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Technical Tips from Mark W. Persons
Updates for Continental 802A and B FM Exciters

Continental 802A FM exciter

Continental 802B FM Exciter
If you have one of these exciters, then the factory recommends you install a kit to cut down on the heat load experienced by a series regulator transistor for the RF power amplifier.  Continental bulletin 200230-1 shows a 4 ohm/100 watt resistor connected in series with the voltage regulator.  We typically use two 2 ohm/50 watt or two 3.3 ohm/50 watt resistors in series.  They mount on the power amplifier module sides next to the heat sink where they get plenty of cooling from the fan. 

Here is the underside of the power amplifier showing a newly installed resistor (gold colored) near the top and machine screws that anchor the second resistor on the closest side (nearest the bottom of the photo) of the assembly.  Both resistors have heat sink compound under them to better transfer heat to the aluminum sides. 

A ruler is lying on the heat sink so you can get a sense of size.

A switch has been installed near the top of the power amplifier so users can select high or low power.

The low power switch position, in this case 0 to 23 Watts, enables the two resistors to dissipate much of the heat that the regulator transistor would have.  That transistor can be seen on the left as a metal diamond-shaped device mounted to the heat sink.  Without this modification, the transistor runs hot and has a high failure rate especially when exciters are set to between 10 and 20 watts of RF output. 

Below is a hand-modified schematic showing the modification.  The actual low power maximum output power will vary depending on the RF transistor in the amplifier.  Add more resistance to lower the power or reduce the resistance to raise power.  

Here is a modification to help prevent black rubber shock mounts from failing.  These are the ones under the modulated oscillator in Continental 802A and 802B FM exciters.  Drill a 1/4" hole half way between the shock mounts in the base of the modulated oscillator.  Drill a 9/64" hole in the exciter chassis and insert a 4-40 x 1-1/2" machine screw.  Add flat washers, a lock washer and a nut.  Cover much of the screw with heat shrink tubing and top it with a locking nut.  Do this at both ends of the modulated oscillator. 

This will prevent the modulated oscillator from breaking loose while it is in transit, which could cause damage to the oscillator and other circuits in the exciter.  If you carefully drill the holes and move the bolts around in the holes, you can get it so the extra 4-40 screw does not touch the modulated oscillator when it is running. 

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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