This is the solid state driver amplifier out of a Broadcast Electronics FM transmitter. It’s job is to take 10 watts from the exciter and bring it up to 150 watts to drive the final tube.
This one had significant damage to the power supply, control board and RF amplifier.
It’s going to be a long day when you see this. This is one of the pass transistors in the power supply and it’s met a violent death. There are three in parallel with an equalizing resistor network. All three were shorted. Fortunately 2N3055’s are still a common, cheap part.
The bridge rectifier blew a hole in the side of the case.
It’s the RF amplifier that was most worrisome as the matched pair transistors it uses haven’t been available for years. Back when BE was able to repair these, the bill started at $850 and went up from there. Let’s see if we can do better.
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The RF amplifier assembly with a close up view of the shorted final transistors.
Borrowing an idea from Mark Persons, we’ve ordered a 350W Pallet amplifier from Progressive Concepts in Florida. It will replace almost everything on this board, with a modern, reliable update that uses parts that are easy to find. Mark used these in all sorts of FM gear to get around unobtainable/obsolete parts. It’s a brilliant retrofit. Mark’s contributions to the industry cannot be overstated.
Old amplifier removed, we’ll clean off the mounting plate and use it as a heat sink for the Pallet amp.
Pallet amplifier mounted in place. Very thin layer of non-silicone thermal compound between the pallet and the heat sink will help keep it cool. We’ve added a resistive pad at the input as the pallet only requires about 2 watts of drive, but the exciter driving it will be happier set somewhat higher. Some won’t adjust below 5 watts. The original forward/reflected power metering circuit has been retained.
Everything put back together. The original power supply is capable of 30Vdc, which is enough to run the pallet up to 200 watts. 48V would be required to generate the full 350, but as 100-150w is more than enough to drive this transmitter to rated power, there is no need to alter the supply.
We’ll burn it in 24 hours at 150 watts, then back to the customer it goes. A fraction of the cost of replacement.