I have now been able to start with the 'noise injection' test:
I first connected the wire to TP21 and could hear noise increase
(VOL set to high). This was not much of a surprise because I
had already established that the LM386 is working.
Then I connected the wire to TP20 (output of the de-modulator?)
and found that I could not hear any difference in noise level.
If I read the uBitx v5 schematic correctly, this could mean that
the Q70 transistor (type 2N3904) or the relay K3 might be damaged.
I assume Q70 acts as an audio pre-amplifier(?).
I continued by measuring continuity between C51 (M1) and TP21 (M2)
with the uBitx turned off: continuity exists (expected)
with the uBitx turned on: continuity does not exist (?) (not expected)
I expected that continuity continues to exist when the uBitx is
with the uBitx turned on *or* off:
continuity exists between TP8 (TX path) and ANT (antenna center conductor)
and TP1/TP2 and ANT
This is strange - based on the schematic it should be:
Either there is continuity between TP1/TP2 and ANT *or* TP8 and ANT - not
Maybe the relay K3 is damaged (or somehow 'stuck').
I was almost ready to send this email when I tried one more thing ...
I noticed in the past that a relay in the uBitx rapidly switches back and forth
when I startup the radio *and* have the MIC cable (coming from the
soundcard) already plugged in.
So I figured, if the relay is 'stuck' - maybe I can 'free' it by plugging
the MIC cable in again and force the rapid back and forth switching.
Bingo - it worked, RX is working again.
Next up: fixing TX ... (don't have time for this today as this will require some surgery ...)
I am very thankful for everybodies help in this endeavor,