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Philco 16 voltage divider resistor questions - Printable Version

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Philco 16 voltage divider resistor questions - 37silverstreak - 12-08-2014

Hey guys,
Well the work continues on my 16 chassis. The thing was in tougher shape than I thought and a mess!! The 10uf electrolytic cap that ties to ground shorted out at some time and overheated the power transformer which caused wax to leak out all over the chassis and also took out a couple of resistors. the good news is the transformer ohms out okay and with some rewiring should be okay.  the question I have is with the voltage divider resistor (part 81 on the schematic).  It ohms out okay, but almost all of the covering is gone from it, leaving just the resistance wire on the insulating tube. My questions are- can I use the resistor as is? I'm assuming the missing covering was some kind of heat sink for the wire. Second, my alternative is to make a new divider resistor using modern power resistors in series, so does any one know the wattage rating for the original resistor? Any input is always appreciated. thanks!
Kevin

RE: Philco 16 voltage divider resistor questions - mikethedruid - 12-09-2014

OK.... I looked at the schematic. Resistor 81, the tapped wirewound is used to establish negative bias voltages. Note that electrolytic cap 75 should be POSITIVE to ground. The voltage across this resistor is about - 34 volts, since that is what appears on the control grid of the final amplifiers as the bias voltage, and is the full voltage that R81 sees. R81 is a total of 20+100+130 ohms, or 250 ohms... Ohm's law lets us figure that a bit less then 0.14 amps will be flowing through it.... so, for the 20 ohm you'll need a half watt resistor minimum for that one, I'd use a 1 watt. For the 100 ohm part you'll need a 3 watt minimum, and for the 130 ohm a 3 watt should do also. (remember, I squared x R) The higher wattage resistors you use, the cooler they will run to touch, so that is also a consideration. If you have the room go to 3 and 5 watts. As for the old resistor, the coating is there to prevent oxidation and shorting. You can make a paste of sodium silicate and re-coat it, and let it dry. You can find sodium silicate in auto stores as the goo they use for muffler patches. you can use that paste as is for the purpose. Let it dry very well before you power up the radio. Looking at the picture of the underside of the chassis in the schematic, and the size of R81, I would think you might have room to mount a 5 lug terminal strip along there to which you can mount the 20, 100, 1nd 130 ohm resistors, if you decide to replace it. Good luck, and keep us appraised.

RE: Philco 16 voltage divider resistor questions - 37silverstreak - 12-09-2014

Thanks mikethedruid for your reply! That's just what I needed to know, and thanks also for the tip about the sodium silicate. I may just give that a try as I don't really have anything to lose and I would like to keep the original part if possible.
Kevin