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Set bias on sbitx

 

I know this has been talked about before but I see two different ways to adjust the bias. Here is what I have seen. First one adjust two pots and sets it 400 mA higher when set. Second one adjusts one pot and sets it 250 mA higher. Which one is the best way to adjust the bias.
 
First one
How to set the bias
Siegfried Jackstien, DG9BFC
 
Set both the POTS to minimum current
Select Mode as USB. Reduce the MIC to zero.
Press the PTT.
Without speaking into the microphone, carefully increase the DRIVE_BIAS POT until the current increases by 200 mA. Note the current.
Keeping the PTT on the microphone pressed, now increase the PA_BIAS POT until the current increases by another 200 mA.
Release the PTT.
 
Second one 
PA bias current
1 Set Mode to USB
2 Set Mic to 1
3 Set Drive to 1
4 Set PA_BIAS1 preset to zero (fully counter-clockwiseBias)
5 Note DC current on PTT
6.. Slowly increase the bias current until the total current consumed by teh radio is increased by approximately 250 mA.
 
 

Hi -- you might note that some versions of Ashhar's radios have an individual potentiometer for each of the pushpull final amplifier MOSFETS, while others have only one (which sets both gates simultaneously).   Take a look at the schematic for YOUR hardware (or open up the case and examine the motherboard) to see which might apply, perhaps?

73
Gordon KX4Z


On Mon, Dec 30, 2024 at 8:36 AM bretnemeth via groups.io <bretnemeth=yahoo.com@groups.io> wrote:
I know this has been talked about before but I see two different ways to adjust the bias. Here is what I have seen. First one adjust two pots and sets it 400 mA higher when set. Second one adjusts one pot and sets it 250 mA higher. Which one is the best way to adjust the bias.
 
First one
How to set the bias
Siegfried Jackstien, DG9BFC
 
Set both the POTS to minimum current
Select Mode as USB. Reduce the MIC to zero.
Press the PTT.
Without speaking into the microphone, carefully increase the DRIVE_BIAS POT until the current increases by 200 mA. Note the current.
Keeping the PTT on the microphone pressed, now increase the PA_BIAS POT until the current increases by another 200 mA.
Release the PTT.
 
Second one 
PA bias current
1 Set Mode to USB
2 Set Mic to 1
3 Set Drive to 1
4 Set PA_BIAS1 preset to zero (fully counter-clockwiseBias)
5 Note DC current on PTT
6.. Slowly increase the bias current until the total current consumed by teh radio is increased by approximately 250 mA.
 
 

I didn't know that some have only one pot. Mine has two but I have also read that one is set to half way and not adjusted. The information is confusing.
 

Here is a photo of my board. One pot is not near the mosfets.
Does that mean I only adjust the one near the fets? I'm not a
big electronics guy but know just enough to get it done with
a little help.
 
 

Looking at the schematic my guess I only adjust pabias1 near the fets. 
 

 
The blue pot. left from the MOSFET’s on your picture.
 
 
 

From: bretnemeth via groups.io
Sent: Monday, December 30, 2024 3:38 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Set bias on sbitx
 
Here is a photo of my board. One pot is not near the mosfets.
Does that mean I only adjust the one near the fets? I'm not a
big electronics guy but know just enough to get it done with
a little help.
 
 

I think I understand. The other pot looks like it is in the receive circuit.
 
 

On Mon, Dec 30, 2024, at 08:57 AM, bretnemeth wrote:
I think I understand. The other pot looks like it is in the receive circuit.
 
 
 
 
The second pot is used in both the receiver and transmitter.  It sets the midpoint for the input to the mixer that is used to translate to the operating frequency.  The setting is non-critical.  Midpoint works well.
 
73
Evan
AC9TU
 

 
Ah, what a great hobby this is!!   Even when we don't "start out" as electronics experts, we slowly BECOME ONE!!!  And that is part of why the Amateur Radio Service was Created (see 97.1)   So thanks for the photo, and I marked the one that is the bias pot.  BE CAREFUL   I don't know which way is "safe" -- youll need to read from someone else.   I   **always**  use a current limited supply (e.g. a real 12V wall wart or current limited supply) when adjusting these bias pots because if you max them out the WRONG WAY, you will get maximum current, instead of minimum when you apply power and hit "transmit".   So a little CARE is advised.   
 
Remember, Ashhar does not design $10-thousand radios      He shows us how to make very simple stuff do AMAZING outcomes.    His designs are elegant.     You do have to read a bit (or ask questions, like you did, good job!)
 
Gordon KX4Z
 
 

Thanks guys. I purchased this radio not working figuring that it was the fets that were bad.
I removed them and tested them and found that only one was bad. I am replacing both of
them today with new ones that came with the radio. That's the reason for all the questions.
Little by little I am learning. You guys are great and so helpful. Thank you all.
 

Thanks Gordon. I have read that you turn the pot counter clockwise then bring it up slowly.
I have adjusted the fets on my Hardrock-50 amp when I put it together so I understand what
you are saying. I'll use a little 12v 2 amp meanwell supply. I'm guessing that the power is less
than 2 amps when making the adjustment.

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