[Amps] Weird Problem (what problem isn't?)

[Kimberly Elmore]

I have an early AL-80A (S/N 095) with a power supply that I modified for another 1000 v of B+. I did that over ten years ago and all was well. However, I've moved a couple of times since then and changed rigs from a TS-930 S to an Orion II. In the process, I noticesd thatthe plate choke coil had de-bonded from the ceramic form at the top. That and the peirodic cleanings took their toll and, as I was putting new tuning indicators on the dial fouds that it arced across a couple od the de-bonded turns near the top while setting the marks for 12 m. Separated them as best I could, used some Q-dope on 'em, decided not to use it on 12 m (10 m is fine) and ordered a new one plate choke. I decided I'd put it in the next time I opened it up.

That ahppened dueing the ARRL DX contest. I almost never use the amp on 15 or 10 m, but really wanted something or other on 15 m. Since I wasn't going to submit a log and the contest was nearing its end, I fired up the amp on 15 m and began to bang away. I then heard and saw an RF arc from someplace in the RF deck. A brief inspection showed it to be from a toriod inductor I'd added to the 160 m tank coil. This was done because the plate impedance changed when I increased the B+ and I wanted to maintain the overal loaded Q and tunig characteristics. 

the inductor is 28 turns of 18 ga magnet wire wrapped on T-200 powdered iron core, similar to the original one used on 80 and 160 m. The arc had occrred between the magnet wore and core at the ends of the winding. This hadn't happened before, but the amp has been moved aroud a faor bit since then and is selom used above 20 m. It had been epoxied to a 0.125" peice of acrylic plastic right beneath the 160 m end of the tank coil. Obviously, on 15 m, all other inductors are shorted out of the circuit but there is still a very stronng RF field in the RF deck.

I orederd up some 18 ga teflon wire for a new winding and used RTV to glue it to the bottom of the chassis in the same place. I fired it up again this morning and this time, RF poked through the teflon and RTV to the chassis from somewhere in the middle half of the coil (no B+ is on the tank components). I carefully inspected the bandswitch and everything is clean as a whsitle: no sign of arcing or overheating anywhere.

I pulled outthe coil, and thisn time I placed about four layers of electrical tape around the core, rewound the inductor and am ready to put it back in. This has, however, made the toriod "thicker" and it sits closer to the 160 m tank coil, so I can't comfortably place it back on the 0.125" acrylic. I have some scotch 23 high-voltage splicing tape I'll try setting it on.

When I did this, I naively thought that since it's a toriod it would be self-shielding, but it sure "sees" 15 m. I'm gussing that I'm dealing with some kind fo self-resonance. I may have moved it enough by winding it on the tape-wrapped coil, but maybe not. Does anyone have any insights into what I'm experienceing and what I might do differently to prevent it (short of undoing the PS mod).


Kim, N5OP