14-watt FM Stereo Broadcast Transmitter
- North Country Radio's MPX96 gets some help -
The blue thing on the right is the exciter/stereo generator. The gray thing is the 14-watt power amplifier.
Exciter/stereo generator interior.
The modified MPX96 is the larger board in the center. The input audio filter/pre-emphasis network is the smaller board on the left. The modulation limiter is the upper board. The unit's carrier output power is about 2 milliwatts.
MPX96 Frequency Programming Switches
Power amplifier interior.
The intermediate power amplifier (IPA), a modified Ramsey LPA-1, is under the shield on the left. The power amplifier, salvaged from a Repco VHF land mobile transceiver, is the small board in the center. Components of the unit's 13.8-volt power supply occupy the balance the of interior space.
The power amplifier with the IPA shield removed.
The IPA has a gain of about 27 dB. It amplifies the carrier from the exciter to a level sufficient to drive the power amplifier. Both the IPA and PA stages are tuned with lengths of coax transmission line (the black coils). A multimeter can be plugged into the red and black pin jacks to monitor final amplifier current. The operating frequency is 97.1 MHz.
Modified LPA-1 schematic
Modified Repco power amp schematic
The exciter/stereo generator and power amplifier viewed from the rear.
A switch on the rear of the exciter/stereo generator selects the unit's operating mode - mono or stereo. The small aluminum box houses a 120 MHz low-pass filter.