[R-390] R390A audio amplifier- Update

[Eric Gauja]

Dear Group,
                     After noting the various comments made by other group 
members about the audio stage local gain control, and a detailed physical 
examination of my R390A, I have now come to the following conclusions: 
1)  Turning the local gain pot to its minimum setting still allows a low 
level of  audio through to the speaker.
                         2)  If I temporarily connect a jumper wire from the 
local gain pot wiper terminal to ground , then audio disappears, at any 
setting of the local gain pot.
                         3)  This to me indicates that at the minimum local 
gain pot setting, there is some internal resistance between the wiper pot 
terminal and the end of the pot track connected to ground.
                         4)  This could be due to the original design of the 
local gain pot, or it could be a contact resistance in the wiper  pot 
connection internally which has increased with time and use.
                               Have any group members recollection of the 
local pot minimum gain behaviour in a new R390A?
                          5) I could further investigate by opening up the 
local gain pot, but am reluctant to do this because the pot does not seem 
intermittent and apart from the minimum
                              gain anomaly works well.
                          6) There is no interaction in my receiver between 
the local and line audio channels - the gain pot setting in one does not 
affect the other.
                          7) The break-in function works perfectly, and in 
practice can be used to completely silence the receiver audio.
Thanks for the comments received from the group members.
Best Regards,
                         Eric.

-----Original Message----- 
From: Eric Gauja
Sent: Monday, May 27, 2013 9:44 AM
To: r-390 at mailman.qth.net
Subject: [R-390] R390A audio amplifier

Dear Group Members,
                                     I have a question about the R390A audio 
stage.  The AF cathode follower V601B is connected to the grid of the local 
or line AF amplifiers V602A  , V602B  through the volume control pots R105 
and R104.   The receiver can be muted by the break-in relay switch 
connecting the cathode follower cathode resistor R627 to ground.  Complete 
silence.  Operation of the break –in switch once more restores audio.  Good. 
However, if the gain volume control pot is turned to its minimum position, 
there is still audio present at a low, but noticeable level.  Why does this 
happen? At the gain control’s minimum position, is not the grid of the AF 
amp connected to ground also?

Best Wishes from Eric (Sydney, Australia)
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