05-04-2013, 09:42 AM
Sometimes you can test a capacitor for leakage by disconnecting one lead and putting an ohm meter across it. A badly leaking capacitor will show some resistance on your meter, making it look like a resistor. A "good" capacitor will show an open circuit on an ohm meter. A better way to test it is to use a dedicated capacitor tester that has a "leakage" function.
But, you should assume that any of the paper capacitors is leaky and liable to short at any time. So it is better just to replace them all. All old electrolytics should be changed before even turning on the radio, as they are especially prone to aging and will damage power transformers that are difficult and expensive to replace.
Philcos from the late 1930s through 1942 used wire insulated with rubber, which becomes brittle and often breaks off over the years, creating a danger of shorting. All of that wire should be replaced or sleeved. I replace it by desoldering both ends of the wire while leaving it in place. Then I tack-solder a new wire to one end of the old one and gently pull the old wire out of the radio. This pulls the new wire through the exact routing of the old one. Routing of a few of the wires is critical to avoid interference of the signal on one wire with the signal of another. Other restorers desolder one end of the old wire and then slip a piece of heat shrink tubing over the old wire. You can either crumble the brittle insulation off the old wire or not, as you choose, but if you do it makes a better appearance.
Testing resistors is easy but a little tricky, because if it is connected in the circuit it resistance can be changed (usually lowered) by other components in the circuit. When you disconnect wire or capacitor connections you often can remove one end of a resistor connection also. Put an ohm meter across the resistor and compare the reading with the specified value in the parts list, or read the stripes to see what the original resistance was. If it varies more than 10% from the original reading, I replace them. Other restorers say 20% is good enough, and they are right. I'm just a little picky. Bad resistors almost always read higher than the original value.
You can replace resistors and capacitors with minimal soldering by cutting the part out of the circuit, leaving the leads in the radio as long as possible. Bend the leads into a loop or hook, and solder the new part the the hook. I usually desolder the old part on a radio I prize, but use the easier method on those less important to me.
If you need to switch the band switch to get good signal, your switch probably has more oxide on it that needs cleaning. Often a good cleaning job will work for a while but then it needs to be done again and again. It's a little discouraging at times, but that's life with ancient electronics.
Check out the detailed restoration threads with pictures that member Bob Anderson has posted on the phorum. (Use the search function to look for threads by "Bob Anderson.") I seem to remember he did one on a 41-255, which is similar enough to your chassis to give you many valuable hints, although the arrangement of the electrolytic capacitors is significantly different.
But, you should assume that any of the paper capacitors is leaky and liable to short at any time. So it is better just to replace them all. All old electrolytics should be changed before even turning on the radio, as they are especially prone to aging and will damage power transformers that are difficult and expensive to replace.
Philcos from the late 1930s through 1942 used wire insulated with rubber, which becomes brittle and often breaks off over the years, creating a danger of shorting. All of that wire should be replaced or sleeved. I replace it by desoldering both ends of the wire while leaving it in place. Then I tack-solder a new wire to one end of the old one and gently pull the old wire out of the radio. This pulls the new wire through the exact routing of the old one. Routing of a few of the wires is critical to avoid interference of the signal on one wire with the signal of another. Other restorers desolder one end of the old wire and then slip a piece of heat shrink tubing over the old wire. You can either crumble the brittle insulation off the old wire or not, as you choose, but if you do it makes a better appearance.
Testing resistors is easy but a little tricky, because if it is connected in the circuit it resistance can be changed (usually lowered) by other components in the circuit. When you disconnect wire or capacitor connections you often can remove one end of a resistor connection also. Put an ohm meter across the resistor and compare the reading with the specified value in the parts list, or read the stripes to see what the original resistance was. If it varies more than 10% from the original reading, I replace them. Other restorers say 20% is good enough, and they are right. I'm just a little picky. Bad resistors almost always read higher than the original value.
You can replace resistors and capacitors with minimal soldering by cutting the part out of the circuit, leaving the leads in the radio as long as possible. Bend the leads into a loop or hook, and solder the new part the the hook. I usually desolder the old part on a radio I prize, but use the easier method on those less important to me.
If you need to switch the band switch to get good signal, your switch probably has more oxide on it that needs cleaning. Often a good cleaning job will work for a while but then it needs to be done again and again. It's a little discouraging at times, but that's life with ancient electronics.
Check out the detailed restoration threads with pictures that member Bob Anderson has posted on the phorum. (Use the search function to look for threads by "Bob Anderson.") I seem to remember he did one on a 41-255, which is similar enough to your chassis to give you many valuable hints, although the arrangement of the electrolytic capacitors is significantly different.
John Honeycutt
