A typical American picnic radio from the late 30ies, a Fada P22 Portable.

The radio is build for operating on 117VAC or VDC or from batteries. For rectification a 35Z5 is used, which is normally a rectifier tube for tube sets which are series heated. This was a typical approach for transformerless radios, to use the 117V line voltage to heat a couple of tubes which are series connected. But being also a battery radio the P22 has no other tubes for 150mA heater current, so the 35Z5 stands alone against the line voltage. To solve this problem the radio is using a "hot tail", a resistance line cord of 560 Ohms.

In generally this was not satisfying, so roughly at the same time the P22 has been build a new rectifier tube with 117V heaters was introduced, the 117Z6. But my Fada P22 is still using the 35V tube. 

From the Riders' files I got the schematic for the radio: P22 schematic (.gif 80k).

First thing to make the radio live again was to think about a replacement for the defective resistance line cord. A quick calculation to drop down 117V to 35V at 150mA current results in 550 ohms consuming a power of 12.3 watts. Looks too much to me, so I decided to use a combination of a diode and a resistor. The new calculation said: 318 ohms with a power of 7.2 watts. Much better, so I did: took a small alu housing, and a quick grip in my resistor junk box. Ready: the "hot tail" replacement. Small enough to fit in the Radio cabinet, and not really going hot ;-)

Going forward with restoring: as usual I had to replace all capacitors. Doing that I checked the resistors, found a few of them out of tolerance, which forced me to change also all resistors. Fine, the Fada started working. It was a bit sensible regarding the line voltage provided by the variac on my workbench: on 120V it was doing well, but below 115V it was hard to tune in a station. Ok, the battery tubes are very critical regarding the heater voltage. In an earlier time somebody had bypassed the resistor which is dropping-down the plate voltage to 1.4V for heating the battery tubes with a second resistor. First I was not aware why he did that and I removed this resistor, but now I understood. I took a new 10k 6 watts resistor in parallel to the original 2k 10 watts resistor, resulting in a combined value of 1.7k. Pretty better now, and in a 2nd step I changed the 100µF caps of the heater supply to 220µF caps, making the heater voltage more stable.

This is the part of the schematic with my modifications: P22 modifications

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Last update: 13/September/2003