jshorva65
10-29-2007, 05:22 AM
When restoring a set, overload protection is a very good idea which was neglected to at least some degree by most manufacturers in their original designs. Most sets, if protected at all, are only protected in the B+ Boost or AC Line circuit. Seldom does one find both circuits protected except in Dumont receivers.
I prefer to provide triple protection (AC Line, B+, and B+ Boost). Under-chassis fuse holders secured with a single rivet, pigtail fuses with "shrink tubing" covering each of the metal end caps, or subminiature "Pico" fuses about the size of a 1/4W resistor work well, depending on the model being restored. In RCA sets, such as the 630 or 721 and "clones" of these models, I've found that moving the B+ feed from pin 8 of the 5U4 to pin 1 and placing an appropriately sized Pico between pin 1 and 8 works well, and the same strategy can be used on the 5V4 for B+ Boost. Single 5U4 uses a 500mA Pico, dual 5U4's calls for 1A. The 5V4 socket gets a 250mA Pico. On thye 630, there are two small unused holes in the side of the chassis beside the filter capacitor bracket. Add an AES p/n P-0100H single-terminal no-ground tie point in one of those holes, move the AC Line wire from its junction with the On/Off switch wire to the new terminal, leave the Switch wire in its original place. Prepare a 5A fast-acting pigtail fuse by covering the end caps with heat shrink tubing or (if you have some clear shrink tubing) cover the entire fuse body. Solder the pigtails of the fuse to the incoming AC Line terminal (the new strip) and to the old terminal where the switch lead is still connected.
When troubleshooting a possible open fuse, remove the 5U4 or 5V4 and check for continuity between pins 1 and 8 of the socket by inserting the probes from above the chassis. Check continuity between the "interlock" pins with the cord removed and switch in the "On" position. If one of the fuses opens, it generally means the set needs to be disassembled to investigate and correct the trouble anyway. Don't use pins 3, 5, or 7 on the 5U4 or 5V4 as these are too close to the plate pins.
Photos:
5U4 factory wiring
5U4 with "hidden" fuse
AC Line fuse
I prefer to provide triple protection (AC Line, B+, and B+ Boost). Under-chassis fuse holders secured with a single rivet, pigtail fuses with "shrink tubing" covering each of the metal end caps, or subminiature "Pico" fuses about the size of a 1/4W resistor work well, depending on the model being restored. In RCA sets, such as the 630 or 721 and "clones" of these models, I've found that moving the B+ feed from pin 8 of the 5U4 to pin 1 and placing an appropriately sized Pico between pin 1 and 8 works well, and the same strategy can be used on the 5V4 for B+ Boost. Single 5U4 uses a 500mA Pico, dual 5U4's calls for 1A. The 5V4 socket gets a 250mA Pico. On thye 630, there are two small unused holes in the side of the chassis beside the filter capacitor bracket. Add an AES p/n P-0100H single-terminal no-ground tie point in one of those holes, move the AC Line wire from its junction with the On/Off switch wire to the new terminal, leave the Switch wire in its original place. Prepare a 5A fast-acting pigtail fuse by covering the end caps with heat shrink tubing or (if you have some clear shrink tubing) cover the entire fuse body. Solder the pigtails of the fuse to the incoming AC Line terminal (the new strip) and to the old terminal where the switch lead is still connected.
When troubleshooting a possible open fuse, remove the 5U4 or 5V4 and check for continuity between pins 1 and 8 of the socket by inserting the probes from above the chassis. Check continuity between the "interlock" pins with the cord removed and switch in the "On" position. If one of the fuses opens, it generally means the set needs to be disassembled to investigate and correct the trouble anyway. Don't use pins 3, 5, or 7 on the 5U4 or 5V4 as these are too close to the plate pins.
Photos:
5U4 factory wiring
5U4 with "hidden" fuse
AC Line fuse