09-08-2012, 03:35 PM
TRANSMITTER MAINTENANCE
TRANSMITTER MAINTENANCE.pdf (Size: 1.11 MB / Downloads: 156)
Transmitter RF sections are ever more reliable; most problem that will
keep you off the air are control-circuit problems. The time to learn the
control circuit is before you have a problem. Don't bypass overloads
or other protection devices. Post a control-ladder for you station with
all external interlocks shown. In my experience, the problems that
keep you off the air are control circuit problems. If that ratio has any
relation to reality, it makes much more sense to concentrate your
familiarization of the transmitter to its control circuits. Memorize the
control circuit if you can. Remember, there are really two stages to get
a transmitter on the air. Close the interlocks. That turns on the bias in
a tube-type transmitter and puts a solid-state transmitter in the ready
mode. The second stage is actually getting the high-power circuits to
operate. Once the first step has been completed, the second step is
almost trivial.
Visual inspections can resolve or localize over half of all control circuit and
RF circuit problems. The first step in trouble shooting is a careful visual
inspection. An inspection mirror is invaluable, keep one handy. Keep
trouble lights and flashlights handy and ready. Most RF problems have
some physical manifestations. If it failed, i.e., there are probably broken
pieces lying around. Control circuits also should be inspected first since most
problems start there. Don't forget built-in diagnostic help. For example, the
802D Digital Exciter has a full help section built in with answer to questions
slightly more important than whether the power cord is plugged in.
Prepare for problems. Are your critical schematics enlarged and
covered in plastic? When you’re working on a transmitter, you
don’t want to tear a critical schematic. Learn the control circuit!
Use control ladder diagrams. Put a copy of the control ladder
diagram on the wall. Be sure to add any external connections to the
control ladder drawing. Learn how the computer monitors the
control line and the limitations of the computer's diagnostic ability.
Understand the RF-flow path. Know the normal currents and signal
levels throughout the transmitter. Keep tools specific to the
transmitter on site. Keep a separate VOM at each site.
What not to do, i.e. what you
don’t do, is almost as important as
what you do. Don’t change the
tube. One of my Rules of Radio
is: “The problem is never the tube,
and if you change the tube, you’ll
break something.“ It is also clear
that if you change the tube and don’t solve the problem, you’ve
introduced a new variable into the situation. Don’t retune the
transmitter. If the tuning has changed, you need to determine why.
More than likely it’s a load problem.
Don’t replace components that aren’t bad. Frustrated engineers
sometimes replace items that are perfectly good and you may wonder
why. They just want to try something--anything. One time in a
thousand it works. Which means that nine hundred and ninety-nine times out a thousand, it doesn’t. Not
very good odds are they? Don’t re-design the transmitter to avoid the problem. One engineer added a
slave relay to his interlock system because the low current in the transmitter control circuit wasn’t
sufficient to keep his interlock contacts clean. The easy way would simply have been to increase the
current in the interlock circuit, a change that would use one resistor. Instead, he wound up with another
relay and a new control circuit.
What to do when trouble strikes is straightforward: Make sure you have good primary power in advance.
Make full inspections of the transmitter and load. Perform basic circuit checks. Formulate a clear
description of the problem in with all the details. Compare your meter reading to the factory test data--
you do have copies on the wall handy don’t you. Call the Manufacturers field service Department for help
if you get in over your head. When you talk to a service engineer, identify the transmitter model clearly:
18 kW FM isn’t a type number that the engineer will immediately identify with. I spent a good fifteen
minutes one night trying to communicate with an engineer who accidentally gave me the model number of
an AM transmitter. He really had an FM transmitter and he thought I had lost my mind when my
questions didn’t make sense. Brace yourself for the possibility that the engineer on call may already be on
a telephone call with another customer. No one wants to wait when he has a problem. But sometimes it’s
inevitable. Don’t get mad. Just get better prepared.
A clean, cool transmitter will work better and
last longer than a hot, dirty transmitter. Inlet
air volume must meet the exhaust volume
requirements. Positive pressure is a positive
idea. Avoid directly attaching the transmitter
to an exhaust s system. If something happens
to obstruct the exhaust line or the building air
inlet, the transmitter can be starved for airflow
even though the air pressure is normal. A
good alternative is to put a duct for the air
exhaust one-Foot above the transmitter. If the
transmitter room is adequately supplied with
air, the hot exhaust air will be ducted out of the
room. If something blocks airflow, the
transmitter can still maintain airflow over the
tube anode or the transistor heat sinks. Watch
for obvious indications of air starvation. The
notion of positive pressure is simply that you try to push more air into a transmitter room than the
transmitter can exhaust. This tends to push dirt and dust out of the transmitter building as it settles on
the floor.