[R-390] Trouble shooting the R390A

[Tim Shoppa]

> the 1/8A fuse comes "after" the caps so I don't know what was causing the
> current draw.  Maybe the fuse was old?  Don't know.

Was the wire in the fuse just broken or was there vaporized metal
deposited inside the glass?

If just broken, it could've been an "old" fuse.  I put "old" in quotes
because if you look at fuse specs, they are allowed to blow occasionally
while carrying rated current.

If there was vaporized metal deposited inside the glass, I think the
most likely cause would be an arc/short inside a tube that cleared
itself (mostly...) after the event.  Sometimes you can find a charred
2.2K plate or cathode resistor too.  Note that B+ divided by 2.2K is
nearly 1/8A itself!

Every R-390A I ever worked on had a charred/swollen 2.2K resistor or
two.

Tim.