K2 U10C Oscillating

[DAVID POWELL-11]

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
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[Don Wilhelm-4]

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
>  
>

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[DAVID POWELL-11]

Don,

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

73,
David, NM5U
  ----- Original Message -----
  From: Don Wilhelm<mailto:[hidden email]>
  To: DAVID POWELL<mailto:[hidden email]>
  Cc: [hidden email]<mailto:[hidden email]>
  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
  >  
  >
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