[Raleigh]

I opened up my 40-150 to fix a problem with the dial string. After that I turned the chassis over to make sure all was well within. I noticed that resistor # 16 (4700 ohm) looks a little burned, but still works. I restored this radio about 5 years ago but haven't used it that much. During restoration I put in a 1 Watt resistor instead of the original 1/2 W. I can't remember why.

The burned resistor measures about 4880 ohms, well within 5%. The DC voltage across it is 19.6 volts worst case. My VOM measures zero AC voltage drop. Now, if I calculate the current, it works out to about 4 ma DC, and power to 0.08 Watts.

This amount of power shouldn't come close to burning a 1 Watt resistor, but it's burned all the same. In operation it is cool to the touch. Can anyone think of something I've missed? Possibly I put a surge across it somehow when I was restoring it. It sure doesn't seem like it ought to burn now.

Whatever it is, I'd rather catch it now before I button it all back together, so any ideas would be welcome.

[TA Forbes]

Check R36, C17, C14, and R15 for value..

[codefox1]

Yup, check the capacitors first. They most often fail first, but could be anything else along the voltage divider including tubes, but least likely.

[Raleigh]

Thanks. Will do & let you know.

[Raleigh]

Checked resistors and caps as suggested. All measure correctly and caps don't leak. I replaced the burned resistor and put the other parts back in the radio as they were. Everything works well, as before.

While the components were unsoldered, I measured the current through resistor #18, and the reading confirmed my calculations. The DC current through #18 is just below 4ma, and the AC (measured by my VOM) is a few microamps. Just for fun I measured the current through the RF coupling transformer #13. Readings are about the same, or a little lower.

I guess the resistor burning might be a fluke or maybe a result of something I might have done when I was originally recapping and testing the radio. I don't remember shorting anything or anything like that, but it was a while ago.

Since I have the radio open I was thinking of adding a power resistor to drop the line power back to 117 VAC or so. Could anyone suggest where I could get the kind that people attach to the side of the chassis, and what value to use?

[TA Forbes]

Try Mouser Electronics.

[Ron Ramirez]

We need Ohm's law plus power calculations to find the answer.

Using the calculator found here:
http://www.sengpielaudio.com/calculator-ohm.htm

I punched in 115 volts and 60 watts, the values given on the original Philco Service Bulletin for the 40-150 (and 40-155). It gave me a total current of roughly 522 mA, (0.522 amps).

So let's say we want to drop 125 volts to 115, a drop of 10 volts. Since R = E/I, that means R = 10/0.522 = 19.2 ohms.

Now to find the power the resistor will dissipate: P = I squared * R = (0.522 * 0.522)19.2 = 0.273 * 19.2 = 5.23 watts, so you would want to use a 10 watt resistor at a minimum; 25 would be better.

And as Tom mentioned, Mouser sells the resistors with built-in heat sinks that are designed for chassis mounting so the chassis helps act as a heat sink.

[Raleigh]

Thanks, Ron. That's a great little calculator.

I found several types of chassis-mount power resistors at Allied and Mouser, for about the same price. The Vishay-Dale types look like the ones you and others on the phorum have used. After glancing at the spec sheet I see why you derate the wattage you recommend so much.

[gary rabbitt]

Hi Raleigh,
You mention the caps "didn't leak". You do know that there is physical leakage, and electrical leakage. A cap that looks perfectly intact, could be shorted inside.
I hope you get the issue resolved.

[codefox1]

Agree with all, no decent way to check old caps, and much less how to predict how many more hours of service they will have left. Same with carbon resistors, who knows what they will do under voltage and heat after all these years, much less how out of tolerence they are already. Can't imagine you could buy a modest but decent dinner for two for what parts cost to renew an average radio. Do it right and they will be playing after we are gone.

[Raleigh]

All caps in this circuit are new (when I recapped 5 years ago.) I left a couple of original micas elsewhere, but not here. I also replaced all the rubber wiring.

When I started this thread I pulled off one lead of each cap and resistor T A Forbes suggested and tested for leakage with a vacuum tube capacitor tester at 600 VDC for the polys and 500 for the mica. None leaked under that test, so I reconnected them. Like I said, the current through the resistor is steady at about .4 ma, dissipating about .08 watts. It is a one watt resistor, and it's cool to the touch after running a while.

Just for fun I put replaced the burned one with 2 resistors in series (a 2200 ohm and a 2400 ohm to give 4600 ohm) to effectively make a 2 watt resistor, and I ran the radio for hours and hours over a couple of days monitoring voltage drop, current, and temperature (by the finger method). Everything was dead on every time I checked it, so I'm pretty satisfied that I might have burned the resistor by touching something wrong with a meter lead or something silly like that when I recapped and tested the the thing a long time ago.

I do remember that when I got the radio somebody had totally bollixed it trying to recap & rewire it before I got it. A lot of wiring was missing or just wrong. I made some mistakes trying to straighten our his faulty wiring (gave me fits for a couple of weeks because I had to figure out routing and connections without referring to the original) so maybe I did something to stress that resistor when I was working on it. I just wanted to ask you guys if I'd missed something essential.

Thanks, Gary, Ron, Codefox, and TA Forbes for your responses.