Re: bitx40: low RF ower an dead part in RX path?

Jerry Gaffke
 

Stephan,

Driving 1khz into C31 is not going to give much response because the amplifier at Q3
is operating at 12mhz, and the 0.1uF at C33 will mostly block the 1khz audio frequency. 
The signals through Q3 remain at 12mhz until after going through the mixer at D15,D16,T4,
after which they are at audio frequencies.

Here's an easy way to do a rough check of the gain of each stage of a receiver:
    http://bitxhacks.blogspot.com/2016/12/troubleshooting-bitx40.html
Farhan uses a piece of wire that's maybe a meter or two long as an antenna, nothing attached to the far end of the wire.
Your household environment will have 60hz power hum and lots of RF signals from digital clocks and light dimmers
and natural atmospheric disturbances, all of which get picked up by the wire to provide the test signal.

If the Bitx40 is getting signals strong enough that you need the AGC circuit to prevent the audio from distorting,
that distortion is probably coming from too large of a signal into the Q12 audio pre-amp stage. 
The LM386 audio amplifier should then give plenty of volume.
I suggest you triple check the way the volume control and speaker have been wired up,
perhaps try a larger speaker.
    
The three terminal Low-Dropout-Regulator should work fine if used with the recommended bypass caps.
You don't say which LDO you are using, here's a datasheet for the popular LM7812 (and many variants of it).
They recommend a 0.22uF cap to ground at the input pin of the regulator to avoid oscillation,
which may very well be the source of your noise problem.  (Maybe just two common 0.1uF caps in parallel.)
A 0.1uF cap from output pin to ground is not a bad idea.
 
Jerry, KE7ER


On Sun, Jun 9, 2019 at 09:32 AM, Stephan Lauffer wrote:

Hello!

With the AGC mod from Don nd6t I have a lot of fun with my bitx40 again. Without this strong signals overdrive my RX path somewhere (in LM386?) that much that I had strong distortion with volume poti in round about the middle position and above.

But there are two other problems (maybe default bitx40 behaviour or a broken part??):

1st: My RX signal is very weak.
I checked out several small speakers but I need to set volume to max in any case. The bitx is then not as loud as a normal human voice nearby you. Maybe a transistor in the RX path is a problem? I feed a nf signal ~1kHz with some few mV into C116, C107 and C33. Everytime I get a loud and strong signal. On C31 I did not get a good/strong signal. Maybe in this stage is a problem? Is there a paper/wiki how I could nail this problem down? First I thought about another NF stage witha LM380. But after I get the loud signal with my testsignal I guess this is not needed.

2nd: "White noise"/hiss in the NF signal:
I tried to power the PA independend with > 12V for more HF output. This means: After the power plug I split the PWR IN with a LDO for the main 12V circuit and another wire without the LDO into the PA in. So fare, so good. The very interesting part is that I get high freq hiss in the RX signal path as soon as the input voltage exceeds the 12.0V out plus the drop voltage of the LDO. F.e. this means: Clear RX signal up to 12.7V and high freq noise up from here. The noise reaches its max volume about 13.0V. Is this a general problem as soon as I drive the PA with higher volume or is there a change to solve this issui with better groundings/shields..?

Best regards, 73 an thanks for help,
Stephan dc8lz

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