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My first Bitx40 kit suffered through my attempts to learn to desolder smd components and various hack attempts.? I did the c113 removal and subsequent replacement, replacing the 100 ohm resistors with 220 ohm to reduce drift, L4 attempted repair and replacement, subsequent replacement with the Raduino and replacement of the audio potentiometer.? So there are many ways that this radio could be messed up.? There may be multiple problems.

The symptoms are no power on transmit: the relay clicks but no power shows on my swr/power meter, low audio volume and audio works for a few minutes then fades out.

I went through the Tips and Mods Q voltage table and compared my radio's voltage readings.? My radio has an odd difference in Q1-Q2-Q3 and Q4-Q5-Q6. Each of these sets are three transistors that should have the same reading or very close to it.? In my case Q1 voltage while receiving is significantly higher than the other two (Q2-Q3). Q4-Q5-Q6 have a similar pattern while transmitting with Q4 being about 23% higher than Q5 and Q6.

For example:

Collector Voltage Receiving, battery power at 12.2V
Q1: 10.73
Q2: 8.70
Q3: 8.70

According to the Tips sheet, these should all be at about 9.8V.

I have gone through the board looking for solder bridges and testing resistances but have pretty much reached the end of my knowledge of circuit diagnosis.? Any advice would be appreciated.

Replacing the 100 ohms at the six emitter resistors of the bidi amps with 220 would explain the higher collector voltage at Q2, Q3. ?No reason for Q1 to be different than Q2,Q3 on receive, signal levels are down in the microvolts and the quiescient voltages and currents are well isolated by caps C11 and C13. ?So I'd zero in on Q1, there's only about a half dozen parts that might be at fault. ?Primary suspect is your new 220 ohms at R12, is it really 220 ohms? ?Is it really soldered down properly with no cold solder joints? ?With power off, compare ohmmeter readings across R12 and R22, but avoid direct contact with R12 because those solder joints are suspect (so maybe put the probes on the R12 side of C12 and the ground end of R11). ? How do the DC voltages at the base and emitter of Q1 compare with those at Q2? ?Q4 likely has the same problem as Q1, whatever that is.?

Trouble at Q1 and Q4 could explain low audio volume and no transmit power. ?And possibly even why the audio fades out, though that seems more likely to be at C113. ?I'd simply remove C113 entirely till all this gets sorted out. ?Without C113 the audio out will be somewhat reduced, but still good enough to hear. ?If audio still fades out after a few minutes, maybe watch the voltage at C110, see if that drops with your audio volume. ?Maybe wire up some other audio amplifier or some high impedance headhones to M1, see if audio is good there.

Thank you very much for your help!

I made a list of the resistors in the three blocks of the circuit, and using the schematic tested and wrote down the resistance from the component on either side of the resistor.? I was able to find a cold solder joint in R45, which was fixed with a little bit of heat.? I went back and checked the Q voltages and things look much better with Q1-3 and Q4-6 all giving similar results.? The audio sounds good and seems stable.? I am very happy with the receiver so I call the receiver fixed.

There is no change in the transmit problem, unfortunately.? I don't believe I ever transmitted with this radio earlier, so I don't know for sure if it worked originally.

The relay clicks, and I hear a hum while transmitting, but no output showing on the swr/power meter.

Very weird that catching a cold solder joint at R45 fixed the receiver. ?R45 is the emitter resistor at Q4, in the transmit chain. ?I'd get some good magnification and bright light, heat up any other joints that look questionable. ?Flex the board lightly while trying to transmit, see if that momentarily fixes it, suggesting another cold solder joint. ?Cold solder joints can come and go, I can believe that the board worked just fine at hfsigs before being shipped. ?You might yet have a problem in the receiver, but after all the probing it is something is now making contact. ?That's in addition to an apparent problem on transmit.

You might check for transmit audio between T4 pins 1 and 6 using some other audio amp to a speaker, or high impedance headphones.

I assume you have PA 12v into the IRF510 hooked up to your 12 volt supply when testing that transmitter. ?What are the DC voltages on the gate and drain of Q15 when you try to transmit? ?You might look over the notes at the top of the files section under "1 docs of Tips n Mods" ?if you have not already. ?Pots RV1 (bias voltage onto the gate of IRF510, carefully follow the hfsigs instructions or you may blow the IRF510) and R136 (determines the drive level into the final amplifier) apparently get set properly at hfsigs before shipping, but might be worth checking.

Yet another case where we need a cheap and standard way of measuring AC voltages through the transmit RF chain.

No info here suggests hfsigs is pretty good at setting it properly, so I'd carefully measure with an ohmmeter before monkeying with it. ?You will probably want to set it exactly back to where it was when done.

Pot R136 determines the signal level going into the IRF510 final. ?Should have about the same effect as mike gain on a commercial SSB transceiver. ?Unless it got nailed all the way back minimum, you should have at least some output from the transmitter whatever the setting. ?Advancing the pot will increase power out, but advance it too far and you will get distortion.

A bit ambiguous. ?Should read: ?"I haven't seen any posts in this forum suggesting anybody having to adjust R136, so hfsigs must be pretty good at setting that pot before shipping."

> "No info here suggests hfsigs is pretty good at setting it properly,"

Q15 RX B: 0, E: 12.27, C: 0

Q15 TX B: 4.50, E: 11.71, C: 0.01

The above voltages were measured prior to heating up cold joint in R45.

I have a simple RF probe kit, which is a capacitor, resistor and diode which is connected to a voltmeter.? It looks like it works as a rectifier and allows a voltage to be read from AC.? It is an older kit from the QRPp International Radio Club.? I have never used it.? Are these helpful?

This is the RF Probe circuit: http://www.n5ese.com/rf_probe_schem.jpg