Farmhouse AM / Short Wave Radio
This radio was particularly interesting because it was built to be completely battery operated. It requires three separate battery packs “A”, “B” and “C” configured with tapped voltages of -1.5V, -3V, high-current 1.5V (for the tube filaments), 45V and 90V. Yes, you read it right, 90V DC of battery power! Common at the time, but non-existent today.
The restoration was challenging because of the age and level of degradation the components sustained over time. All tubes needed replacing, both transformers were open-circuit, several tubes were completely shot and of course the numerous capacitors. After that was sorted out, as well as replacing rubber isolation mounts, frequency needle and glass… the question of how to power it in the long term needed to be addressed.
Fortunately, I did not need to re-invent the wheel and found a reference to a simple schematic for an isolated power supply. With some modifications to get the correct voltages, it worked like a charm. Still working after 80 years.
Here is a view of back of the cabinet, ready to go:
The final test on the bench:
All of the replaced components:
Some more detail on the power supply:
This restoration was a bit tricky. Initially, the symptoms were loud humming and distorted sound which usually means old capacitors. After replacing all capacitors, the humming was gone but the distortion issues continued. Next tubes were tested and replaced as needed. Still, after the unit warmed up only a very low volume was achievable and anything above that was completely distorted.
Finally with some encouragement to keep at it, it was traced to an open-circuit resistor on the emitter bias for the pre-amp before the final amplifier stage. Now, this unit is just like new complete with new needles and rejuvenated mechanicals on the turntable. Amazing how nice 1940’s technology can sound – rich bass!
Prior to replacing components, someone had already been at one of the capacitors:
Finally, a new set of tubes, capacitors and a new resistor as well: