[Elecraft] "Noise" on VRFDET line

[Baker]

After a number of months of perfect operation my K2 has developed=
 a 
"VOX chopping" problem.

While checking various signals that are used by the KSB2 board  I=
 
discovered the VRFDET line has some "noise" on it which may be=
 the 
cause of my VOX problem.

Setting the K2 to TUNE mode on any band with the power set to=
 minimum 
(0.1W) I observe on a scope a sawtooth waveform of about 0.5v P-P=
 at 
about 2mS rep rate.  If I turn the power control up this ripple=
 is 
still there superimposed of the DC level.

I have metered the VRFDET line on the RF and Control boards,=
 together 
with checking values and resoldering connections, but there's not=
 
much there anyway. The measured power indication hovers around=
 0.2W 
to 0.3W  I thought that when I first had my K2 up and running it=
 was 
around  0.1W.

Shorting the RF side of the detector diode D9 to ground does not=
 
change the signal, therefore the noise is on the cathode end. If=
 I 
remove pin 2 of U6 on the control board from it's socket then the=
 
noise is absent from VRFDET.

I would have thought that pin 2 (RA1) of U6 should always be an=
 input 
for the ADC in that chip and that it should not be driving VRFDET=
 at 
any time.

I would be grateful for any advice,  scope measurements etc, as I=
 am 
beginning to suspect that U6 may have gone faulty in a strange=
 way.

73

Stewart G3RXQ
-- The Bakers' Home
-- baker@nildram.co.uk on 27/10/2003