[Elecraft] K2 U10C Oscillating

[DAVID POWELL]

Don,

Thanks. The feedback capacitor solved the LMC660's instability problem. 

73,
David, NM5U
  ----- Original Message ----- 
  From: Don Wilhelm<mailto:w3fpr at embarqmail.com> 
  To: DAVID POWELL<mailto:dwpowell68 at msn.com> 
  Cc: elecraft at mailman.qth.net<mailto:elecraft at 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
  >   
  >