[R-390] Audio HUM WAS: B+ short in RF deck

[Charles Steinmetz]

>Can see with the scope very large 120 cycle ripple on RF/IF B+ line at the
>B+ fuse and plate of  V201 as well as the plate of the 1st audio amp.

So, there's your problem.  (But, what is "very large"??  How many 
volts peak to peak?)

>V601 and 602 were both swapped out for new tubes with no change.

Do you really mean the audio tubes, V601 and 602, or the rectifiers, 
V801 and 802??  (The accumulated observations do not implicate 
V601/602, so there would be no reason to pull them.)

>Checked L601, 602 and 603 for DC resistance with a VTVM, they are within
>spec.  Can't check the inductance.
>
>Caps C603 and 606 were previously replaced by axial caps installed on octal
>headers and plugged into the original sockets.  33uf/350 and 45uf/350 caps
>were used.  They were checked with the VTVM and do not appear shorted or
>open. Can't test for leakage at rated voltage.

If there is large ripple at the B+ fuse, it means either (1) 
something is wrong with V801, V802, L601, L602, C606A, or C606B, OR 
(2) something is drawing massively too much RF/IF B+ current, OR (3) 
something is drawing massively too much AF B+ current.

Any RF/IF B+ current drain sufficiently large to cause excessive 
ripple should blow the 1/8A RF/IF B+ fuse, and would also drag the B+ 
voltage down.  So, (2) seems unlikely.

Similarly, any sufficiently large AF B+ current drain would also drag 
the B+ voltage down.  Does your AF B+ still measure about right?  If 
excessive current were being drawn by V601A or B, it would fry R606, 
so V601 is not the problem.  Excessive current could conceivably be 
going through V603 or V604, or there could be leakage from the 
primary of T601 or T602 to ground.  So, if the AF B+ voltage is low 
(or just to be sure), pull the AF B+ feed from Terminal 2 of L603 and 
see where you stand.

If L601 and 602 are within spec for resistance, they are almost 
certainly not the problem.  L603 is not implicated by the symptoms.

SO:  By elimination, you most likely have a problem with excessive 
current drain on the AF B+ supply, OR a problem with V801, V802, 
C606A, or C606B.

Just in case there is excessive current being drawn somewhere, I'd 
limit how long you leave it on to 2 or 3 minutes at a time until you 
know there isn't.

Best regards,

Charles