Hi Al -
There IS a certain amount of hiss in this type of stage. The
transistor and tube stage are both running at full gain, and the audio
level is controlled by how much audio is fed to the base of Q5. So just
"shot" or "thermal" noise is present. The S/N ratio is good at normal
listening volume, (typically 9 - 10 O'clock on the AF GAIN control,) but
degrades at low volume since the noise stays constant while the signal
decreases to zero. 0/N isn't a very good "ratio"! :-) This is a
common complaint when phones are used at greatly reduced AF GAIN
settings because of impedance mismatch.
It's also possible that C212 or C213 are leaky. There is a fairly high
voltage on C213. Less likely but possible is C136.
You could try lifting one end of C213, which will increase the HF
response, but may be able to tell if that's helping. If not, lift one
end of C212 to determine if the noise is coming from the transistor or
tube stage.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
ALAN PHILLIPS wrote:
Greetings and happy hollidays to all.
I have a late model TR4 serial 32000 series. The audio board for the
6AQ5 has many components on it plus the 2n3877 transistor. This is the
version before the TR4C which has far less components on the board.
Now the problem.
With the volume all the way down there is a frying noise coming from
the speaker. The 6AQ5 has been replaced. the 2n3877 has been replaced.
I pararrelled new caps on the 1mfd and 10 mfd with no change.
I grounded the grid of the 6AQ5 and the speaker, of course went silent
with that test. The RF gain turned minimum has no effect. Checked for
loose grounds, cold solder joints, dirty tube sockets, some light
tapping and still no change. All power supply caps have been replaced.
both in the rig and the aC4r
Any ideas would be appreciated, or if you have had this problem
before. With the volume at normal levels you don't hear the frying
noise. but sometimes I have to work and just turn the AF gain to
minimum and thats when you hear it,\\
73'S AL KC9YS
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