[R-390] FW: Thursday's Line & Local report

[Charles Steinmetz]

Craig wrote:

>Now the voltages on terminal #1 of the Line & Local pots agree with the
>normal Motorola. The audio hasn't changed.

OK, so everything before the pots is OK.  The problem must be from 
the wipers (terminal 2) of the pots forward.

Just as a sanity check, measure the audio on terminal 2 of the pots 
and verify that it goes from 0 (just a few mV) to the same as 
terminal 1 as the pots are rotated from full CCW to full CW.  (For 
example, if the carbon tracks were broken internally at terminal 3, 
the radio would be near full volume all the time.  It's very unlikely 
this is the problem, but it needs to be ruled out.)

If that is OK (essentially no audio on terminal 2 of the pots when 
they are fully CCW), the question is how audio is getting to the 
amplifiers (V602A, 602B, 603, and 604) to be amplified when the pots 
are all the way CCW.  A second question is whether the amplifiers are 
working properly.  For example, if the NFB resistors R612 and R626 
were missing or open circuit, the amplifiers would have too much 
gain.  Still, if there were essentially no audio on terminal 2 of the 
pots (and, therefore, no audio on the grids of V602A and 602B (pins 2 
and 7) with the pots fully CCW), even if the amps had too much gain 
there still should not be very much audio at the plates of V602A and B.

So, the task is to find out (i) how audio is getting into V602A, 
602B, 603, and 604, and (ii) whether V602A, 602B, 603, and 604 are 
operating properly.

If the wipers of the pots have essentially no audio on them, the 
grids of V602A and 602B should have essentially no audio on 
them.  The only other obvious way for audio to get from V601A or B 
into V602A, 602B, 603, and 604 is through R606, to the grids and 
screens of the 6AK6s (V603 and 604) -- but this would only occur if 
C603B were not bypassing the audio to ground. You say you have 
checked C603B, and that it is a new cap.  How did you check it?  Did 
you measure it in-circuit (at the junction of C603B and 
R606)?  Again, as a sanity check it is probably worth looking at that 
junction with a scope to see if there is any audio on it.  (There 
will be some 120 Hz hum, so a voltmeter reading could mislead 
you.)  Is it possible when the cap was rebuilt the + or - end was 
connected to the wrong terminal of the octal plug?

If all that checks out, is it possible someone rewired the audio deck 
for some unknown reason, and added another path for audio to get into 
V602A, 602B, 603, and 604?

Best regards,

Charles