The All-band HF Amateur Radio Transceiver: Just waiting to be modified
Nik VK4PLN has now received his 700Hz CW audio filter boards and built up the board as shown below:
He put these on the spectrum analyser. The spectrum before the filter is included in circuit:
And after:
Seems to work! To Nik’s ears an LM324 gives a better result than the TLC274 op amp. You mileage may vary.
Gian, I7SWX, has shared a mod for replacing the TDA2822 audio amplifier. He intends to apply this circuit after other mods he is experimenting with on his uBITX.
The audio PA is the TDA2003, an amplifier where the bandwidth can be modified. His circuit is designed for 3kHz.
This circuit, with reduced bandwidth, has been tried on an FT920. It is possible that some components values may need to be changed.
It is important to note that Gian has not tried this mod as yet, but others might like to beat him to it!
Nigel G4ZAL has just finished his µBITx build and modded it a little in readiness for adding additional boards/hacks.
He cut the track feeding the TDA2822 and fed it with 5v from a little Radio Control UBEC. At the same time, he removed the 5v regulator on the Raduino and fed that from the UBEC as well. You can see from the photos that he has added a little ‘bus’ for picking off further 5v supplies as required.
His installation looks pretty snappy in the well used Banggood case
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And the µBITx is already hard at work on 40m using the FT8 digital mode:
Nik VK4PLN has yet another board design – this time for an audio CW filter similar to the QCX and HiPerMite. Nik says “he is loving the cheap and easy EasyEDA/JLPCB boards and simple web designer”.
He “takes no credit for the design, just copied it and added a switching relay. He has ordered 10 boards at $10 posted”.
Center Frequency: 700 Hz
Bandwidth: 200 Hz
DC Power: 5VDC
GAIN: Upto 20dB via R11 and R12 + trimpot.
Resistors 1206 SMD:
R1 33k
R2 33k
R3 1M
R4 47k
R5 47k
R6 36k
R7 36k
R8 10k
R9 750k
R10 10
R11 100K
R12 100K
Capacitors 1206 SMD:
C1 0.1u 104
C2 47n 473
C3 36n 363 (33n + 3n3 mount on side together)
C4 1n 102
C5 39n 393 (33n + 5n8 mount on side together)
C6 1n 102
C7 2n2 222
C8 2n2 222
C11 0.1u 104
C14 220u Electrolytic
Other:
IC TLC274
Relay OMRON G6S-2-DC5
200K/100k trimpot
R11 and R12 are optional to parallel the 200k trimpot to make it a logarithmic 100k. Or just use a linear 100k pot.
Use a switch connected to 5V to turn on the filter.
This powers the opamp and switches the relay feeding the audio into the circuit. When off the relay just passes the audio straight through unfiltered.
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.”
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!
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.
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.
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
Generally speaking, internal speakers in a QRP transceiver, like the µBITx will be inferior. Fragile, rattly and thin sounding, and inefficient. Don Cantrell, ND6T, has written an article about how he dealt with this problem. He incorporates a headphone jack in the speaker with a switch and an associated signal limiter circuit to prevent the RX from damaging his eardrums with loud stations and static bursts.
See his blog page for details.
The TDA2822 is used as the µBITx final audio amplifier to drive headphones or speakers. The kit provides wiring instructions to wire to a panel mount “see through” stereo jack.
Several constructors have found that this little IC has gone up in smoke, much to their surprise and frustration!
It is not entirely clear why the TDA2822 is failing. In some cases it seems to be the result of inserting a plug into the stereo headphone socket. Inserting a mono headphone plug in the stereo jack could result in a short from the ring to sleeve. Even inserting a stereo plug could result in shorts.
However, there are also reports of the device going up in smoke spontaneously. One theory, from Jim Sheldon W0EB, is that this is because loud pops or extra loud signals cause the 470µF capacitor (C77) in the output circuit to draw high current during charge up, damaging the chip. However, it could equally as well be a run of bad chips, or the fact that the device is running near its voltage maximum (the original chip was rated for 15v maximum).
The first batch of µBITx shows an FCI PI1 TDA2822M chip:
These FCI branded devices have yet to show up with issues.
Raj VU2ZAP tested the current draw from the FCI chip with normal audio use and current was 60-70mA. A dead short with varied drive shows the current draw was between 300 and 800mA. At 800mA the chip got hot! The current did not go above 1A at any time.
The original ST parts (now obsolete) claimed an Absolute Max of 15v, and also gave that as the maximum operating voltage. This agrees with the specs for the NJ2073D and NTE7155 clones. So running it at 12v should be legal, though nowhere else in the ST datasheet is there mention of operation over 9 volts. A bit of a red flag. ST continues to build the SOIC8 variant, the TDA2822D.
Other manufacturers are making 8 pin DIP packages. These clones may or may not be marked. They are readily obtainable on eBay and AliExpress at very low cost (you can buy 10 for around US$1).
It is likely that cloned versions made in China have been used in some second batch µBITx products. These items may not be an exact copy of the original and may not be as robust. They may, in fact, be a low voltage version of the chip that is incorrectly labelled.
Several constructors have confirmed that their Batch 2 µBITx come with a WX branded TDA2822M as shown in the photo below.
Not all Batch 2 µBITx have this chip installed. It is likely that all current Batch 3 kits are affected. These WX chips seem to spontaneously combust at some point when used in a circuit with 12v DC applied, as illustrated in the photos below:
Testing by the GQRP club suggests that Chinese chips purchased online may not take more than about 6v DC (at around 45mA), and get very toasty at the original’s rated maximum voltage of 15v.
First reports of fried TDA2822’s seemed to be due to a shorted audio output, e.g. when plugging a mono plug into a stereo jack. In series with a proper 8 ohm load, the 470uF cap should be fine. With a short to ground, there will be a quick surge current of unspecified amps from the TDA2822 till the cap is charged. Though if that’s a failure mode, it is not the only one. More recent reports suggest that these chips are being fried spontaneously, suggesting an over-voltage issue.
Protecting your TDA2822 against short circuits
There are several possible approaches to short circuit protection for your TDA2822.
To reduce current in rush, the easiest approach would be to insert a 4 ohm 1/2 watt resistor in the output line to the speaker or headphone socket. While this reduces audio output slightly, it also protects the chip against short circuits, and slows the current inrush to the DC isolating 470µF capacitor in the output circuit.
Another approach would be to reduce the size of C77, the DC isolation capacitor – perhaps to 100µF or even 47µF. Circuits for the TDA2822 often use a 100µF capacitor. Experimentation may be required if the audio begins to sound a bit constrained.
One or other or both of these fixes is recommended fix for FCI chips, but these fixes won’t address the issues with a WX branded chip.
Protecting your TDA2822 against high voltage failure
The best option for all chip types would be to reduce the voltage feed into this chip to bewteen 5v and 9v. Since the audio stage is connected to +12v (rather than to the relay switched RX line) makes this a bit easier to achieve. A regulator or buck power supply is recommended for the feed to the TDA2822.
If you have a WX branded part in your µBITx, a mod to reduce the voltage to this chip is considered ESSENTIAL. The first step is to cut the short trace on the back of the board into the square pad of C76 (near X2).
Add an LM7805 regulator, pin 3 going to that square pad of C76, pin 1 going to
the feedthrough at the other end of the cut trace, and pin 2 going to ground. Maybe glue the LM7805 face down onto the back of the board, with the leads in the vicinity of C76.
Ideally, add a 0.33µF capacitor (and realistically any value of capacitor from 0.1 to 10µF) from pin 1 to ground.
This modification has now been shown to work, and details on how straightforward this is can be seen in Raj VU2ZAP’s experiment where he adds an 78L09 surface mount chip.
If you need to replace the TDA2822, it should not be difficult to find a replacement IC. If you don’t mind waiting these can be procured very cheaply from the Far East. If you want it more quickly, then they can be order through a local supplier.
Make sure that you also purchase a socket (or use machine pin headers). This will make it easier to replace the chip in future if it blows again.
Removal of the IC is easily achieved by snipping the pins above the board, and then removing them one by one using a soldering iron, solder wick and needle nose pliers.
You could use an LM386 module as a replacement to the audio amplifier on the µBITx board. LM386 modules are readily available on the internet for well under a US$1. Buy several so you have spares in the junk box. Source the audio feed from the volume control output and wire up the stereo jack or speaker to the output of the module.
The TDA2822 and LM386 are not pin compatible, but it is possible to make an adapter to plug into the TDA2822 DIL socket (using two further DIL sockets).
The pinout map supplied byClaude HB9CGL is as follows:
LM386 pin – TDA2822 corresponding pin
2 – 4
3 – 7
4 – 4
5 – 1
6 – 2
Howard K4LXY shows his “adapter” using the LM386 to replace the TDA2822:
Claude has left pins 1 and 8 of the LM386 unattached, and in this configuration the LM386 datasheet states that it has a voltage gain of 20, or 20*log10(20) = 26dB. The LM386 on the Bitx40 has a 0.1uF cap between pins 1 and 8 to increase the gain to 46dB. The original TDA2822 of the uBitx has a gain of 40dB, so you may want to try adding a cap between LM386 pins 1 and 8 if you need more gain there. A larger value cap (up to 10uF) would improve low frequency response. Howard decided to include this capacitor.