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I thought I'd give an update on the battery eliminator. This radio has long been fixed (just needed the usual bad caps, some resistors, and some rotten wiring replaced) and it works well. However, I just got around to repairing this particular battery eliminator.
The circuit design is such that it uses a power transformer with one 5V winding for the B+ rectifier (5Y3) and a 3.5V winding for the filament supply. The plates of the 5Y3 are tied together, making it a half wave rectifier; and, the tube gets it's plate voltage directly from the AC line. In this case, the B+ ground is "hot". Every cap was, of course, bad (a .05 uf "across the line" cap, a 20-10uf@160V electrolytic can, and a dual 1000uf,6.3V cardboard tubular cap). Also, the B+ filter resistor had risen from 2.5 Kohms to nearly 4Kohms and the bridge rectifier for the filament supply was dead. I replaced the 2.5Kohm resistor with a 2.2Kohm, 2W and I made a bridge rectifier from standard 1N4007 silicon diodes.
There is a tapped wirewound resistor in the filament line, which is connected to the three screw terminals on the front. The screw terminals are for a jumper which can be connected to bypass part, or all, of the resistor in order to obtain proper filament voltage for various tube counts. I had to bypass the resistor completely in order to achieve 1.5 VDC on an ordinary 4 tube RCA radio.
I did replace the power cord with a polarized type, with the neutral side going to ground. This power supply originally had an in-line power switch and I will place such a switch in the hot side of the line.
The only other thing that I might end up doing is replace the 5Y3 with a 1N4007 and a suitable dropping resistor. That should cut down on the amount of heat produced.
This now makes 3 battery eliminators. The other one being a similar Perma-Power unit and then there's one that I crudely built using a transformer from AES and an LM317 for the filament voltage regulator. The one I built is safer and more stable, since it uses a transformer both B+ and filament supplies and has voltage regulation.
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