I am just getting started in radio restoration and have successfully completed the repairs of two RCA's AA-5 radios. I am now getting started on a Philco 48-300. There is a 2-section resistor in the power supply. Each section has a design value of 875 ohms. When checking these values I found that R100B is 660 ohms and R100A is 925 ohms. My question is do these need to be replaced and if so, what wattage should they be? I can't find any specifications on the circuit diagram or parts list regarding wattage. Also, there is something installed parallel with R100B and I don't recognize it as either a resistor or a capacitor. Nothing is shown on the circuit diagram. It has flaking red paint on it and is about 1/4 inch in diameter and about 1 1/2 inches long. Does anyone know what this is?
Check and see if your filament voltages are ok. If so I'd leave them be unless they are discolored or burnt looking. Your total resistance is reasonably close to the 875+875 total that is being used for the 7.5v filament string which is the most critical. You don't want to stray too far from 7.5 volts.
Thanks for the response exray. I don't want to plug the set in until I have the capacitors changed out. This model radio is a portable unit with a 117Z3 rectifier and 3V4 output. The other tubes are a 1R5, 1T4, and 1U5 with low filament voltages. The two part resistor is all in one long narrow unit affixed to the side of the chassis - not the typical looking resistor.
Bob
Agree with all, if the resistor is not discolored or shorted out, it probably is fine. I've done a number of the 46 and 48 portables, and they are great performers.
My suggestion would to bring this one up slowly on a variac and measure the voltage on the filament string. It will probably be too high with today's higher supply voltages and your recap job. And if the rectifier fails and you replace it with a a diode, definitely you must beware.
I have added a couple of hundred ohm 10 watt resistors to the string of the filament supply to tame this down with the original rectifier in place and have had good luck with it. You would want to dress this close to the chassis as it would get warm.
I will follow the suggestions about the filament voltages. Codefox 1: Since these resistors are enclosed in a long metal case does the case get discolored if there is a problem? Also, do you have any idea what the item is that is installed parallel with R100B that I mentioned in my first post? Thanks, Bob
There should be nothing in parallel with R100B. If you unsolder one end and measure , you will probably find that it is a resistor, and that in fact R100B is open. You can't always tell by the look of a component.
My best guess is this indicates a repair was done. If this is the case I would remove the the R100A/B resistor and the "extra" resistor, and replace it with 2 1000 ohm ten watt sand resistors. You might have to put in a terminal strip to connect things up, but try to dress the resistors right up to the chassis, as they do generate a little heat. Add a fuse while you are at it. One half amp should be around right.
The reason I would use 1000 ohms and not 850 is because the line voltage is a lot higher than it was in 1948, and the battery set tube filaments are very delicate. If you want to ditch the 117Z3 and use a silicon diode, then I'd probably use 1500 ohms. The 117 volt tubes are not that reliable, and not so easy to find. Add a fuse while you are at it. One half amp should be around right.
Good luck
Codefox1: Thanks for the response. I'm sure you are correct about R100B being open. I will check it out tomorrow. The replacement did not look like any resistor that I am familiar with. I will follow your suggestions to replace with 2 10 watt 1000 ohm resistors. Bob
I finally got back to working on this radio. I removed the extra resistor and found that R100B was not open. Resistance measured about 950 ohms. Apparently someone had put another resistor parallel with 100B to lower the effective resistance. My inclination is to leave R100A and R100B alone and not put in the 1000 ohm replacements. Any comments?
I agree with Xray, if the filiment voltage is correct, leave well enough alone. If you lift one leg of the added resistor and the set stilll plays well, you will enjoy more years on the tubes. I can't imagine why this addition was done, but there had to be a resason.
I have replaced the electrolytic caps and the other paper caps and the out of spec resistors. The radio is now working and here are the filament voltages. 1R5 - 1.8v, 1T4 - 1.5, 3V4 - 2.8, 1U5 - 1.5, 117Z3 - 122. Are these acceptable? Bob
Probably OK. I assume you are using a digital meter. I'd still consider putting a 22 or 47 ohm 10 watt resistor in series of the hot side of the line cord to tame the overall voltage down a bit. Line voltage is higher nowadays.