Rebuilding the Collins 516F-2 Power Supply

The Collins 516F-2 is a heavy-duty power supply for the KWM-2/2A transceivers and the 32S-1,2,3 series of Collins transmitters. Over the years, power cords become frayed, the electrolytic capacitors dry out, and the potted bias rectifier modules give up the ghost. These problems can be easily solved by replacing the bad components with modern equivalents.

More serious problems are inherent in the power supply's design itself.The heavy startup surge current can damage the on/off switch in the controlling rig, and the output voltages are too high for today's higher line voltages, causing radios to run hot. Furthermore, one cannot simply substitute solid state rectifiers for the 5U4 and 5R4 rectifier tubes, because the resulting voltages will be higher still, typically in excess of 1000V for the HV, and well over 350V for the LV. The nominal B+ voltage requirements of the Collins radios are 800V and 275V.

Here is the original "junker" 516F-2 that was the starting point for this project. It had a bad power cord and interconnecting cable, a poorly installed replacement tube socket, chipped connectors and innards that were, well, best not talked about. I decided there wasn't much point in trying to turn this ugly duckling back into the Cedar Rapids swan it once had been.

The new replacement transformer and filter chokes were designed and sold by Bill Abate K3PGB and manufactured by Heyboer Transformer Co of Grand Haven, MI.

A couple of years ago I acquired an excellent, newly designed replacement power transformer and pair of filter chokes for the 516F-2. These beauties were sold by Bill Abate K3PGB, as a service to the Collins collectors' community. This new iron has significantly improved specifications: the transformer is designed to produce the proper voltages with solid-state rectifiers and has a tapped primary to accomodate line voltage variations. The new transformer runs cooler and the chokes have higher current ratings than the failure-prone originals. At last, the project worked its way to the top of my "to-do" list and I took the new parts off the shelf.

I have several 516F-2 power supplies and considered simply swapping the new iron for the old in one of them as part of a restoration. Eventually, however, I decided to "rebuild" rather than "restore." I needed a supply that I would use, that had all the features I wanted, but that would be hidden out of sight behind the desk or in its cabinet. I didn't care if it resembled the original.

As it turns out, one of my 516F-2s was a near-junker. It had a sloppily enlarged tube socket hole, broken connectors, worn-out power cord and connecting cable, and a mish-mash of capacitors and resistors hanging from the bottom. There was no way this baby could be made presentable. It was the perfect candidate for a rebuild. I started by stripping it down to the bare chassis.

A trip to the kitchen and two dishwasher cycles later, the grunge on the chassis came off nicely. (Funny how the meat loaf the next day tasted like burned resistor.) Shorn of all its components, however, the chassis' gouged tube socket cutout really looked awful.

A new rectangular opening with a cover plate replaces the original gouged tube socket cutout.

Out came the nibbler and a file, and in a half-hour the cutout looked a whole lot better. A small aluminum cover plate provided the finishing touch. (Okay, I know, no one could see the original ugly hole with the cover plate in place. But I'd know it was there and I'd never be happy with the supply if I didn't fix it.)

There's nothing quite as classy as the silver-ceramic terminal strips from old Tektronix oscilloscopes, and I used several of them to mount the two diode rectifier banks. Twelve 1N4007 diodes (in four strings of three) replace the 5U4 and 5R4 rectifier tubes. The diodes lay between the Tektronix strips, and the whole assembly sits on a small piece of glass-epoxy circuit board.

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Here is the completed supply, with its shiny new iron, control relay and mostly new parts. About the only components remaining from the original supply are the small 1H filter choke, the bleeder resistors and cage, and the bias adjustment pot. After the photo was taken, I added a bottom plate to keep fingers out of the innards.