02-27-2013, 09:56 PM
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.
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.
John Honeycutt
