Roman provides a no-soldering hack to reduce the audio-stage gain on a v5 (current model) µBITx.
Power off the BitX before starting…
1. Observe which end the notch on the body of the 386 is facing
2. Remove 386 carefully from socket (i.e. use tiny screwdriver pull up chip body incrementally back and forth from each side)
3. Gently lift either pin 1 or 8 90 degrees so it is horizontal. These are the pins closest to the notch, that is, in each line of four pins, the pin at the end of each line closes to the notch.
4. Reinsert the 386 into the socket in a straight down manner until firmly in place, ensuring that all remaining 7 pins go into their proper slot (and don’t bend accidentally due to misalignment). Make sure that the notch on the 386 is facing the same direction that it was when you removed it.
Try it out by turning on the BitX – there will be less hiss added to the audio (this is especially noticeable in headphones)
The v5 uBITx has an LM386 for the audio amplifier stage. Philip G7JUR suggests connecting a 10 microFarad capacitor to the LM386. The +ve end should go to pin1 and the -ve end of the capacitor to pin 8. The easiest place to access is to solder to C75. This will give a bit more gain, and bass.
Ashhar Farhan VU2ESE commented that the LM386 distortion really increases with increased gain. He also notes that more gain is not increased sensitivity. He thinks a better idea might be to bypass pin 7 to ground with the 10 uf. Either way, he suggests that Philip keep experimenting!
Jerry KE7ER suggests for uniform gain from 300 to 2300 Hz
with no capacitor gives at least 26dB but under 46dB with a 10uF capacitor.
Adding a resistor in series with the 10uF cap would reduce the gain.
Barrett K5SSO points out that these speaker covers may be just the trick for your µBITx!
Tim AB0WR used some fine mesh black hardware cloth i.e. screen door wire. He cut a piece out of the hardware cloth that is a little larger than the speaker diameter and run a mounting bolt through from the top of the cabinet, through the hardware cloth, using the speaker to clamp the hardware cloth against the underside of the cabinet.
Tom W1EAT has noted that Q70, the audio pre-amp, has a really big job in the uBITX and sometimes gets overdriven by very strong signals. Tom also replaced the 2822 audio amp with an LM380 and lost around 8db in audio gain.
So he though he would try another transistor to try and improve this situaiton.
He had some 2SC945 transistors marked with the highest “beta”(AKA hfe) rating, so I took one with a 360 hfe and replaced Q70. He made no other component changes. The 2SC945 is supposed to be specially designed for audio pre-amps, so it ought to be a good replacement.
With RX voltage of 13.6, the Q70 collector voltage is 4.67 on the 2sc945.
I should mention that I added a 16K resistor from the base to ground when Q70 was a 2n3904, which was suggested by W3JDR to reduce distortion. I did not remove that resistor.
The results were totally positive. Tom has more AF gain, and none of the signals in tje IARU HF contest overloaded it.
Something for experimenters to try out …
Now you can have a look at a partially populated JackAl board thanks to this photo from Jack W8TEE in response to a question about a LA4425 as a replacement to the TDA2822.
The highlighted square in green is the audio amp stage of the JackAl board: a 7W TDA7266M. Seems like you could really blast the neighbours with that one!
In case you haven’t figured out what the JackAl board is about: it is a supplementary board that hsould be released in the next few weeks by Jack and Al, that adds a Teensy 3.6 processor, and a number of other mods, all on one board. The Teensy will give the µBITx new features like DSP.
An initial report from Mike (callsign not known) of distorted audio on a v4 main circuit board was tracked down to the audio amplifier. The v4 main board is the new board shipping from HF Signals from around the beginning of June 2018.
Mike solved his problem by bypassing the audio amp and using an output amp as indicated bottom left in the photo above. You can see that he has also incorporated a mic amp board in his prototype.
We will be watching out (or rather listening out) with interest to see if others are experiencing the same distortion issues from the audio amplifier section. Hopefully this is just a one-off!
Mike has now fixed the problem with crossover distortion. It was a biasing issue in the audio final.
Raj VU2ZAP suggested lifting up D15 or 16 and disconnecting one end from the board. Then, solder the center and one end of a 47 or 100 Ohm preset between the two diodes. Make sure the preset is at minimum. Power up and slowly increase the preset till distortion goes away, while also checking that Q72 and Q73 don’t get hot when the volume is low.
It is possible that this problem may reappear on other new v4 rigs.
Redoing the biassing as suggested by Raj VU2ZAP solved the issue. Mike took a 500 ohm pot and paralleled that with a 125 ohm resistor. He says, “The mod doesn’t look so good (LOL) but it works. One of the transistors became warm if the voltage difference due to the pot became too large. I adjusted this while feeling the temp and listening to a CW carrier”. The setting point was where he observed low/no distortion AND low temperature.
The photo below shows how Mike fixed the issue.
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:
- Cut off the legs of the chip as close as possible to the chip itself
- Hold each leg tightly with pliers, while heating the soldered end on the board. Wiggle the leg until it comes out.
- Clean up the holes using a solder sucker.
- Install an 8-pin DIL socket to hold a new TDA2822 or one of the potential replacement chips.
Nick VK4PLN has been working on a new board that will give a few extra features to a stock uBitx and plugs into the audio loom socket.
Its an Audio board providing easy access to Audio I/O pins. (add in your own AGC board, External amplifier…) It includes an area for adding an SSM2167 Mic Pre-amp module (with filtering for feedback and shutdown on TX). It also includes the simple 4 component PTT POP fix. (BS170) and a switchable 200hz CW filter. (LM324) that Nick already produces as a board for purchase.
The board has a bonus “snap off” section with a 3.5MHZ BCI filter for the RX chain.
Here is a pre-view, NOTE this is a WORK-IN-PROGRESS.