From: "Lee Bahr" 
Subject: Fw: [drakelist] L4-PS R12 burned - .825 ohm 2W resistor on pin 6
of cable harness


"Lee Bahr"  made an utterence to
the drakelist gang
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One more thing to look at when the resistor blows!  Many years ago I had a
3-500Z short and take out the resistor in my L4B.  I called Drake and they
told me to be sure and check the pi-wound RF chokes at the base of the
tubes. They said they "COLLAPSE" when a tube shorts.  I looked the chokes
over and they looked good.  I then replaced both tubes and the resistor and
fired up the amp.  POW! the resistor blew again.  I called Drake and they
told me to change the pi choke or chokes.  They sent them to me and I
replaced them and the resistor fuse again.  I then fired up the amp with the
new tubes and never had another problem.  I called Drake about this and they
told me my choke had "collapsed" internally when it drew too much current
even though it looked perfectly fine on the outside with no burned wires or
black carbon marks.  Each of the wires on the choke looked good but they
must have melted and shorted way down inside the pies.  I measured the
removed chokes from the amp with a ohmmeter and indeed one choke had less
resistance then the other one.  This was the choke in line with the bad
tube.  Apparently Drake saw a lot of pi RF chokes go bad when 3-500Zs
shorted to tell me this before I even tried to repair my amp!

One side note.  The tube that shorted never did give me a short indication
on an ohmmeter.  It apparently had a saged element and would flash to ground
only with high voltage applied to the tube.

Lee, w0vt


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