Repairs Extend Lives of FM Exciters
     

Fig. 1: The exciter, with the original RF amplifier section visible at left rear.
We don’t need to throw away equipment that is capable of serving for many years to come if it is updated on occasion.

The Continental 802A FM exciter was produced from 1983 to 1992. Many remain in use but some have been sidelined because of a failure in the 50 watt RF amplifier section.

This model was made with a Motorola MRF315A transistor. Well, Motorola is out of the semiconductor business, and no company, to my knowledge, is making a direct substitute. Continental can replace the RF amplifier module, including heat sink and voltage regulator, with a plug-in compatible module; but this will cost around $2,000, which is a lot of money considering the exciter may not be worth that much when operational.

An 802A crossed my service bench recently with a problem. My answer was to remove most of the RF power amplifier circuit card and replace it with a pallet amplifier from Broadcast Concepts (www.broadcastconcepts.com).

Because the exciter’s modulated oscillator puts out only about 20 milliwatts, they provided me with a modified version of a television amplifier and called it an “80 Watt FM Pallet, 40 dB gain, MTF173, 28 Volt (Custom)” amplifier at $249 plus shipping.

Fig. 2: The 802A with the new pallet amplifier.
This dropped in nicely after most of the original amplifier card was removed. I left the directional coupler at the output of this assembly so forward and reflected power could be monitored.

One cautionary note. The pallet amplifier, any amplifier, needs good cooling. To do that, I carefully marked mounting holes on the original Continental heat sink. Then I drilled and tapped holes so six 4-40 machine screws could tightly secure the pallet’s aluminum base to the aluminum heat sink.

Silicon heat sink transfer compound (Radio Shack 276-1372A) was used to ensure good thermal contact between the two. Both surfaces needed to be absolutely flat before they were joined. No accidental aluminum drilling burrs were allowed.

Another case

The Continental 802B FM Exciter is a later, updated version of the A offering. One arrived with serious burn damage on its RF amplifier circuit card, which could not be repaired. Investigation revealed that there were two or possibly three versions of RF amplifiers used in this model. Yes, another candidate for a pallet amplifier.

Fig. 3: Closeup of burnt components in an 802B.
Doing the change on an 802B was a bit more complicated. In this model, an RF low-pass filter was required for restoration of the exciter to its former glory. The A model had a filter as an outboard option. That didn’t apply here. So, I installed a Broadcast Concepts 1500-watt low-pass filter with directional coupler on the same module. The cost was only $125 plus shipping.

One problem I had was that the filter coils stood high enough above the circuit card so they would short when the top cover was put on the module. The answer was some 1/4-inch-high aluminum bar stock that raised the cover of the module by that amount. It seems much of my work while repairing and/or restoring equipment is as a machinist.

It all worked out, although I had to adjust component values on the directional coupler and exciter meter board to bring the forward and reflected DC sample levels up high enough to get proper metering.

A known problem with these exciters is seen in the case of the Darlington voltage regulator transistor on the RF power amplifier assembly. It will fail, causing the exciter to go to a full 50 watts or more of RF output. A couple years ago, in the pages of Radio World, I mentioned a modification that adds a 4-ohm resistor in series with the regulator, to reduce its heat dissipation.

Fig. 4: The rehabbed 802B with the Broadcast Concepts module at right.
I use two 2-ohm/50 watt resistors in series because 4-ohm resistors are not a standard item. Sometimes I put a switch in to short the resistors when full power is required from the exciter.

My latest revision to that module also replaces the original MJ3001 regulator transistor with an MJ11023G. This newer transistor is much more capable of handling voltage and current. When installing one, I remove the socket it belongs to, because the new transistor has larger diameter leads.

Fig. 5: Aluminum bar stock was added to clear low-pass filter coils.
Yes, soldering is required here.

In one instance, the regulator transistor broke into oscillation at about 200 kHz, causing the exciter to transmit on three frequencies simultaneously. A 0.39 mfd poly capacitor from base to emitter on the transistor put a stop to that problem.

Mark Persons, WØMH, holds CPBE certification from the Society of Broadcast Engineers and has more than 30 years’ experience. His website is www.mwpersons.com. Find past Tech Tips under the News & Technology tab of radioworld.com.


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