KX1 Receiver issue

[Len Chesler-2]

I have had my KX1 for close to ten years now, and it has been a great traveling companion with lots of fun QSO’s.  Two nights ago, I got the rig all hooked up and discovered that no audio was emanating from the headphones. The AF gain control, filter pot, and RF gain control made no difference.  I hooked up the paddle and found that the TX is working fine, and I could hear the side tone through the headphones.

To see if a loud signal had any effect on the KX1 receiver, I put a dummy load on my K3 and sent some test tones, none of which were being received at close proximity.  This problem occurs across all 3 bands.  The antenna tuner is working, and my SWR meter show a perfect match.

Any suggestions as to where I should start troubleshooting?  Tnx.  Len, K6LEN
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[Don Wilhelm-4]

Len,

If you can hear sidetone, the audio amplifier is working, that means you
can start with the Product detector and/or the muting transistors.
Check the voltage on the gate of Q2 and Q3 - if it is zero (or very
low), then check U1 pin 6  (the /MUTE signal) - If that pin is also low,
you may have a bad output from the firmware IC - best check is to
substitute another firmware IC.

If the /MUTE line is OK, then try tapping on pin 1 of U5 - do you hear
any clicks in the headphone?  If so, the product detector is working.  
If you have something to measure RF voltage, measure the RF voltage at
U5 pin 6 - you should have at least 200 mV peak to peak or 40 mV rms (as
read by an RF Probe).

The next place to check is U6 pin 4 - the output of the Rcv Mixer. If
you have nothing there, the only components between there and the
product detector input is Q8 and the Xtal IF filter.

All OK so far, tap on U6 pin 1 (input to the RX Mixer).  If you have
noise there, all but the front end if the RX is OK.  You can check the
RF voltage level at pin 6 - like U5 pin 6, you should have at least 40
mV rms or 200 mV peak to peak.

Still all well to that point - there is a possibility that Q7 is either
shorted or conducting during receive.  Check the DC voltage on the base
- it should be zero volts.
If you suspect Q7, it can be removed for receive (but not transmit).  If
taking it out of the circuit restores receive, replace it.

Good luck,
73,
Don W3FPR

On 7/17/2014 3:55 PM, Len wrote:
> I have had my KX1 for close to ten years now, and it has been a great traveling companion with lots of fun QSO’s.  Two nights ago, I got the rig all hooked up and discovered that no audio was emanating from the headphones. The AF gain control, filter pot, and RF gain control made no difference.  I hooked up the paddle and found that the TX is working fine, and I could hear the side tone through the headphones.
>
> To see if a loud signal had any effect on the KX1 receiver, I put a dummy load on my K3 and sent some test tones, none of which were being received at close proximity.  This problem occurs across all 3 bands.  The antenna tuner is working, and my SWR meter show a perfect match.
>
> Any suggestions as to where I should start troubleshooting?  Tnx.  Len, K6LEN
>

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