From: "Gene McCalmont" 
Subject: RE: [drakelist] L4-PS R12 burned - .825 ohm 2W resistor on pin 6
of cable harness
Organization: RowdyHD Inc.


"Gene McCalmont"  made an utterence to the
drakelist gang
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Good Morning Jason;
R12 is a "protection" resistor that is the last line of defense between the
L-4PS HV section and the L-4B RF deck. The resistor was meant to "blow" if
there are problems at the RF deck level to protect the power supply from
damage. Since R12 did let go, suspect problems "down stream" from the HV
section of the L-4PS rather than the power supply itself. I've seen these go
under the following conditions:

1. Overdriving the amp wherein one or both tubes pull too much current
causing the resistor to let go.
2. Arching either in the tube(s) or tank circuit caused by either a
defective 3-500z or improper tuning and loading of the amp. The arc
momentarily draws excessive power through the HV section.
3. Arching at the antenna changeover relay caused by "hot switching". That
is, a momentary "stutter" in the keying circuit causing the relay contacts
to "bounce". I did this once when mindlessly letting up pressure on a Shure
444 PTT switch while speaking into the microphone. I lost both finals in the
T-4Xc and R12.
4. One or more components in the RF deck have shorted to ground causing
excessive HV current. Frankly, from your description, I would suspect that
this is the most probable problem.

I would first unplug everything, remove the bottom cover of the RF deck and
do a visual inspection to see if something obvious jumps out. Checking
things like bypass capacitors and looking for burnt or cracked resistors are
obvious. Also check the antenna changeover relay and other components in the
tank circuit for obvious damage. You might just get lucky.

Let me know how you do and best regards;
Gene

W5DDW
Gene McCalmont
270 Oak Ridge Ln.
Argyle Texas 76226


"Jason Buchanan"  made an utterence to
the drakelist gang
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hi all,

R12 in my L4-PS burned to a crisp.  Any ideas why and what I should look for
as to why this happened?

Amp was working fine when this happened - I powered it off as soon as I
noticed the smell.


73 Jason N1SU 

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