Dave Schwarberg
CINCINATTI —WCPO-TV, the ABC affiliate here, uses a 54.4 kW TPO Axcera Visionary UHF two-cabinet transmitter that first went into service in 2003 on Channel 55. It was converted to Channel 22 in 2010 and upgraded from IOTD2100 tubes (23 kW) to the higher power IOTD2130 (30 kW). An interesting problem occurred in April when we had a primary fuse blow in the circuit feeding the transmitter, shutting off gun and cavity air blowers. After restoring power, we found that we couldn’t bring the transmitter back up due to constant crowbarring.
COULDN’T HAPPEN AT A WORSE TIME
Unfortunately, this outage occurred shortly before ratings sweeps. While we were able to stay on the air at half power with the other amplifier, we really needed get back to 100 percent operation as soon as possible.
After several attempts to solve the problem ourselves, we called the good folks at e2v. (And this is where that company shines the most, as when you give them a call there’s always a live person on the other end. ) In no time we were connected with a couple of superior troubleshooters in e2v’s New York office, Dominic Piarulli and Mark Strohecker, who offered their services.
They suggested that we roll out the heavyduty test equipment, including a Hipotronics high voltage tester, to try and rule out failure of several transmitter components that could be causing the crowbarring. However, after completing these tests nothing unusual was found, pointing us back to the IOT with a suggestion that a thorough grid outgassing might be in order.
Dominic and Mark had theorized that the tube’s grid had overheated due to loss of the blower and that the high temperature liberated some entrapped gas.
(For those not familiar with grid outgassing, it involves connecting an external DC power supply to send current in the reverse direction through the grid element in a slow and controlled manner so as not to cause it to overheat or overload the ion pump associated with the tube.)
CURE FOR A BIG GAS PAIN
After our instruments showed that the outgassing process was successfully completed we were able to get the tube to take high voltage, and after a little retuning we were making power and back in business.
The wonderful phone support from Mark and Dominic couldn’t have been more helpful at a critical time. In addition to offering troubleshooting tips, they even sent us a loaner IOT to help us locate the problem.
In my experience, I’ve found that in happier (normal) times, e2v’s IOTs are really a dream to tune and operate. They feature a typical gain of some 22 dB, a very low noise floor and a beautifully linear sweep throughout the entire channel bandpass. This sort of performance, along with the level of customer support and service supplied by e2v, will certainly be factors to consider when it finally comes time to retube the transmitter (and based on our experience, this will probably be quite a ways into the future).
David Schwarberg is a transmitter engineer at WCPO-TV and has 46 years of RF experience. He may be contacted atdschwarberg@wcpo.com.
For additional information, contact e2v at 800-342-5338 or visitwww.e2v.com.
