Reliability of modern equipment can reduce our maintenance duties significantly; some gear needs only to have its dust blown out once a year. But not so with older tube transmission equipment that we may need as our backup or even our main transmitter.
Cleaning out dirt and dust is fine, but your rig also needs a thorough visual inspection if you are to assure reliability. Use a strong trouble light to inspect all the dark recesses.
Fig. 1: Damaged components are lurking. Give your transmitter a thorough inspection. Before you begin, bring someone with you for safety’s sake. Also disable the remote control. There’s nothing scarier than hearing the click of relays as the air talent tries to bring up a transmitter you’re working on.
Watch the high voltage meter as you turn off the transmitter. The meter indication should drop quickly. If it does not, or if the needle hangs around the normal reading when the transmitter is on, suspect bad bleeder resistors. This indication makes the next few steps particularly important.
Turn off all breakers for the transmitter and use a shorting stick to discharge all components. The bleeder resistors and the transmitter interlocks are supposed to do this job, but the interlocks may have been defeated, or the bleeders are bad.
If anything live exists, the shorting stick will find it —and you’ll know it. One crack of a high-voltage discharge will be all you need to be reminded to use this safety device. Many seasoned engineers nickname the shorting stick a “Jesus stick” — because if you don’t use it, that’s who you’re going to meet. Yes, it’s that serious.
The stick’s purpose is primarily to discharge large power supply capacitors, usually found in the bottom of the transmitter. But I recommend you touch all components. You can’t do any harm, because the transmitter is off, right? One engineer told me about a transmitter that was fed by three separate circuit breakers. Two were in one box marked LV (low voltage) and HV (for high voltage). In another box, across the room, was another circuit that the shorting stick discovered. The arc took a chunk out of the shorting stick! Better the stick than your hand or arm.
So what will your inspection disclose? Well, with trouble light in hand, maybe nothing. Then again, you may see damage as in Fig. 1.
Contract Engineer Mike McGowan found this arced grid tuning variable capacitor during one of his inspections. In this case, the rig still worked, but wouldn’t tune properly.
As you inspect, look for anything unusual. Check wiring terminals — are they tight and not corroded? Check those oil-filled power supply capacitors for leaks. And speaking of leaks, is the blower motor in good shape and not leaking oil?
You can buy clean T-shirt material online, or purchase Scott Rags in a Box at a hardware store. Clean rags are great for cleaning when paired with isopropyl alcohol. As with all cleaning, use plenty of ventilation.
As you clean, look at those bleeder resistors — the foot-long, tubular wire-wound resistors, usually with a ceramic finish. Corrosion on the terminals or along the exposed wire can prevent these resistors from doing their job, which is to discharge the high-voltage power supply when the transmitter is turned off.
Fig. 2: You can use sliding doors to hide equipment racks. Remember when we checked the plate voltage meter while shutting the rig off? Suspect one or more bad bleeders if that voltage indication didn’t drop to zero quickly. After you get the coating of dirt and grease off the wiring and components, suspect anything that doesn’t look right.
Your camera phone is a real asset in the inspection task, not only for pictures to submit to Workbench, but also to identify parts or sub-assemblies that need repair or replacement. The phone can also serve as your eyes in hard to reach places, ensuring a thorough inspection. More than one engineer has sent his picture to the repair/parts depot to identify a particular component.
As you clean, be careful not to jar or mis-form coils. Especially at FM frequencies, even a slight bend of wire can mistune a circuit. If you’ve never cleaned your rig, get some help before you begin. Someone who is experienced with that particular transmitter can teach you a lot.
Lloyd Collins, director of engineering for GoodRadio, offers an alternative to hinged doors when you want to hide your equipment racks. The sliding door panels seen in Fig. 2 keep things hidden but can be slid to one side or another for maintenance.
Should you need all the racks exposed, you can simply lift the doors off their track and replace them when your work is completed.
Reading Workbench is like taking a college course in hands-on radio problem-solving! Contribute your ideas, help your fellow engineers and qualify for SBE recertification credit. Send tips to firstname.lastname@example.org. Fax to (603) 472-4944.
John Bisset has spent 44 years in the broadcasting industry He handles West Coast sales for the Telos Alliance. He is SBE certified and is a past recipient of the SBE’s Educator of the Year Award.