Re: K2 signal tracing progress and questions arising

Posted by Paul Fletcher on Nov 25, 2009; 10:22pm
URL: http://elecraft.85.s1.nabble.com/K2-signal-tracing-progress-and-questions-arising-tp3982841p4068078.html

Don Wilhelm-4 wrote

Paul,

Rather than trying to do your proposed test, I would suggest you look
carefully at the T/R switch area first.  Typical problems are diodes
oriented wrong (check against the Parts Placement diagram in the back of
the manual).  A broken lead (or unsoldered, poorly tinned, etc.) of RFC3
could cause similar problems  - be certain RFC3 is wound on a ferrite
core (dark gray) and not one of the black cores.

73,
Don W3FPR

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.