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Radio
Broadcast Technical Consulting and Sales
10032 Island Drive, Brainerd, MN
56401
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Radio
Broadcast Technical Consulting and Sales |
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Technical Tips from Mark W. Persons |
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Continental 815A Transmitter IPA Amplifier |
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On the left is an original
Continental 815A IPA (Intermediate Power Amplifier). The 815A is a 5 KW FM
broadcast transmitter. Its 4CX3500A PA (Power Amplifier) tube needs about
100 watts RF of drive to produce 5 KW of RF output. The original amplifier
was made using two Motorola SRF3999 transistors. The "S" in the part
number indicates they were specially made for a particular manufacturer.
Unfortunately, the transistors are no longer made.
One fix for the situation is to replace the palate amplifier with a newer version such as the Broadcast Concepts FM150. This uses only one power MOSFET transistor that runs cooler. Efficiency of the original amplifier was about 61%. This new amplifier is closer to 75%. You can see the replacement amplifier on the right in this photo. |
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My
solution was to remove the original electronics from the heat-sink
palate plate and bolt the new amplifier on top. Four 6-32 threaded
holes were drilled and tapped in the original palate plate and silicone
heat sink compound was used to thermally bond the two plates together
Since the replacement amplifier is the same width, but shorter,
extensions were needed for the input and output cables. The good
part is that the revised amplifier fits the same footprint as the
original.
The new amplifier runs on the same 43 to 48 VDC that powered the original amplifier. RF output is controlled by drive. The original amplifier needed 10 to 14 Watts of RF drive. This new amplifier requires less than 2 Watts of RF input to get 100 Watts of output at 95 MHz. The actual drive will vary with frequency. Since it is much less drive than the original design, you need to be careful doing a changeover as you could accidentally overdrive the new amplifier and cause a transistor failure. |
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The stories go on and on.
Stop in again sometime. I'll leave the soldering iron on for you.
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page last edited 09/24/2009
