I'm beginning chassis work on a fairly decent model 41-295, and looking through the schematic, I noticed the bias resistors are listed as: 82B=150 ohms, 82A=15 ohms, and 82=10 ohms. The parts list shows 82B as 115 ohms. Which one is correct? Also, what should the wattage of these resistors be?
Thanks in advance!
I believe the parts list is incorrect. The unit (41-295) that I have are coded on the actual hardware as 10, 15, and 150. Additionally, the measured resistance values check out to be very close to 10, 15, and 150.
TFB
That's what I thought, but I wanted to get a second opinion. I've re-done a 40-201, which is very similar underneath, but it has a much more elegant tapped BC resistor handling the bias duties. I wonder why Philco moved away from that design to individual carbon resistors around the power supply? Cheaper, I suppose, but better? I don't know. The 40-201 and 41-295 seem comparable in many ways. I guess I'll get my chance to decide which one is actually better. I will say that the paper-covered tuned oval "muffler" antenna on the 40-201 is FANTASTIC! It not only pulls in distant stations loud and clear (I'm listening to the Rockies in extra innings against the Red Sox on KOA, Denver, while I'm in Minneapolis), but it also does a superior job of nulling noise and unwanted signals. Does the 41-295 built-in loop work as well? I don't know because mine came in D.O.A.
I'm curious to hear opinions on the subject.
Mike
Mike, that was one of my first console sets I purchased. Had it from 1981 until around 89. It was quite sensitive and I loved that set (never electrically restored it except for the filter caps). I generally shy away from most sets after the 39 model year, but I miss that one. I remember the antenna has limited movement compared to the "muffler" design. Now ya got me thinking about finding another.
You can closely estimate the wattage of the resistors if you know the voltage drop across the field coil and bias resistors. Unfortunately, the Rider's schematic for the 41-295 on Nostalgia Air doesn't include enough information to make the calculation. Maybe the Philco schematic does.
Subtract the voltage across the second filter capacitor from the voltage across the first filter capacitor to get the voltage drop, E. (The voltage across the first filter cap is missing from the Rider's schematic.) Then add the resistance in ohms of the field coil to the resistance in ohms of the three bias resistors for the total resistance, R. Use Ohm's law, E=IR, to get the approximate current, I, in amps. The bias current is so small that you can ignore it for the purposes of this calculation, so you can assume that the current across all three bias resistors is the same. In my experience, this calculation gives the B current within a very few milliamps, and any error is on the safe side.
Power in watts = I squared times R, so plug the value in ohms of each bias resistor into this formula to get the power each resistor must dissipate. I doubt that you'll need more than 2 watt resistors in that circuit, but since I don't know the voltage drop I can't say for sure.
It is hard to find power resistors in the exact values needed, so when replacing a candohm resistor, I'll usually put two or even three 1 watt resistors in series to get as close as I can to the right value and to be sure that each is dissipating no more than about 1/2 or 3/4 watt. Even then they can get pretty hot, so I make sure they are well separated and sticking up on long leads.
The only time I've had a problem doing this is when the resistors are squeezed in tight, as in the 46-350. The candohm resistors use the steel chassis as a heat sink, and I don't know any way to do that when replacing them with modern resistors. I know candohm resistors have a bad reputation, but now I usually won't replace them if they are still OK.
Another advantage to the candohms is that the wire-wound construction provides a little bit of needed inductance to the circuit. Of course if you can find modern power resistors of close to the right value, that problem goes away.