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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