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.
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