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T-368 – HV On RF – How To Correct

The Problem

Much has been written about the design that puts HV on various components on the T-3s RF deck. I too have often wondered why Barker and Williamson chose this design, it seems so temperamental. Failures of the Plate Current meter, band switch, and numerous arcing problems have been attributed to this design. Ott Kazott offers an excellent explanation of why B&W chose this design, as well as a suggested solution. Tim WA1HLR also has a suggestion for this, listed under the WA1HLR Mod.

How and Why

The broadband PI-L output circuit in this rig has a great deal of frequency coverage and adjustment for SWR. A rig with all this going on when fed at the hi-point at these power levels requires a substantial plate choke for RF containment and broadbanding compounds the problem. The choke has to be able to handle that level of power and frequency coverage and not have a serious series resonance problem at the same time.

Anyway, substantial chokes also have mass and ballistics… a bad combination on the back of a radio truck. The R-175 (about 6″ tall) I used in place of what was in there (about 2 3/4″ tall) would break off in the first second the radio was mobile again.

So by feeding it at the low impedance point, a short, stocky, mechanically/electrically strong, with low ballistic profile, all frequency solenoid choke can be used… and that’s why it was done. Very Rugged.

But, at that, it wasn’t all that good an idea anyway. Later T-3 MWO’s have what looks like a small Johnson pi-wound choke hung on in series with the solenoid choke…. and a lot of by-pass capacity as well. They must have been having a hell of a time keeping RF out of the power supply.

The other disturbing thing about that setup was that there was HV all over the tank, band switch and associated wiring. With that going on you would have to keep things very clean.

And feeding at the low point also required hefty capacitors with a lot of capacity for the DC blocking function. Had it been fed at the conventional point they would not have been necessary.

The original blocking caps carried full RF tank current and caught fire. At one point mine had 3 in there. Mine had the 3 caps removed during an MWO and a very large cap placed in there instead. The one in there now is rated for 10 amperes current flow. It was left there during my conversion as it wasn’t bothering anything and is a pain to remove.

The B&W solenoid and the Johnson look alike chokes were given to WA2NPL, HV leads were redressed and the old existing bathtub bypass was used as the “new” DC plate blocking cap. Recycling of components is practiced here at OTT HQ .

The other point of apparent ballistic concern was the plate parasitic choke. Unless yours was changed by a civilian, the plate parasitic choke is mounted on the tank coil, not on the tube as shown in the book.

Another Issue

One other problem out there is that the tank coil form is often broken on the end nearest the ant changeover switch. There are two fasteners there are supposed to be somewhat loose. Often they have been “re-torqued” and if you “flex” the RF deck while moving it the form breaks.

I don’t have access to an “F” unit but I heard that in those models, they use Nylon screws with interference fit nuts so they can be set perfectly without danger of over captivating the form. (I found this to be true in both my “E” and “F” models. -KG2IC)