I've been trying to get my old 250TH rig back on the air. for the most part, I've been rather successful. but, I've run into a kink... I can't seem to neutralize the final.
The exciter is a Johnson Viking II - plenty of drive, and at the same time, variable. This is good. I'm not gonna have my fingers in there, twisting on the big disks of the neut. caps while there's 100w of RF flowing through there :-) but, things just aren't as they should be - or so it seems. I was talking to John/WA5BXO about this, and he suggested putting it here in the reflector. Here goes. ================================================== well, I'm more frustrated than I was before... removed the B+ lead to the final, got my meter out, connected a clip lead from the center conductor of the coax connector of the link output to a diode, then to the meter probe, and a clip lead from the shield of the coax to the other meter probe. We're ready to measure output voltage! And, measuring voltage wasn't a problem. the problem was, that every time I thought I was getting the output down to (as low as 50!) millivolts, I'd run the main tuning condenser through resonance again, and the voltage would shoot back to nearly where it was, when I started. I thought "well, maybe the grid current through the relay is causing problems." So, took another clip lead from the grid bias voltage input on the back of the final, and ran it to ground. ==========(insertion)=================== (John came up with the 'relay' idea. quite simple. Instead of a bias supply, attach a relay from the bias point to ground. The relay should require about the same amount of current to close, as do the tubes. In the case of 250TH's, around 90mA for 2) ========(continuing)==================== Fired up the exciter again, (grid meter don't work, unless it's got a ground return) and ran the excitation level back down low, and started again. Same thing - I'd get to a point where it looked low, then I'd run the final capacitor through resonance again, and the output would go back to nearly where I started from. So, I chased that down, for another hour. I never got the output voltage from the link lower than a volt or two DC (as rectified by the diode) and as a result, while I'm not seeing a dip in grid current anymore (with no B+ applied) I am seeing a difference of about 100mA's between dip and resonance (max power out) What the heck is going on? -=-=-=-=-=-=-=-=- that was the first time I tried. I've since been told, that there needs to be a load, on the output of the link, so I plugged it into my 50ohm dummy load and measured the output there, at the dummy load. but, I got the same results. Here are the facts: The final uses Cross Neutralization. The final "C" section is a B&W Butterfly tuning capacitor Neutralizing caps are on the end of it. Thoughts? Regards, -Jeff
