Dirty switch contacts for the 40 m position?
-----Original Message----- From: Jim Shorney <[EMAIL PROTECTED]> To: drakelist@zerobeat.net <drakelist@zerobeat.net> Sent: Sun, 6 Jul 2008 10:58 pm Subject: [drakelist] Taming the shrew "Jim Shorney" <[EMAIL PROTECTED]> made an utterance to the drakelist gang ---------------------------------------------------------------------- Well, another problem cropped up today. Earlier this evening I was helping a local ham air check a new antenna on HF. Ironically, he seemed to be suffering from RF feedback on 75 meters, so we went to 40. As soon as I keyed the mic and started talking, the TR7 took off with close to 100 watts of SOMETHING that looked like a carrier. Subsequent tests on 20 and 10 revealed that his antenna and my radio were both working fine on those bands ( he said my audio was EXCELLENT). Good thing the radio was still sitting in the lab, or I would have really been annoyed. Testing into the dummy load revealed that under full power operating conditions, the TR7 was oscillating at about 9.6 MHz, in ONLY the 7.0-10.0 MHz bandswitch position. All other bands were rock solid, anywhere I checked. I looked at all the obvious things, HP filter, LP filter, grounding, PIN diodes, etc.; everything seemed to be in order. (BTW, an MFJ-259 antenna analyzer will drive the TR7 to 200+ watts output when injected through the HP filter board). The good news is that the new PA transistors survived this abuse with flying colors, and the heatsink was never more than warm to the touch. I finally backed the predriver gain pot down a little, and that seemed to kill the oscillation. I didn't go back and reset power levels, because it was getting late and I wanted to knock off and ponder this a little bit more. Any thoughts from the gurus on how to tame this beast? Even with power reduced, it makes me a little nervous that the transmitter stip could be that close to the edge on 40 meters. 73 -Jim NU0C ---------------------------------------------------------------------- Submissions: drakelist@zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
