Evan -
I wonder if this is a symptom of the 'new and improved' resistors offered to 'replace' the IRC "BWH"
resistor originally used. Drake originally specified to use _ONLY_ type BWH resistors for
replacement.
IRC sells the "SPH" series as a replacement, and it is a wirewound resistor like the "BWH", and
_should_ work as well.
Harbach offers "RES400" resistors as replacements, which also seem to be
wirewound.
Like you, my experience, at least with the original resistors, is
'vaporization'! :-)
I'm sure you know Collins used a 6" long cartridge fuse designed for HV in the KWS-1. They're about
$10 each when you can find them. Obviously a regular 3AG type glass fuse is useless in this
application.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
K9sqg wrote:
Fellow enthusiasts,
Had a question about the 0.82 protection resistor in an L7 power supply, which is identical to the
supply used for the L4B and almost identical to the one used with the L4. I've been using this
configuration for almost 10 years without any problems. I've had experiences with these resistors
in my amplifiers as well as those I've serviced for others. Typically, a high voltage short will
pop that resistor very fast and protect the diode strings and transformer, if the short is
external to the power supply (i.e. in the cable or RF deck). Yesterday I had a failure of that
resistor with a failure mode that is unlike anything I've ever experienced before and wonder if
anybody else has seen this phenomenon as well.
When tuning up my L4B that used the L7PS with a brief over current situation, followed by a brief
arc in the RF deck followed by a sizzling sound in the power supply caused by the protection
resistor entering a failure mode. To make a long story short, that resistor did not fail
instantaneously. Instead, the body of the resistor had maybe 25% of it blown away and the carbon
"element" down the center was fractured and not making continuous contact. As long as the amp was
in standby, there was no problem. When going into transmit, the resistor would arc and light up
the inside of the case but would not fail in a typical, rapid manner. Fortunately I shut things
down and eliminated further damage. I've repaired supplies where the paper insulation on the
transformer was burned or carbonized in some cases indicating there might have been a small fire;
details from the owners were sketchy since the supplies were always out of sight and direct
observation was not possible during the failures. It is possible that the protection resistor was
placed too close to the transformer; in these supplies it was obvious that somebody had replaced
the resistors previously and the leads seemed to indicate close placement to the transformer winding.
In any case, I wonder how many of you might have experienced a similar situation. I hope this is
an isolated situation and it serves to remind us that when there is a problem like this, shutting
things down quickly is the best course of action.
Enjoy those Drakes.
73,
Evan, K9SQG
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