There is quite a vibrant local FM radio scene here in Spain. If you pick a quite spot on the dial and don’t interfere with existing stations and keep under a few hundred Watts, the authorities turn a blind eye. As a result, I get asked to repair, modify, supply all sorts of equipment including FM transmitters and associated gear.
Typical work over the last couple of weeks was an 800 Watt transmitter with a badly damaged output stage due to faulty antenna connections and a rebuild of a home brew 300 Watt transmitter.
The first job was a PCS Electronics Cybermax 800 transmitter. The final stage pallet amplifier is in its own shielded housing with a filter stage. The output MOSFET transistor had put up a respectable fight before breaking into two, along with damage to the PCB and supporting components. PCS have an upgraded pallet module which is rated over 1000 Watts. So, given the cost of a repair and replacement transistor vs a new pallet, a new pallet was fitted. 800 Watts could not be exceeded given the limitation of the power supply. Tweaking the power meter and output of the exciter in order that front panel power and reflected power readings were correct, a final test and job done.
The 300 Watt transmitter was far more time consuming. Actually, a pain in the backside. The unit consisted of a 5 Watt budget transmitter, used as an exciter. A 12V power block power supply, an external 48V 15 Amp power supply and a Broadcast Warehouse 300 Watt pallet amp. Sadly Broadcast Warehouse no longer trade. The equipment was always high quality, at a price. I had replaced the power MOSFET in the pallet amp a couple of times, the last time being as a result of the transmitter being left in the Spanish sun whilst operating. The customer didn’t want to spend too much but also wanted to up the power a little. Again, turning to PCS Electronics, we obtained a STMAX3015 module and display. This provides a 15 watt transmitter combined with stereo encoding and RDS. A very nice quality unit for the price, great audio conditioning, compression etc. For the power stage a 600 watt pallet amp was used. Given the extra power and stray RF inside the enclosure, the pallet amplifier has to be fitted in its own case, with power and inputs being fed via feedthrough capacitors etc. A large heatsink fitted with a couple of fans to provide good cooling. The case had to be considerable modified. The exciter adjusted so that a maximum power output of 400 Watts could not be exceeded. This gives a good margin of reserve for the output stage and keeps things within the limits of the power supply. All tested and seems to work well.
PCS Electronics provide a wide range of prebuilt and modular equipment. The range covers FM, AM and TV equipment, with supporting components and information. The smaller items are quite straight forward to deal with, but once to start to exceed the 200.Watt mark, lets say, you need to have a good understanding of RF technology and working practice. The modular aspect of the equipment is well thought out, as you can build custom equipment and or modify – upgrade existing systems to your own specification. The support staff such as Marko are very knowledgeable, will always advise.
