Hello group,

I've had my Bitx for a while and am just starting to put it together. I've been reading this list off and on but now so much has happened with the mods I almost don't know where to start.

I thought of just build it see what happens and search for answers as issues come up but maybe that's not the best way as it sees that there are a lot of interesting and necessary mods out there.? Any suggestions as to an appropriate approach??

Thanks and 73, Peter?
ve3poa?

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Yes, Just build it stock, don't hook up transmitter power.

Get Rec working first.? Get tunning working with Pot and or modifications with Sketch and 10T pot.

Figure out a box and where everything needs to go.? I used a Jameco metal box, a little to small but nice.

Might next work on Audio amp.? Like putting a resistor to the pot for noise at near zero volume.

I went to 16 Ohm 4" (Big) speaker from an 8 Ohm 2" speaker.? That and 10uF caps on the 2 places on LM386

made my audio MUCH better.? My advise is to use a 16 Ohm speaker.

Go from there.

Mike, WA6ISP

Hello group,

I've had my Bitx for a while and am just starting to put it together. I've been reading this list off and on but now so much has happened with the mods I almost don't know where to start.

I thought of just build it see what happens and search for answers as issues come up but maybe that's not the best way as it sees that there are a lot of interesting and necessary mods out there.? Any suggestions as to an appropriate approach??

Thanks and 73, Peter?
ve3poa?

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

Hi Peter, several of us in the Barrie WAX Group have our BITX40v3 on the air now.
One mod that really helps is to use a 10 turn pot for tuning (the single turn that is included is just too touchy when tuning).
Allard PE1NWL makes one of the better firmware upgrades and that is the one we all use: ??
Be sure to install the two push buttons for Function and for Fine Tune (this gives 1 Hz tuning resolution) in Allard's sketch.
Take a look at Minimum BITX40v3 Hacks at ?
There are a number of useful links in the TCA article 'BITX40v3 - an Affordable and Fun "Kit" Transceiver' in the September/October issue.

73, AL? VE3RRD

It should work right out of the box, once you have followed the wireup instructions from hfsigs.com.
If it doesn't, check all those coils for cold solder joints, broken wires, and shorts, as that seems to be
the primary source of trouble on these rigs.
Check out the receiver first, but will take a halfway decent antenna to get any signals since gain is
kept low to avoid overload when using a "real" antenna.
Avoid feeding the rig more than 12v, I'd recommend using a 1A fuse into the main rig,
and a 2A fuse into the IRF510. ?If you don't have an RF wattmeter,?
use a 2w 50ohm resistor as a dummy load,?it should get hot when transmitting?
while yelling into the mike. ?Better yet, build a diode RF?detector, and measure?
the RF voltage across the dummy load.
With the original firmware, the operating frequency may jump around a hundred hz or so,
enough that I wouldn't want to put it on the air.
So once you think the rig is working, next step is to figure out how to download Allard's code:
? ??
I suggest you just add the function button, and avoid the other mods till you know the rig is working well.

Here's a couple other mods to consider:
??/g/BITX20/message/33933

Some insist that everyone avoid any chance of the 2'd harmonic not meeting the letter of the law.
It's good to add that 100pf C0G/NP0/SiverMica cap to?the output low pass filter, rated for at least 100v.

The typical 100w ssb transmission may well have a 2'd harmonic that's 10db worse than our 5w bitx40.
FCC regs state that we should use "the minimum transmitter power necessary to carry out the desired communications".
Ignoring that directive (as many operators do) seems a far greater sin. ?

Q15 can be blown by several volts at the base if your bitx40 is hooked up to an antenna while a nearby rig
is dumping 100w into some other antenna. ?If that's in your future you should do the double-diode mod.?
?
Good luck!

Jerry, KE7ER

Thanks for all the quick responses. Good information and leads to a solid plan.

73, Peter,
ve3poa

Mike's right, the LM386 can oscillate when the volume pot is set for minimum volume.
The fix is to insert a resistor, any value between 1k and 10k, between the volume pot pin 2 and the LM386 pin 3.

Also, R111 at 100 ohms limits current into the LM386, this can clip audio peaks.
Can fix this be adding a 10 ohm resistor in parallel.with R111.

I believe both of the above come from posts Raj has made to this forum.

I have no idea where the "2 places on the LM386" are that Mike is putting his 10uF caps.

Jerry, KE7ER

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I put a small 10uF/16V electrolytic from 7 to 4 (Gnd), another across C113.

And a big change when I got a Big 4" 16 Ohm speaker.? This made me very happy with the audio!

Mike, WA6ISP

3/2017 9:00 AM, Jerry Gaffke via Groups.Io wrote:

Mike's right, the LM386 can oscillate when the volume pot is set for minimum volume.
The fix is to insert a resistor, any value between 1k and 10k, between the volume pot pin 2 and the LM386 pin 3.

Also, R111 at 100 ohms limits current into the LM386, this can clip audio peaks.
Can fix this be adding a 10 ohm resistor in parallel.with R111.

I believe both of the above come from posts Raj has made to this forum.

I have no idea where the "2 places on the LM386" are that Mike is putting his 10uF caps.

Jerry, KE7ER


On Mon, Nov 13, 2017 at 07:36 am, Michael Hagen wrote:

Might next work on Audio amp.? Like putting a resistor to the pot for noise at near zero volume.

I went to 16 Ohm 4" (Big) speaker from an 8 Ohm 2" speaker.? That and 10uF caps on the 2 places on LM386

made my audio MUCH better.? My advise is to use a 16 Ohm speaker.

?

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

开云体育

Good Ideas!

Thanks, I will decouple the DC off the pot first.

Mike, WA6ISP

Hi.

I do not have a BITX but have been following most of the discussions on the group from time to time.

I have seen a number of posts on the LM383 oscillating when the volume control is set to minimum value and the suggestions being made.

I downloaded the LM36s data sheet and had a look at it and also on the BITX circuit diagram.

According the to BITX circuit the volume control pin 2 is directly coupled to the LM386 pin 3 (non-inverting input) which is the base of one of the transistors.

?When the volume control is at minimum it effectively shorts the internal 50KΩ biasing resistor, upsetting the LM386 internal biasing.

Presently the volume control also has a DC voltage across it (pin 2 centre pin of the volume control being connected directly to pin 3 of the LM386) which will lead to a scratchy control in a short time.

A simple modification that I suggest is to put an electrolytic (you can play with the value to get the best audio without too much bass. You can start with a 1μF) with the + side to pin 3 of the LM386 and the - side to the middle contact (pin 2) of the volume control.

You can put the capacitor on the volume control itself if you don't want to play around with the pcb.

This would couple the audio to the LM386 input without upsetting the internal biasing and without having any DC voltage on the volume control.

As I said I don't have a BITX and presently I do not have time to breadboard an LM386 amplifier to test it but I am sure that this will not only solve the problem of the LM386 oscillating at minimum volume but also prevent the volume control becoming scratchy as there will be no DC voltage passing through it.

Another thing that I noticed that crops up in the comments is that you can hear your own voice for a couple of seconds after you start transmitting.

A look at the BITX circuit shows that there is a 470μF (C110) on pin 6 of the LM386 to ground, decoupling the supply with a 100Ω (R111) resistor to the supply.

There is no need for such a high value capacitor if you are using a decently smoothed full wave rectification power supply, although it is mostly used to smooth the voltage variations at the LM386 supply which is limited by R111 due to the changing current in the LM386 with audio.

The reactance of a 47μF capacitor at 100Hz?is 3.37Ω so effectively it forms a potential divider with the 100Ω (R111) resistor, effectively decoupling any residual hum from the power supply by nearly 30 times (29.67).

So another modification I suggest is to replace the 470μF (C110) with a 47μF capacitor which would reduce the discharge time at the LM386 supply and less time hearing your own voice when you change over to transmit as the only discharge path for C110 is through the LM386 R111 R113 and Q16 which also has its own 47μF?supply decoupling capacitor

This should also work if you change R111 to 10Ω which you may have to to prevent clipping as the reduced value capacitor may not be enough to smooth the supply voltage variations due to the audio input signal to the LM386 and R111 limiting the current, but be careful if your supply goes too high not to exceed the maximum voltage for the LM386.?

So I would suggest that if you lower R111 in value you should start decreasing it in steps until no clipping can be heard or seen on a scope.

Give these mods a try and please report back.

Regards

:Lawrence

On Mon, Nov 13, 2017 at 8:34 PM, Michael Hagen <motdog@...> wrote:

I put a small 10uF/16V electrolytic from 7 to 4 (Gnd), another across C113.

And a big change when I got a Big 4" 16 Ohm speaker.? This made me very happy with the audio!

Mike, WA6ISP

3/2017 9:00 AM, Jerry Gaffke via Groups.Io wrote:

Mike's right, the LM386 can oscillate when the volume pot is set for minimum volume.
The fix is to insert a resistor, any value between 1k and 10k, between the volume pot pin 2 and the LM386 pin 3.

Also, R111 at 100 ohms limits current into the LM386, this can clip audio peaks.
Can fix this be adding a 10 ohm resistor in parallel.with R111.

I believe both of the above come from posts Raj has made to this forum.

I have no idea where the "2 places on the LM386" are that Mike is putting his 10uF caps.

Jerry, KE7ER


On Mon, Nov 13, 2017 at 07:36 am, Michael Hagen wrote:

Might next work on Audio amp.? Like putting a resistor to the pot for noise at near zero volume.

I went to 16 Ohm 4" (Big) speaker from an 8 Ohm 2" speaker.? That and 10uF caps on the 2 places on LM386

made my audio MUCH better.? My advise is to use a 16 Ohm speaker.

?

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...

Mike,

Using 16 ohm speaker reduces the power from LM386 and thus the current.

Just parallel 10 ohms across R111 and then see the sound. R111 at 100 ohms drops too much
voltage and causes distortion.

Raj

At 14-11-2017, you wrote:

Reducing R111 to 10 ohms sounds good, that audio power amp can require significant current.

Mike's cap from LM386-7 to ground is also a good idea,the LM386 datasheet recommends
a 10uF cap there to?improve the power supply rejection ratio. ?This is far more effective than
putting a large cap at the power supply pin, perhaps C110 could be reduced to from 470uF to 10uF
with the new cap present at pin 7, and thus reduce the audio pop on ptt.

A larger cap at C113 increases the gain, 10uF there is not unreasonable.
With nothing at all at C113 gain is around 26dB, with the 10uF cap it is 46dB.
Some have found it best to remove C113 entirely as the LM386 was getting overloaded.

Speaker quality varies wildly, good to try several.
Bigger is often better, but by no means always.

Jerry, KE7ER

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I got 2 speakers from Goldmine or somewhere like that.

They are "Golden Voice".? I realized 16 ohms would be better liked by the LM386.

I think they are a lot more efficient than the 2" I was using?

I think I have plenty of audio without bypassing the 100 OHM?

Mike, WA6ISP

Reducing R111 to 10 ohms sounds good, that audio power amp can require significant current.

Mike's cap from LM386-7 to ground is also a good idea,the LM386 datasheet recommends
a 10uF cap there to?improve the power supply rejection ratio. ?This is far more effective than
putting a large cap at the power supply pin, perhaps C110 could be reduced to from 470uF to 10uF
with the new cap present at pin 7, and thus reduce the audio pop on ptt.

A larger cap at C113 increases the gain, 10uF there is not unreasonable.
With nothing at all at C113 gain is around 26dB, with the 10uF cap it is 46dB.
Some have found it best to remove C113 entirely as the LM386 was getting overloaded.

Speaker quality varies wildly, good to try several.
Bigger is often better, but by no means always.

Jerry, KE7ER



On Mon, Nov 13, 2017 at 08:56 pm, Raj vu2zap wrote:

Mike,
Using 16 ohm speaker reduces the power from LM386 and thus the current.
Just parallel 10 ohms across R111 and then see the sound. R111 at 100 ohms drops too much
voltage and causes distortion.
Raj

At 14-11-2017, Mike wrote:

I put a small 10uF/16V electrolytic from 7 to 4 (Gnd), another across C113.
And a big change when I got a Big 4" 16 Ohm speaker.?? This made me very happy with the audio!
Mike, WA6ISP

-- 
Mike Hagen, WA6ISP
10917 Bryant Street
Yucaipa, Ca. 92399
(909) 918-0058
PayPal ID  "MotDog@..."
Mike@...