Hi Don (again!) I was following the steps in the signal tracing checklist that say with 25mV in to the AF amp I should get 1.6V out with the speaker disconnected and I don't. I've checked the signal path from the audio amp to the speaker connector and there are no shorts or open circuits.
The BFO is around 1V pk pk so that looks healthy as do the other oscillators. The post mixer amp is working OK as are the pre-amp and attenuator. Loss through the CW filter checks out also. That leaves the IF amp, the detector and the AF amp. I can peg the S meter with the generator. AGC voltage with no signal is set at 3.8V (this is a rev A K2 remember). Something is definitely wrong somewhere though as a 50uV signal is quiet even with AGC off. One thing I have noticed with AGC off is that if I set the RF gain so that the IF chain is just out of clipping there is still low audio (if that helps) Lots of head scratching going on here. Cheers, Paul M1PAF ----- Original Message ----- From: "Don Wilhelm" <w3...@embarqmail.com> To: "Paul Fletcher" <p...@melreed.demon.co.uk> Cc: <elecraft@mailman.qth.net> Sent: Thursday, November 26, 2009 12:05 AM Subject: Re: [Elecraft] [K2] K2 signal tracing progress and questions arising > Paul, > > Do you have a sufficient signal level to give you 1.6 volts out of the AF > amplifier? > The impedance of the amplifier's input is considerably higher than the > impedance connected to the output, so the AC voltages may actually be > similar in and out. Can you peg the S-meter with your signal generator? > > Most problems with "low audio" are actually caused earlier in the receive > chain. Check to see if you have at least 200 mV peak to peak (70 mV RMS) > of BFO at RF Board U11 pin 6. The diodes (D40 and D41) across the input > of the IF amplifier will clip at a level lower than the "Saturated" test, > so that item in the Signal Tracing section is no longer possible to do. > > What is the input vs. output from the IF amplifier U12. If there is not a > significant increase, you may have a bad IF amplifier or T7 may be mounted > backwards (red winding goes closest to U12), or the AGC voltage to the IF > amplifier may be too high (check U12 pin 5, it should be no higher than > 3.9 volts). > > 73, > Don W3FPR > > Paul Fletcher wrote: >> >> OK - have done the checks and re-soldered a few connections and I'm now >> happy that the attenuation in the pre-mixer stages is OK. I've progressed >> through the receive chain and the only obvious thing is the audio amp >> that >> is causing problems. The signal out of the amp (with or without the >> speaker >> connected) is not significantly higher than the signal in (in fact it's >> actually attenuated if I compare the signal across PD1 and PD2 and the >> output). I've replaced the LM380 and both mute FET's to no avail. There >> are >> no shorts on the output and the sidetone is healthy (same output as input >> measured on the bypass pin) and I'm completely flummoxed. I've checked >> out >> the AF gain control and it varies from 5k (max volume) to a few ohms (min >> volume) and I've confirmed this measurement right up to the chip. >> >> According to the signal tracing fault finding I should be able to get >> 1.6V >> RMS out of the audio amp but no chance - nowhere near. >> >> Help! >> >> Cheers, >> Paul. >> ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
