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I have a Hendrix kit built Bit-x 20 (assembled myself) and after about a year of playing with it, finally started making contacts.
It has the 10K pot mod instead of a tuning cap. and tunes from 3.152 Mhz to 3.350 Mhz.
It has 2 IRF 510's as final output and is adjusted to about 8-10 watts out.
I strung an LNR end fed multiband antenna about 25 ft up and managed to make contacts while on vacation in Sturbridge, Ma. ?My first contact was Kansas, where the other ham said that I was on frequency, but "FMing". ?The second contact was Tennessee, where the ham there said that I had "RF in my audio". ?He thought my antenna was causing the problems, but previously at home I had tuned to 10 watts out and 1.2 SWR on my Carolina Windom, and experienced the same comments of " just barely intelligible audio" while making 1000 mile contact.
Any suggestions, thoughts or musings would be appreciated.
Andy G
KM4TRT

I had similar audio reports on a BITX40 kit.? What I believe fixed the problem was to adjust R136.? If you can find the comparable variable resistor on your kit, it may help.
It is at the base of Q14, near the bottom of the schematic here: http://www.hfsigs.com/bitx40v3_circuit.html

R136 is "drive level". Over-driving can produce distortion at the driver stage (cause of "barely intelligible audio", likely mistaken as FMing?), and if it continues can burn out the driver's emitter resistor. For the BITX40 this is R141, 10 ohms. It seems the original surface-mount resistor's power rating is too close to the maximum normal drive level, so some users are experiencing this burn-out. There's not that much actual material to it, so it won't give off a significant smoke or smell as it fails, therefore there's not likely to be much warning ... The BITX's Final is Class A, hence the hefty quiescent current. The Hendrix may be Class B or an AB, so likely to be more efficient that way. Summary: don't push it - these wee devices ain't 4CX250Bs! ? ;)

Thanks for reply all.
I have the equivalent drive level resistor. ?The procedure call for pre setting the IRF 510s to 50 ma each, and then adjusting the drive level pot until the increase in voltage slows down. ?This should occur at about 12 watts. ? Then back down about a half watts. ?Originally I was able to see about 15 watts if I kept going
but now that point seems to have dropped to about 10 watts, so I back down about 1/2 watt from there. (measured on a scope and then using formula)
I guess I have to start measuring voltages on all the transistors.
Andy
Km4TRT