What the Kit Replaces
The FR-100 Kit replaces 3 hard to find parts in the final compartment. These parts are likely to be damaged when the rig is severely overloaded, or moderately overloaded for extended periods of time. A TS-830S is shown, but essentially the same parts were used for all the Kenwood hybrid radios. The symbol names (L1, L2, C7, C8) may be different from rig to rig, but the parts are the same.
L1 is a 6.8uh RF choke, It needs to have low series resistance, high current handling capacity of at least 500ma, and have a self resonance of at least 70 MHz. On the TS-530S and 830S it is a small molded green part that looks something like a resistor, and on the 520 and 820 models it looks like the J. W. Miller part supplied in this kit.
C7 is the plate coupling capacitor, it is wired from the top of the ceramic Plate Choke to the plate tuning capacitor. All of the output power of the tubes (up to 120W or more) must pass through that 0.001uf capacitor to reach the tank circuit. It must exhibit very low loss at high frequencies, be able to pass the RF output current safely, and have a 3 KV rating. These are definitely becoming harder (almost impossible) to find! It is essentially the replacement for the "doorknob" cap in higher powered linears and the like. It needs to be a GOOD Class 1 dielectric capacitor or it will destroy itself, sometimes like a firecracker! Z5U, Y5P, X7R and similar caps are definitely NOT appropriate!
What I provide for C7 is a kit of three Class 1 330pf 3KV temperature compensating 5% new production disk ceramics that are wired in parallel. This produces a replacement cap that should last forever!
C8 is the bypass cap between the two plate chokes. It is rated 0.01uf at 2 KV. It often blackens from heat because of its proximity when L1 is damaged.
These parts are suspect when new finals, proper voltages, and adequate drive still produce low output power, perhaps on the order of 50-70 watts.
L2, the plate choke is also suspect, and usually will be discolored, its varnish blackened if it's been overloaded. The plate choke for single wound chokes should measure around 7 Ohms. Shorted turns, which will also corrupt the part's operation, can't really be seen by measuring resistance, and this applies to most coils and chokes. Resistance measurement primarily serves as a continuity check. L2 is not part of this kit. If L2 is damaged, it will probably have to be rewound. Fortunately, they seem to survive very harsh punishment!
