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Having some trouble with my bitx40 output lately. Using a QRP Guys dummy load, I'm measuring only 3V peak on the output (which translates to 90mW). I went back through the bitx setup instructions and tried adjusting the PA bias. When measuring the PA current draw, the most I got was around 200mA.?

I've tried multiple mics to rule out low audio input problems.?

Is there anything else I should try before replacing the IRF510??

Cheers,
David K4DBZ

You should be adjusting the quiescent drain current through the IRF510 for 100ma or so (with PTT pressed so it is trying to transmit).
Did you succeed at this?? No RF need be generated when doing this adjustment, so no need to speak into the mike.
When you get that 100ma quiescent current, the drain of the IRF510 should be at your supply voltage (typically 12 volts dc)
and the gate will be around 4v.? Be careful when making this adjustment, the bias pot is backwards, so fully counter-clockwise is max current
which will be several amps of drain current, and the IRF510 will smoke unless you have a fuse in the power supply line of 2 amps or so.

The instructions to set the bias current are at? ?
under "Alignment of the Bitx40".

Did R136 get twiddled, and left too far clockwise (also backwards, so fully clockwise is zero watts out).

If you can adjust for 100ma of IRF510 drain current, then the IRF510 is probably fine.
Problem is somewhere between the mike and the IRF510 gate, too small of a signal is getting through.
Could be a broken/shorted/cold-soldered wire on one of those torroids?


Here's a bunch of other suggestions, not in any particular order:

Verify your DC power supply is holding up at 12v when you key try to transmit.

Verify your dummy load is correctly connected to the antenna/ground connector into the Bitx40.
The BNC connectors that come with these kits are sometimes not quite machined to spec,
and the center pin of the BNC plug may not be making contact.? Cables can also fail.

Do the mods that Allard suggests (v1 or v2) to implement CW mode.?
If you get sufficient power out in CW mode, that means the trouble is between the mike and the dual diode modulator.

If CW power out is also low, could be trouble in the transmit LPF.? Remove L8 and C160 from the circuit,
couple what was the top of C160 out to your 50 ohm dummy load.
There will be lots of harmonics now, but should see adequate power if the trouble was in the LPF.

Perhaps your power meter is haywire.??
Build a low power dummy load of 3 or 4 watts so it heats up?quickly, perhaps using Mouser 283-51-RC.
Get a feel for how fast it heats up when transmitting CW for a few seconds,
then try to get the same effect by wiring that same 50 ohm resistor to a variable power supply?or a string of batteries.??
For example, watts = volts * volts / 50ohms,? so sqrt(10Watts*50ohms) = 22.4 volts DC?
should also heat up that same 50 ohm resistor at a 10 Watt rate.

Q13 and R141 run hotter than they should.? If you fed your Bitx40 more than 12v, they could have smoked.
Also, atmospheric static or a local QRO station with it's antenna near your Bitx40 antenna can blow Q13.
Check the DC emitter-to-ground voltage at Q13 and Q14 when transmitting with no audio, Randy says about 3.5 and 0.5 volts respectively:
? ? /g/BITX20/files/K7AGE/BITX40%20Voltage%20Measurments.pdf?
Search "Q13 R141" in this bitx20 forum web page for more info.

Check the other voltages Randy gives, see if they more or less? agree with your rig on transmit.?

If power out is still low, time to build a diode RF probe or find a 10MHz (or better) oscilloscope,
see where in the transmit signal chain the signal level is falling off.

So, in answer to your question, there's lots of things to do before blindly replacing the IRF510.
Your trouble could be anywhere between the mike and the power meter inclusive,
and odds are it's not the IRF510.

Jerry, KE7ER

Should not be any need to remove L8 and C160 when going around the LPF.
I'd just put that 3W 51 ohm resistor across C160, nothing on the antenna connector,, see how quickly it gets hot

Jerry, thanks for the tips. I was able to adjust RV1 and see 100mA with the key down and no audio input. However, I do not see the 1A current when shouting/whistling into the mic. My "power meter" is a QRP Labs dummy load with pinouts for the voltage across the resistors (and a formula for calculating watts). I still only see 3V peak across the load which is very low (should be like 20V or more I think).

How can I further inspect the audio chain to see where my signal is dropping off?

The audio chain is just the mike pre-amp at Q12.
A good start would be to check for the DC voltages that Randy gave us.
Key the mike, but no need for audio into the mike.

Next step is to do Allard's mods (either version), and see if you get full power as a CW transmitter.

If it works for CW, then (on the Bitx40) it all comes back to trouble around the mike and Q12.
If you can figure out a way to feed 50mv of 1khz audio into the mike amp, maybe try that.
But mostly just inspect those parts for bad solder joins, bad wiring to the mike, bad mike,
perhaps probe around as much as you can with an ohmmeter to verify that things are still soldered together ok.

Jerry, KE7E