[Elecraft] K2 U10C Oscillating

[Don Wilhelm]

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
>   
>