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. That means a lot of labeling, unwiring and disconnecting to remove it. This time the negative supply voltage failed. It’s derived from the positive regulator with a series of capacitors and diodes. The capacitors fail and the unit shuts down. The PA output device had also failed, see below.
Sometimes you have to dig to find the failed components. Other times you don’t.
The ceramic cover was blown right off the FET.
Fortunately the final device is still available. Not cheap, but available.
Idle bias current needs to be set. There is a small, flimsy trimpot to adjust it on the RF pallet. Those trimpots are peppered all over Nicom equipment and are trouble. Also check every solder joint. The factory apparently doesn’t.
Oh boy. This is an early Armstrong Solid state FM Transmitter. Closing in on 20 years service.
These units were built by RVR in Italy and badged, (and fortunately still 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.
This transmitter is showing 296 watts into the reject load, with no input power applied.
The amplifier modules have shut down due to excessive reflected power.
The combiner is the likely failure. Remove the many, many screws and open the splitter/combiner chassis for a look.
This is the Splitter portion. It takes roughly 65 watts from the exciter and divides it between the five PA amplifiers. 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.
Five metering and reject load assemblies on the underside. Each of these resistors is capable of dissipating 500W. Two in parallel should then be able to handle the entire amplifier output.
Until this happens.
Instead of reading 100 ohms, this one read zero. Armstrong had the resistors in stock.
There. That’s better.
Plenty of fan capacity in the combiner to cool those reject loads. They look much better cleaned up.
Under normal conditions they only dissipate a few watts. This unit is back up running with 7W reject (4500W out).
Like most solid state modular transmitters, this one will still run at reduced power with a failed amplifier, but the combiner unbalances and the fans have more work to do.
These 1kW amplifier modules are heavy, largely due to the two large transformers. About 100# each. They won’t be going back in the rack without a helper.
To the right is the main switching supply, an all-day recap project as there are five of these boards in the transmitter.
Armstrong/RVR uses these fuses everywhere. Each amplifier has Six of them, in addition to two mains breakers each. They are easy to replace, right on the back panel. About $7 each.