[Elecraft] [K2] K2 signal tracing progress and questions arising

[Don Wilhelm]

Paul,

Do you have a sufficient signal level to give you 1.6 volts out of the 
AF amplifier?
The impedance of the amplifier's input is considerably higher than the 
impedance connected to the output, so the AC voltages may actually be 
similar in and out.  Can you peg the S-meter with your signal generator?

Most problems with "low audio" are actually caused earlier in the 
receive chain.  Check to see if you have at least 200 mV peak to peak 
(70 mV RMS) of BFO at RF Board U11 pin 6.  The diodes (D40 and D41) 
across the input of the IF amplifier will clip at a level lower than the 
"Saturated" test, so that item in the Signal Tracing section is no 
longer possible to do.

What is the input vs. output from the IF amplifier U12.  If there is not 
a significant increase, you may have a bad IF amplifier or T7 may be 
mounted backwards (red winding goes closest to U12), or the AGC voltage 
to the IF amplifier may be too high (check U12 pin 5, it should be no 
higher than 3.9 volts).

73,
Don W3FPR

Paul Fletcher wrote:
>   
>
> OK - have done the checks and re-soldered a few connections and I'm now
> happy that the attenuation in the pre-mixer stages is OK. I've progressed
> through the receive chain and the only obvious thing is the audio amp that
> is causing problems. The signal out of the amp (with or without the speaker
> connected) is not significantly higher than the signal in (in fact it's
> actually attenuated if I compare the signal across PD1 and PD2 and the
> output). I've replaced the LM380 and both mute FET's to no avail. There are
> no shorts on the output and the sidetone is healthy (same output as input
> measured on the bypass pin) and I'm completely flummoxed. I've checked out
> the AF gain control and it varies from 5k (max volume) to a few ohms (min
> volume) and I've confirmed this measurement right up to the chip.
>
> According to the signal tracing fault finding I should be able to get 1.6V
> RMS out of the audio amp but no chance - nowhere near.
>
> Help!
>
> Cheers,
> Paul.
>   
>