When you find a circuit breaker that has failed open, it
is improper to bypass it with a jumper wire.
Fig. 1: Temporary fuses to replace a
failed circuit breaker.
Instead, use a fuse in place of a breaker on a temporary
basis. After all, the breaker was there for a reason: to
protect wiring and circuits from catastrophic failure!
Inline fuse holders are great for fuses up to 20
amperes. I've soldered wires to larger fuses when a fuse
holder was not available. Attach a paper note on or near
the breaker handle, indicating to leave the handle down
and that a fuse is being used instead on the back side.
Remember, this is a temporary fix
until a replacement breaker is installed.
Also, when you make a change or have information that
might be helpful in the future, make a notation in the
equipment manual. Do not use
an ink pen, use a pencil so you can erase the note, if
the change you make doesn't work out as planned.
AIR PRESSURE SWITCHES
One of the most common problems in a transmitter is an
air pressure switch that won?t let the transmitter run,
even when the cooling fan is working. Those switches
were put there for a reason. They protect the
transmitter from lack of cooling. We know that all
blower motors eventually will fail from bearing or
electrical problems. That is why transmitters are
protected from catastrophic failure by air flow or air
Bypassing a switch leaves the transmitter open to
serious problems when the blower does quit entirely. The
cost can be very high, not to mention the bad reputation
you might get for doing the wrong thing.
Fig. 2: A clogged honeycomb air
filter from a Continental FM
Fig. 3: Cleaning cups on squirrel
Don't assume that the air pressure or air flow switch is
bad when it might be trying to tell you there is a real
Check the air path before making any attempt to
recalibrate an air switch. The metal honeycomb air
filter in Fig. 2 is a common example of how airflow can
be restricted due to lack of maintenance. Use a mirror
to examine the air holes to verify all is well. Airflow
is also often reduced by dirty blades in squirrel cage
The cupped blades fill with dirt, thus reducing fan
efficiency and allowing transmitter component
temperatures to rise. I typically use a flat screwdriver
to scrape dirt away.
Check the transmitter's instruction manual for a
procedure to set air flow or pressure switches. Lacking
good directions, my approach is to adjust an air sensor
so the transmitter air light shows normal when the
blower is running and the high voltage is off. Then open
a PA cabinet door about 1 inch. The air light should go
out. Adjust as necessary to get it right. Test it
I recommend doing this whenever you replace a tube in a
transmitter. It just takes a couple of minutes to save a
major headache and expense later.
CHECK THE TEMPERATURE
Fig. 4: A thermometer atop a transmitter
Fig. 4 shows a thermometer on the air output port of a
transmitter. It is just one more performance indicator
like a voltmeter, ammeter or power output meter.
I typically use an inexpensive Taylor brand zero to 220
degree F thermometer. They are available in the cooking
utensil department of many local stores for under $10.
My recommendation is to purchase two thermometers for
each transmitter. Use one at the transmitter's air inlet
and one at the air outlet. The temperature difference
should stay constant. Incoming will likely be cooler in
the winter and warmer in the summer. Just add the normal
difference to find what the output stack temperature
should be. Look in the transmitter's instruction manual
or check with the manufacturer for normal heat rise
across the transmitter at the power level it is running.
Don't be surprised if the temperature differential on a
20 kW FM tube transmitter is 100 degrees F. Make notes
on what temperature is normal and use the thermometer as
one of the many readings you take at the transmitter
site on a regular basis.
Fig. 5: Four mice dead from high
MICE IN THE BUILDING
You've seen it before. Dead mice in the high voltage
section of a transmitter. Remember, you could be just as
dead if you touch high voltage!
Use a high voltage shorting stick, as I described in an
RW article in April 2016 (radioworld.com,
key term arc gaps). You will be surprised and still
alive when a loud flash and bang happen on a circuit
you just shorted to ground and thought was dead. Safety
Do your best to keep mice out of a transmitter building
by plugging any openings as little as 1/4 inch wide.
Mouse traps, in my opinion, are a Band-Aid for the
problem. Keep the mice out, so you don't have to clean
up their messes afterward. Scout out and cover any
openings that can invite mice and other critters.
When checking a diode, don't assume that failed diodes
always short. Yes, that is the most common failure mode,
but they can fail open.
Fig. 6: A transistor with excessive heat
Case in point: a Collins 5 kW AM transmitter that would
not turn on. The problem was an open steering diode in
the logic circuit. A glass-cased diode opened after 40
years of age and temperature cycling. It happens!
A similar situation occurs with transistors. I often
find open or ?no gain? transistors in the exciter of
Continental 315F/ 316F 5/10 kW AM transmitter. Checking
for shorts with an ohmmeter does not find the bad ones.
Harris MW-1 AM transmitters have a similar problem. I
use a Sencore TF-46 Super Cricket Transistor & FET
Tester. With it, I check for leakage and even match
transistors for gain in the RF section of a module for
Heat sink compound is used to help transfer heat from a
transistor to the heat sink it is mounted to. More is not better.
Best to put a bit on and rub it around so there is a
thin layer on the transistor before it is bolted down.
The photo shows the white compound on transistor leads.
Bad choice! Keep the leads clean so they can make good
contact with the socket they mate with.
Fig. 7: Too little heat transfer
Fig. 8: Burned Harris MW-1 PA module
Fig. 7 shows just the opposite. Too little heat sink
compound. In either case, the transistor will likely run
hotter than it should and will fail early. Heat the bane
Many radio broadcast engineers have worked with the
Harris MW-1 AM transmitter. There are 12 output modules,
each of which plugs into a circuit card edge connector.
You need to not over-tighten the mounting screws that
hold the module in place but make sure they're snug.
They also provide an RF ground to the module. If you
forget, RF current will attempt to flow through the
socket, then burn the edge connector and socket it plugs
into. If the damage isn't too great, the circuit card
can be repaired using copper tape over the burned
connector, as shown in the photo.
By the way, you are caught in the middle on this one.
Over-tightening the module screws will strip them out of
the threaded aluminum sheet they go in. Just use common
sense and don't overdo it.
On a related matter, I learned from a factory
representative years ago that the transmitter will have
about one module failure per year. That is normal.
Losing more is likely a symptom of other problems.
It makes perfect sense to think these things through
before jumping into mistakes.
By the way, there is a free audio version of my father's
book, "Where Have All the Broadcasters Gone" available
on my website at: http://mwpersons.com/books.
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