Transmitters! These days you can put 10kW of FM on the bench to repair. Fun times.
Nicom NT250. In this case the NT stood for “not transmitting”.
Nicom packs almost everything but the PA on a single board. This time it’s the derived negative supply voltage that has failed. (bad capacitors and a failed choke)…and the PA output device, below.
Sometimes you have to do some digging to find the failed components. Other times they find you.
The ceramic cover was blown off the FET. looking inside, we can see why.
Fortunately this device is still available. Not cheap, but available.
Check the bias idle current before running it very long. There is a small, flimsy trimpot to adjust it. If it’s off the device will fail sooner than later.
Oh boy. This is an early Armstrong Solid state FM Transmitter. Closing in on 20 years service.
These units were built by RVR and badged, and fortunately supported by Armstrong.
A 100W exciter feeds 5- 1kW amplifier modules through a splitter. The amplifier modules come back into a 5 port wilkinson combiner and then out to the antenna. Simple, simple.
This one however has…..Issues.
Yes you’re reading correctly. This transmitter is showing 296 watts into the Reject load, with no input power at all.
The amplifier modules meanwhile, have shut down due to excessive reflected power.
If you’ve guessed “combiner issues”..you’re well on your way. After a quick stop for Tylenol (trust me, you’ll need it)..we remove the 10,597 screws and open the splitter/combiner.
This is the Splitter. It takes roughly 65 watts from the exciter and divides it between the five PA amplifiers. Just another Wilkinson design with some metering and ceramic reject load resistors.
It was working fine, so we’ll leave it alone.
Here’s the PA combiner. Not much to see here. But if we flip the assembly over…
Five metering and reject load assemblies. Each of these resistors is capable of dissipating 500W. Two in parallel should then be able to handle the entire amplifier output, at least for a while.
Until lightning comes along and you get this..
Armstrong had the resistors in stock, although you can also find them on Ebay and other places.
There. That’s better.
Plenty of fan in the combiner to cool those reject loads.
Under normal conditions they only dissipate a few watts. This unit is running with 7W reject (4500W out).
If one of the amplifiers shuts down however the combiner becomes unbalanced and those fans have work to do.
These 1kW amplifier modules are HEAVY. About 100# each. Of course you don’t know that until you slide it out of the rack and… well, it won’t be going back in without a helper.
Two big torridal transformers each add a lot of weight. That is an enormous switching supply right next them, an all-week recap project as there are five of these boards in the transmitter.
Armstrong/RVR uses these everywhere. And I mean Everywhere. Each amplifier has Six of them.
They are easy to replace, right on the back panel. They are not however inexpensive. $7 each at Mouser.