The original V series Nautel transmitters have required a couple of firmware upgrades in some cases.
Upgrading the PA module firmware on Nautel V-5D transmitter, WDVT, Rutland, VT
The first was for the controller to add a little bit of bias to the PAs during analog operation. The second one I have had to do is to the PA modules themselves which were to keep the power supplies from shutting off during re-transfer from Generator power to commercial power.
I have done several of these and once you get the hang of it, it only takes a few minutes to complete. Still, I remember when transmitters didn’t have firmware. The low voltage control circuits were either 120 or 240 VAC with big relays and contactors that loudly confirmed their closure before any meters began to move.
Regarding Nautel transmitters in general; the newer models are not the same rugged, reliable designs that were common in the past. We have AM ND series transmitters that have been on the air for 20 years without a single failure. The models rolling out of the factory these days often have switching power supplies fail without reason or warning and RF pallets that are fragile things. Ah well, I suppose all things are cyclical.
Not exactly sure how it happened, but one of our Nautel transmitters malfunctioned! It is a pretty rare event, so I thought the exclamation point was needed. One of the PA pallets went bad and the transmitter lost 1/2 a PA module. Since the TPO for this particular station is 7 KW, they remained on the air at full power. In the interest of staying on top of things, we fixed it anyway.
Diagnostics were simple:
Fault lights on the front of the transmitter observed
Press the status button to find out faults, which were Module D failed
To to module sub-menu, find Module D and discover Q1 disabled, and Q3 shutdown.
The problem is with Q3, order a new pallet from the manufacture
Upon removing the module, I did not see the damage at first:
Nautel V series FM transmitter PA module
It is board A3, which for this particular flavor transmitter is a Nautel Part number NAPA16-B. Once I replaced the defective module with a new one, I discovered what looks like a symptom of the greater problem:
Nautel NAPA16-B defective board
Over to the left-middle-lower section of the board, R10 and R8 are burned open. These are surface mount 2-watt, 20-ohm resistors. A glance at the schematic shows that these are part of the bias supply. A quick set of measurements with a DVM shows that Q1 seems to be intact and not shorted. Interesting…
The question is: Is it worth trying to fix this board or should I just trash it an buy a new spare?
In the previous post, the issue with the WVOS-FM transmitter was detailed: The PA feed through/bypass capacitor had arced to the PA cavity causing lots of unwanted off-air time. When I went to order the replacement parts, of course, they were not available. It seems that Broadcast Electronics changed the design of its transmitters in the late 1980s to use a different feed-through arrangement.
They were nice enough to send us a nifty retrofit kit; BE part number 959-0272 which replaces BE part number 959-0115. If interested, the six pages of installation instructions are available here, for your reading pleasure.
The retrofitting itself was quite the job; drilling six mounting holes and one one-inch feed through hole in the PA cavity, mounting the new feed through housing, rewiring the high voltage connection to the tube and back to the HV bleeder assembly, etc. What with all of the drilling, sawing, filing, deburring, and whatnot, I began to wonder if the transmitter would ever run again. This is the transmitter before the modification:
Broadcast Electronics FM3.5A PA cavity
This is the old high voltage feed through hole, arc mark clearly evident.
Broadcast Electronics FM3.5A old high voltage feed though
This is the modified feed through/bypass configuration.
Broadcast Electronics FM3.5A new PA feed through/bypass capacitor
While doing this work, I removed the tube and put a plastic sheet in the bottom of the PA cavity and around the HV parts at the bottom of the transmitter. Somehow, getting aluminum filings in the tube socket seemed like a bad idea. I also thoroughly vacuumed out the entire transmitter once all of the metal work was done.
I removed the Kapton capacitor plates from the old feed-through arrangement and reinstalled the Teflon insulating plates to keep the air flow out of the tube cavity going in the correct direction. The new capacitor looks very beefy, perhaps it will never fail again.
Once the installation work was done, I brought up the transmitter first with no screen and no connection to the tube anode. Then with the tube connected, and finally with the screen supply turned on. The tuning needed a brief touch up but all in all, the transmitter came up and ran well with the new feed-through arrangement.
This Broadcast Electronics Fm 3.5A will be thirty years old in April. We should have a party!
Broadcast Electronics FM 3.5A, WVOS-FM, Liberty NY
Unfortunately, this transmitter is not doing too well these days. The PA high voltage feed through capacitor has arced over to the PA cavity, causing the station to be off the air.
BE FM3.5A HV feed thru capacitor
Naturally, this happened over the weekend, parts will not arrive until Tuesday at the earliest, and the station is without a backup transmitter.
Obviously, trouble shooting this was a two-person job. Never work alone on HV equipment. The symptom was the main circuit breaker was tripping after the HV on command. Starting from the transformer end of the HV power supply circuit and working toward the anode of the PA tube, all of the components were tested by isolating each component then turning the HV on. Special care was taken to discharge all components after each test. The capacitors and bleeder resistors were reconnected at the same time. There is too much risk involved with charged 8 KV capacitors and no way to bleed that charge to ground. Everything worked up until the PA cavity was reconnected (without the tube), then the breaker tripped again. Thus, the above feed-through capacitor was removed and disassembled, revealing the damage.
The question is, how long should transmitting equipment last? After all, if one were running a freight delivery company, you would not be driving around in thirty-year-old trucks, would you? No, not if you wanted to stay in business. Like all electro-mechanical equipment; transmitters, consoles, STLs, antennas, computers, etc wear out. A smart plan would be to have a replacement schedule and be putting money into a capital equipment replacement fund. Equipment life varies with the type. Getting twenty years out of a main transmitter is a pretty good service life, going beyond that is pressing one’s luck. Ten years on any one computer is a very long time. Then there are certain transmitter manufactures that drop support on older units, which makes it difficult to keep them operating. Owners and managers need to be cognizant of the age and condition of critical infrastructure. As field engineers, how much time do we devote to keeping antiquated equipment running, or should we even be servicing it at all? As independent contractors, we incur a liability whenever we touch something. Where does the ownership’s responsibility lay in providing safe, functional equipment for their stations? All interesting questions.
