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Philco 40-100 - Printable Version

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Philco 40-100 - bridkarl - 07-12-2024

I picked this up today. A 1940 battery operated farm radio. Aside from the schematic I haven’t been able to find very much about it.  It was only $20 or so plus shipping.

RE: Philco 40-100 - DaleHCook - 07-13-2024

Do you have the three pages with 40-100 service information from the Philco 1940 RMS Year Book)?

RE: Philco 40-100 - bridkarl - 07-13-2024

I only have page 43 with the schematic and parts list which I found in the library here.

RE: Philco 40-100 - DaleHCook - 07-13-2024

You should also have p. 9 (instructions for setting push buttons) and p. 40 (alignment).

To find all published information on this site pertaining to the service of any Philco radio model, use the Philco Service Index.

RE: Philco 40-100 - MrFixr55 - 07-13-2024

Hi Bridkarl,

Though having lived in Suburban Long Island NY my entire life, farm sets always interested me, as these radios were often a "lifeline" between the rural farmer and civilization. I have several farm sets made between 1936 and 1940, some using the 2V tubes that were first introduced in 1932, others using the octal and miniature 1.4V tubes. Farm sets using the 2V tubes required a 2V storage battery that was brought to town to charge, plus the disposable B batteries. Other farm sets were designed similar to automobile radios to run off a 6V storage battery. A vibrator type B power supply provided B+ voltage similar to car radios. The farmer either swapped the battery between the home and car (or other farm implement, had a windmill "Win-Charger", a 6V power plant or took the battery to town to have it charged. Still other farms had a 32V power plant consisting of a generator that ran by day and a bank of batteries to run the home at night. Radios were designed and built to run on 32VDC also, using low-B+ "space charge" tubes.

For sets that ran on batteries, the farm family used them sparingly due to the expense of running the radio.

Farm radios like yours and portable radios using the Sylvania-designed 1.4V octal tubes became quite popular starting in 1938. Usually, the circuits were rather similar between these "farm" sets and the battery-operated (only, no line power option) portables of the time. Power usually came from a 1.5V (A), 90V (B) battery pack, or 1.5V A battery and 2 45V B batteries in series, depending on make and model. I would think that yours ran from a battery pack.
I have several sets using these tubes including an RCA 14BT2 farm set, and BF Goodrich - Mantola 419, Bulova 220 and "Trav-ler" (model unknown) Battery operated portables. These last 2 use the newer 7 pin miniature tubes. I also have several battery-electric portables including a 1947 Zenith 8G005 Trans-Oceanic and the AM only version 6G001, and the radically newer and lighter 1948 4G800 "Zenette".

I don't play mine for long periods of time, so I use an alkaline D cell for the filaments (A battery) and either 10 9V batteries or my ARBE III for the 90V B supply. I did get a Silvertone "Power Shifter" battery eliminator that I will install in the RCA. These were rather popular, because as rural areas received electric power, the farm owner did not really want to discard the battery set that they paid good money for, and certainly wanted to stop buying batteries.

Unlike the earlier farm sets using 2V tubes which may have used push-pull output, these sets don't require a C battery. Grid bias for the audio section was derived by putting a resistor between the negative terminal of the B battery and B- (chassis ground).

Unless attacked by mice or other vermin, and unless wired with rubber insulated wire, these sets are rather easy to fix, as almost no heat was generated in these sets. All tubes are easily available on eBuy. The non-electrolytic caps and resistors are likely good, but caps should be replaced anyway. It seems strange that electrolytic filter caps exist in sets that are battery powered, but replace them. The radio is unstable without them.

Unfortunately, the schematic does not identify what color power leads go to what. However, as the schematic shows:
A- goes directly to chassis ground.
B- goes to the junction between R17 and R18
B+ goes through the switch to the junction of C22, the output transformer (20) and B+ distribution.
A+ goes through the switch to the filaments of the 4 tubes.

The only downside of these sets other than the need to power them, is the fact that the output power is about 10-20% of the output power of the common AA5 set and fidelity suffers. However, sensitivity is similar to AC-DC AA5 sets.

RE: Philco 40-100 - bridkarl - 07-13-2024

Thanks. This is all really helpful. It’s only the 2nd farm radio I’ve found - I think the other is some 1930s Firestone Air Chief or western auto model. I’ll have to dig it out of storage - I worked on it for a while but I had an issue finding a tube.

RE: Philco 40-100 - Radioroslyn - 07-13-2024

Have had a few and serviced a few this is what I've found. Battery cable and ant/gnd wire the rubber will be flaking off badly. There's a small value e cap across the 90 line. It's there to keep the b+ clean. I've found some that are open. Best off to replace as if it's leaky it will drain your b batteries. 

Set is silent. Chances are the opt pri is open. This is problematic as Philco decided to use an output tube that has a pretty high plate load resistance 25K I think. At 20 or 25K they are difficult to find a small affordable matching transformer. There are other tubes w/the same pin out and a lower load and will make more audio. At 10K or lower you have a lot more choices. I think the 1A5 makes abt 100mw at 25K and the 1C5 makes 260mw at 8k.

If the volume control is noisy try to save it the best you can. They are expensive to replace because of the dual switch on the back.

Even though the output power seems rather punny with 1A5 and a quiet room it's not bad. With 1C5 twice as good but it does run the batteries down a little quicker.