----- Original Message -----
From: "Charles Ring" <w...@roadrunner.com>
To: <drakelist@zerobeat.net>
Sent: Monday, September 10, 2012 6:13 PM
Subject: [Drakelist] 2-C
The received audio in my 2-C is completely dead. This had
happened
before and then resolved itself but is back. The S-meter
still shows the
calibrator signal, and feeding tone into the sidetone jack
produces weak
audio that is not very responsive to the AF gain control
and is
modulated with 60Hz.. No difference between the AM and
either of the two
sideband positions. There is a slight hum in the speaker
and
considerable hum on the 60uF section of the triple filter
cap, which is
a new replacement one. No audio on the mode switch.
Advice?
lets look at the circuit, there are a couple of
handbooks and a schematic at BAMA if you don't already have
them.
The first step as always is to check the power
supplies. Make sure the 12V and 10V busses have the right
voltage on them and that there is little or no ripple. The
10V supply appears to be dropped from the 12V supply so
check the latter. This supply has a 1000 uf cap across it so
it should be pretty pure. The entire audio stage runs off
this supply. The hum bothers me. It could indicate the
power supply is malfunctioning or that there is an open
ground somewhere. The usual voodoo for these (and much
other equipment) is to work all the screws to make sure they
are making good contact because many grounds are returned to
the chassis through them. The 12V rectifier is a full wave
one so the ripple should be 120hz. If you are hearing 60 hz
and the supply seems OK its probably an open ground
somewhere.
The audio is solid state and has three stages, Q4, Q6
and Q7. The volume control is ahead of Q4 and should
control the input to this transistor including the side
tone. The side tone jack is connected right to the wiper
of the volume control so it will work in reverse to tell you
what is there. I would use a scope on this jack and see if
there is audio there. If so, its getting lost further on.
If its not there or is weak I would look at the cap across
the pot first. This is C-62, a 0.05uf cap. Its possible,
although unlikely that its shorted or has developed a low
resistance. I think its more likely that C-65 has problem.
This is the coupling cap to Q4.
Since you have a scope (to look at the ripple) I would
first confirm there is a signal at the side tone jack and
then trace it along the audio chain to see where its getting
lost. Since the position of the function switch (that is AM
or SSB) does not seem to affect the problem its likely after
the switch S-5.
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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