[R-390] AF Deck recap

[2002tii]

Dave wrote:

>Over the weekend I have been working on bringing one of my R-390a's back
>to life. Had not used it in a long time, when I did it blew the fuse. So
>turns out that the filter caps were bad, so I decieded to recap the
>whole deck. After I was done, placed it back in the radio and played for
>an hour or so, then the fuse poped, turned out that C609 which I just
>replaced is bad, I put the original back in and again it plays just
>fine. When I re-capped I used a 10uF 35V tantalum from mouser part
>number 74-173D106X9035XE3. *Looks like I used the correct part. So what
>gives? Just a bad new part? or am I over looking something???*

I do not think you have found what caused the fuse to blow.  C609 is 
just the bypass cap on the cathode of the first audio amp (V601A, 
half of a 5814A a/k/a 12AT7A).  Any current that flows through C609 
has to flow also through V601A and its 56k plate resistor, 
R605.  Even if the tube and the cap were *both* shorted (0 ohms), 
they could only draw around 5 mA from the B+ line through R605 (which 
is only about double the stage's normal operating current) -- nowhere 
near enough to blow the fuse.

Best regards,

Don


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