low pass filter passes everything below 30mhz and if you overdrive the rig slightly you get a spur on the tech window on 10m where the spur is 16.5 to 16.7mhz and there is no filter for that.  What makes this worse is some radios are very poor at 10M putting out maybe 2W so pushing the audio to get more invites the problem to be greatly worse.

There is no setting we can safely give that absolutely assures there will
be no problem that is consistent with maximum achievable power.

As a licensed amateur radio operators we are responsible for signal quality and also not generating signals outside our assigned bands.

There are two solutions one is bandpass the other is high pass filter.
Either way the rig must be modified to allow those and there are side effects.

One side effect is you need extra switching not provided.  The other is any filter has a loss though it and that would further reduce power out.

Short of that, keep the power right down on the higher bands and go for it…


Kees has been busy sending out boards!

Kees K5BCQs has 403 paid orders for AGC and/or Anti-pop Mini-Kits and 399 of these have been shipped.  He has additional boards and parts and will continue to supply these Mini-Kits as long as there is demand. Details are in the “Files” section of the BITX20 webpage under his call K5BCQ.

Kees was somewhat surprised to be able to process such a quantity over a short period of time. That indicates there are opportunities for others to join in with their ideas. As Kees notes, the uBITX is a great vehicle for modifying and improving.


Is a heatsink required on FT8 mode?

One of the challenges with digital modes is their higher duty cycle than phone or CW.   However, Ashhar Farhan VU2ESE, designer of the µBITx, assures us that on FT8 the standard heatsinks supplied with the kit are adequate.  FT8 has a 50% duty cycle.

This may not apply with other digital modes such as WSPR.


Diagnostics for µBITx

There is a Facebook group for the µBITx in addition to the BITX20 list that represents the primary source of news articles on this website.

One of the members of the  Facebook Group (Ufi Auttorri) has produced a diagnostic guide for constructors who are facing problems with their µBITx.   While it is still a work in progress, and won’t cover all issues arising with the rig, it is an excellent starting for constructors who have run into issues.  You can find it here: