[R-390] Today's Line & Local experiment

[Charles Steinmetz]

Craig wrote:

>Checking the electrolytic caps in the Amelco AF module isn't too 
>much of an issue. No extras on hand.

Just swap in C603 from the other unit to see if that is the problem.

>P120 pin #15. Goes from the grid of V602 thru S104 to terminal #1 of 
>both Line & Local pots

???  According to my schematics, the wire at P120/J620 pin 15 does 
not connect to either V602 or to the audio pots.  It connects the 
grid of V601A, through R602, C601, and S104, to the cathode circuit 
of V601B when the AUDIO RESPONSE switch (S104) is in the WIDE 
position (but not when S104 is in the NARROW position).

This provides negative feedback around V601A and B and reduces the 
gain of V601A by about 10dB, to match the audio level in the WIDE 
position to the audio level in the NARROW position (i.e., to match 
the insertion loss through the NARROW filter, FL601).  Even if it 
were not connecting, it would only raise the audio level by 10dB in 
the WIDE position (and not at all in the NARROW position) -- not 
enough to cause unbearably loud sound.  You can check it by switching 
the AUDIO RESPONSE switch to the NARROW position.  If the level 
sounds approximately the same when you do this, the NFB is connecting 
properly through S104 and P120/J620.

Or did you mean P120 pin 2?  That connects the output of the cathode 
follower, V601B, to terminal 1 of both LINE and LOCAL pots (it also 
connects through R608 to the grid of V601B for biasing).  That is 
definitely not your problem -- If it were not connecting, you 
wouldn't have any audio or DC on the LINE and LOCAL pots.

Best regards,

Charles