Repairs Extend Lives of FM Exciters
We don’t need to throw
away equipment that is capable of serving for many years to come if it is
updated on occasion.
Fig. 1: The exciter, with the original RF amplifier section visible at left rear.
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
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
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.
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
Fig. 2: The 802A with the new pallet amplifier.
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.
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.
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.
Fig. 3: Closeup of burnt components in an 802B.
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
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.
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.
Fig. 4: The rehabbed 802B with the Broadcast
Concepts module at right.
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.
Yes, soldering is required
Fig. 5: Aluminum bar stock was added to clear
low-pass filter coils.
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.
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