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Last Update 5/19/04

DRS - Broadcast Technologies
Continental Electronics
Dallas, Texas
Phone: (800) 733-5011

Clay Freinwald shares his plans for modifying the air filters for the 816 series.



Clay Freinwald  -  February 12, 2001




Back in the late 70’s our trusty Collins 831G-2 ingested its factory provided Foam Intake Filter.  At that time I chose to not replace the foam an opted to have constructed what is often called Clay’s Teepee to house Two of the FARR 30/30 pleated air filters.   This method of filtration proved to be superior to the old Foam Pad provided by Collins.   The reason for using Two filters was to reduce the velocity through the filters.   For some time Continental has been showing a sketch of this filter arrangement for use with the 816 Series transmitters in their Transmitter Training Classes.


More recent experiences with newer filters with much greater filtering efficiency (95%) led me to modify all of our previous  “Teepee Equipped”  FM transmitters.   This new method is actually easier to install and maintain and the results are nothing short of outstanding!




1 - Filter Frame, 20 by 20 inches

             FARR Model 079473-002  (You don’t need fasteners to hold the filter in the frame)

 1 - Air Filter, 20 by 20 by 12 inches

             FARR Model Riga-Flo, P-Series 200, 90-95% Efficiency Part # 122556034

             (FARR can be contacted at 1-800-432-7726)

 1 - Manometer

             Dwyer Model 25.




·         Prepare the Filter Frame by attaching a piece of aluminum, centered, on one side of the frame to which you will mount the manometer.  This need only be larger than the Manometer (Use Pop Rivets so as to not intrude on the inside of the frame thereby preventing insertion of the Filter)


·         Mount the Manometer on the Aluminum, using 6-32 hardware.  (Make sure your mounting screws are level as its important that the device be level, also make sure the screws are just long enough to mount the manometer and not interfere with the installation of the filter.) 



·         Remove the old Air Filter from the Transmitter and discard.  If your transmitter has a fan guard (made of expanded mesh) discard this as well as it will add about .1 inch of static pressure.


·         Attach the Filter Frame to the top of the transmitter with RTV or other suitable adhesive/sealant.  There is no need to use fasteners, however you want to make sure that the frame is air tight to the transmitter top.


·         Make sure that Manometer is level, adjust as required.


·         Now that the Frame and Manometer are attached to the transmitter you may now add the Red Gauge Oil.   Start by adjusting the calibrate knob (on the lower Left) to the middle of its range.  Then CAREFULLY pour the oil into the device (WARNING - DO THIS SLOWLY AS ITS EASY TO ADD TOO MUCH)  When you have reach the point where the Oil is just visable on the scale...STOP!!   Adjust the device for a Zero Reading.


·         Discard all but 3 feet of the supplied Manometer Hose.


·         Separate the two hoses so that you have one long piece (about 3 feet) and one short (about 1 foot)


·         Connect the hoses to the top of the Manometer with the Long Hose on the Manometer INPUT and the Short one to the other connection.  




·         The long hose must penetrate the top of the transmitter and then enter the space below the Filter.  (Goal is to sample the Pressure Drop across the Air Filter YOU DO NOT WANT TO SAMPLE PRESSURE UNDER THE FAN BUT RATHER OVER IT)  The end of the hose must be secured and should be dressed in such a manner that it is parallel to one of the side of the transmitter fan housing to avoid picking up the turbulence of the fan.


·         First - Carefully locate a place to drill through the top of the transmitter cabinet close to the edge of the filter frame.  Check under the location you wish to place the hole to make sure that there are no major metal parts or components.  I suggest you have someone hold a damp rag under the area of the hold to catch any stray filings.


·         Drill your hole just larger than the size of the Manometer Hose


·         Next - Drill a hole in the SIDE of the fan housing, near the point where you have drilled the transmitter top.  Drilling from the top.


·         Seal both of these holes, around the hose, using RTV etc.


·         Dress the hose from the point that attaches to the Manometer down along the side of the device and then to the point that you have entered the fan housing.  Remember not to cut the foot long portion of the house (to goal is to have it sample the air pressure within the room and be out of the way of the actual Filter Intake turbulence.  You may secure this with small clamps or a dab of RTV.


·         Set the Air Filter in the Frame.  You will not require any fasteners as the filters weight will keep it in place.


·          Using a Felt Marker etc., note the date of your installation so you can keep track of same.  This is important as the ONLY way to tell when the filter needs changing is by measuring the pressure drop on the manometer.


·         Turn on Transmitter Filaments and note initial pressure drop across Filter.   Depending on the dynamics of your location, fan type, speed, elevation etc. this reading will vary.  The Manometer should come with an adhesive marker to place on the scale of the Manometer to mark the starting point.


·         If you started out with a nice clean transmitter, you will, in short time, notice the DRAMATIC improvement in that much transmitter cleaning will become a thing of the past, with the exception of wiping down some H.V. Insulators....THAT’S IT !   Pollen, Exhaust Carbons, Dust etc. will remain on the outside.


·         The Manometer will tell you when its time to change the filter.  (The factory recommends that you change the filter at 1.5 inches may want to change it sooner)  You will be amazed at how long this filter will last...and how much it will hold.   You need to remember the considerable amount of filter media inside this unit (39 Square Feet)

      I have found that the pressure drop on a new filter will be about .3 inches, I have been changing the filters at .5 inches.


·         If you REALLY want to have a clean plant, consider the installation of a blower system that creates a positive pressure within your transmitter building and pre-filter the air using the same filter.


If you have any thoughts and/or questions, feel free to give me a call @ 206-726-7071 or you may email me at


Good luck and enjoy your new CLEAN transmitter!