[Elecraft] Hum in the Sig mode-KX1

[Charles Vollum]

Hi Steve,

It sounds like exactly what I'm experiencing with S/N 354 (30m option 
not installed yet.)  Maybe we can work it out together...

I looked at the AGC voltage at VSMTR (measured at Q9's collector), and 
found that with no signal, I have the stated 1.4 volts, but with a 
strong signal (2 volt P-P audio on the input side of R5 from a 50uv RF 
signal) I still have 1.2 volts instead of 0.6 as listed.  Not sure what 
this indicates...

Following Troubleshooting step 70 for AGC/S-Meter, I have checked R5, 
D4, C53, Q9, R20, and C8 for value and soldering, and D4 and C53 for 
orientation; I have a hard time reading L9's bands, but think it's also 
right...  voltages for Q9 are correct per Table E-6 (though it takes a 
while for the base to settle down after a loud signal has charged up 
C53.)

The AGC is functioning, and the S-Meter seems OK as well, with SIG 
params of 74L and 3H giving reasonable levels on the display (4 bars 
for the 50uv signal.)

Looking with my scope, the hum or buzz is a spike about every 12.5ms, 
not actually a 60 cycle hum, but a 75-80 Hz buzz.  It is louder than a 
1uv RF signal, and quite a bit louder than the receiver's internal 
noise.

I have noticed that it doesn't always start exactly when I select 
S-Meter mode, but sometimes takes a second or two to kick in.  Once I 
had it go away while I was taking measurements with the scope, and then 
it suddenly came back about 10 sec later.  It is possible that there is 
a flakey connection somewhere that I am tweaking with my probing, but I 
couldn't make the effect come and go at will by any manipulation I 
tried.

My guess is that the buzz is related to U1's operation, but I don't see 
how it is getting into the audio only when S-Meter is active.

Where to next?

73,

Charles
N7BPT

On Jan 1, 2002, at 9:23 AM, SJH wrote:

> I have completed the KX1 including the 30M option and have not yet 
> added
> the ATU module but noticed I here a 60 cycle hum in the audio when I
> switch to the Sig mode which shows the second digit blinking indicating
> this is in the Bar graph mode.   When I switch out of this by pushing 
> in
> combination the  Band and Rit buttons , it goes away.....Any
> suggestions??????  Transmitter section seems to check out at this point
> and I don't seem to have any shorts in the 30M module
>
> Steve
> KQ4UB
>
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----
Charles Vollum
http://xsw.com/boojum -- Boojum, a trailerable offshore tug yacht


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