Don, Thanks. The feedback capacitor solved the LMC660's instability problem.
73, David, NM5U ----- Original Message ----- From: Don Wilhelm<mailto:w3...@embarqmail.com> To: DAVID POWELL<mailto:dwpowel...@msn.com> Cc: elecraft@mailman.qth.net<mailto:elecraft@mailman.qth.net> Sent: Thursday, April 09, 2009 5:17 AM Subject: Re: [Elecraft] K2 U10C Oscillating David, The LMC660 op amp can have a tendency to oscillate when driving a capacitive load (see the LMC660 datasheet) although it is normally stable in the K2. Before replacing it, try the following steps - check after each one and stop if no oscillation. 1) Flush trim the leads of U10 and the nearby R-Paks. 2) Use a cotton swab and alcohol to clean any flux residue from the area of U10, RP2 and RP3. 3) Try adding a 10 pF capacitor between U10 pins 8 and 9 If the oscillation persists, replace U10. 73, Don W3FPR DAVID POWELL wrote: > On my K2 (#3014) I hear signals and noises while tuning across 160m with the antenna disconnected. The worst is a strong but unstable signal that moves from about 1850 kHz to about 1910 kHz during about 15 minutes after I switch on the radio. I accidentally traced the signal to U10C on the control board: the signal jumped when my finger got near the left end of resistor pack RP2. (U10C appears to be a buffer for the V BIAS-XFIL line.) > > The signal is several kHz wide and resembles hash from a dimmer or switching power supply. > > I have verified the U10 circuit resistors and voltages as nominally correct. > > U10D, an identical circuit for the V BFO line has pretty clean dc at its output. > > Has anyone else seen this? Am I correct in concluding that I should replace U10? > > 73, > David, NM5U > > ______________________________________________________________ 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
