I still suspect a phasing problem, perhaps a double error. For example, the feedback may not be crossed correctly at the input transformer, and to compensate, the output transformer could be wired out of phase. Do you have a dual-trace oscilloscope and can you look at phasing throughout the modulator stages?
Another thought is to temporarily disable one half of the push-pull circuitry, perhaps by removing tubes. If you get more output from one-half the circuit, it would verify a phasing problem. Wonder also if you have a large power resistor (200 w) of 2500-5000 ohms you can temporarly substitute across the secondary of the output transformer. Any modulation should heat it up fairly quickly and would proove the function of the modulator circuit. Bill ----- Original Message ----- From: [EMAIL PROTECTED] To: [email protected] Sent: Saturday, February 24, 2007 7:59 PM Subject: Re: [AMRadio] Low modulation My Gates BC-1T still resists my troubleshooting efforts. For the mod reactor I substituted a big power transformer's secondary which measured 45 H - no difference. I temporarily substituted a 1 uF 4000v cap for the 2 uF @ 3 kVon the low end of the modulation xfmr secondary - no difference, still very low mod yet the 833s are working reasonably hard. I then used that big transformer to bench test the mod transformer as per Don's suggestions. I applied its output, 60 Hz 2700 VAC, to the primary of the mod xfmr, then secondary, then each half of the secondary - in all cases I got what I expected on the other end (and no tripped breaker or other bad sign). Then I put it all together again and gave it one more try. I hit the go button and heard a low level, high-pitched squeal and saw a wisp of smoke rise from the back side of the mod transformer. I dropped the plates back off immediately. The squeak made me suspect that I'd accidentally swapped the feedback lines at the mod xfmr primary, and indeed I had. This led me to inspect the transformer's backside a bit more closely. There is a terminal exiting each set of windings with a jumper between. I'd noticed before that the terminals were at the extreme ends of "slots" in the outer cover. Now the terminals were no longer at the end, but about 1/3 of the way back across the slot. The windings had shifted? I bet there were not slots there originally, but something made the windings shift before and cut the slots. And the stress of my feedback error had shifted them back part of the way. In spite of its reasonable behavior in my bench tests, I think I need a mod transformer. Any thoughts or other suggestions? Steve WD8DAS ************************************** AOL now offers free email to everyone. Find out more about what's free from AOL at http://www.aol.com. ______________________________________________________________ AMRadio mailing list List Home: http://mailman.qth.net/mailman/listinfo/amradio Partner Website: http://www.amfone.net Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body. ______________________________________________________________ AMRadio mailing list List Home: http://mailman.qth.net/mailman/listinfo/amradio Partner Website: http://www.amfone.net Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.
