09-22-2023, 10:17 PM
Hi Justin,
In the case of your 4 separate electrolytic caps, all negative terminals connect to "B-". This is not necessarily Chassis Ground, but it is the "Common Negative" for the B supply of the radio. The chassis connects to this common negative through a capacitor and sometimes through a 150K resistor. This is to allow AF and RF energy (the metal chassis, the tuning cap stator, IF transformer case and tube shields) to connect to B- without allowing a lethal amount of electricity to pass through a person contacting the chassis. If your original 4 section filter "cap" (capacitor) was a metal can, you will find that it was mounted to the chassis through a bakelite insulator and secured to that insulator by 4 twisted tabs on the outer diameter that extend to and are electrically connected to the metal can. The can also likely serves as the B- "Mecca Point" where all of the B- connections (1st Audio, IF and convertor cathodes, the output tube cathode bias resistor, etc.) and Detector / 1st Audio filament return connections are made.
Looking at the Schematic, an 80 uF 150 WV (Working Voltage) cap connects between the B- Mecca and the junction between the output transformer red wire and R18, a 22 Ohm resistor. This replaces the "Half Moon symbol" connection to the old can cap. A 40 uF 150WV cap (replacing the square symbol) connects between B- Mecca and the junction of R16 and R17. (The other end of R17 connects to the brown lead of the AF Output transformer (This serves the same purpose as the "hum bucking" coil on the older electrodynamic speakers on pre-war radios.) Another 40 uF 150WV cap (replaces the triangle symbol) connects between B- Mecca and the junction of R16 and the 99V B+ to the output tube screen and the IF transformer primaries.
The final capacitor, the 40uF 25v cap connects between B- Mecca and the junction between the output tube cathode and the output tube's cathode bias resistor, R13, and serves as a "bypass" cap, adding stability to the output stage. The working voltage of this cap is much lower than the various B+ filters because the voltage drop across the resistor is only about 12V or less. Be careful not to accidentally substitute this cap for one of the others. It will go off like a firecracker (been there, done that).
Be sure not to use the positive terminals of the old cap as terminals for the positive terminals of the new cap ("bridging" the original cap with the new caps") unless you hollow out the original cap and "stuff" it with the new caps. Purists do this to maintain the look of originality and also when there is no room under the chassis to add the new caps. Experts on this Phorum have written articles on "stuffing" cans. If ytu have the room under the chassis for the new caps, add a terminal strip and move the wires from the various + terminals to the original caps to the terminal strip and connect the corresponding + connections for the new caps to these terminals. In some cases, the connection for the + terminal can be made to a "junction" point.
Another important component to replace is the selenium rectifier. These tend to suddenly fail catastrophically and create a rotten egg stench.
In the case of your 4 separate electrolytic caps, all negative terminals connect to "B-". This is not necessarily Chassis Ground, but it is the "Common Negative" for the B supply of the radio. The chassis connects to this common negative through a capacitor and sometimes through a 150K resistor. This is to allow AF and RF energy (the metal chassis, the tuning cap stator, IF transformer case and tube shields) to connect to B- without allowing a lethal amount of electricity to pass through a person contacting the chassis. If your original 4 section filter "cap" (capacitor) was a metal can, you will find that it was mounted to the chassis through a bakelite insulator and secured to that insulator by 4 twisted tabs on the outer diameter that extend to and are electrically connected to the metal can. The can also likely serves as the B- "Mecca Point" where all of the B- connections (1st Audio, IF and convertor cathodes, the output tube cathode bias resistor, etc.) and Detector / 1st Audio filament return connections are made.
Looking at the Schematic, an 80 uF 150 WV (Working Voltage) cap connects between the B- Mecca and the junction between the output transformer red wire and R18, a 22 Ohm resistor. This replaces the "Half Moon symbol" connection to the old can cap. A 40 uF 150WV cap (replacing the square symbol) connects between B- Mecca and the junction of R16 and R17. (The other end of R17 connects to the brown lead of the AF Output transformer (This serves the same purpose as the "hum bucking" coil on the older electrodynamic speakers on pre-war radios.) Another 40 uF 150WV cap (replaces the triangle symbol) connects between B- Mecca and the junction of R16 and the 99V B+ to the output tube screen and the IF transformer primaries.
The final capacitor, the 40uF 25v cap connects between B- Mecca and the junction between the output tube cathode and the output tube's cathode bias resistor, R13, and serves as a "bypass" cap, adding stability to the output stage. The working voltage of this cap is much lower than the various B+ filters because the voltage drop across the resistor is only about 12V or less. Be careful not to accidentally substitute this cap for one of the others. It will go off like a firecracker (been there, done that).
Be sure not to use the positive terminals of the old cap as terminals for the positive terminals of the new cap ("bridging" the original cap with the new caps") unless you hollow out the original cap and "stuff" it with the new caps. Purists do this to maintain the look of originality and also when there is no room under the chassis to add the new caps. Experts on this Phorum have written articles on "stuffing" cans. If ytu have the room under the chassis for the new caps, add a terminal strip and move the wires from the various + terminals to the original caps to the terminal strip and connect the corresponding + connections for the new caps to these terminals. In some cases, the connection for the + terminal can be made to a "junction" point.
Another important component to replace is the selenium rectifier. These tend to suddenly fail catastrophically and create a rotten egg stench.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
