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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401

 


Technical Tips from Mark W. Persons
Harris MW-1 PA Module Repair


Those who work on Harris MW-1 or MW-1A AM Transmitters will recognize these 14-inch long power amplifier modules.  It takes thirteen of these to equip a transmitter to run at 1000 watts.  One is an RF driver and the other twelve are power amplifiers.  Each module has two RF transistors and five audio transistors.  Depending on conditions, it is expected that there will be one module failure on average per year in these transmitters.

We repair these modules by first checking each of the transistors for gain and leakage when each device one has been removed from the circuit.  Just checking for a short in the transistors is not enough as some of them go open, and will not show a short.  Then, we check for any shorts across the insulators under each transistor.  Careful attention is paid to putting just the right amount of silicone-based heat transfer compound on both sides of the insulators.  We also check the diodes and fuse.  Any new replacement components are checked before installation.     

When we repair modules for clients, the charge is about an hour of shop time ($85) plus parts.  Module repairs typically cost the client $150 plus shipping.  We test modules in a transmitter to make sure they are working properly.

When reinstalling one of these modules in a transmitter, we recommend making sure it is fully pushed down into the edge-connector socket.  Then, both top mounting screws need to be snug, but not over tightened.  Too tight and the screw will break or strip the threads.  These screws are the RF connection to the transmitter chassis.  Failure to tighten these two screws will result in burned contacts on the bottom edge-connector and socket.  Repairs are definitely more expensive then.     


The above photos are of a module which has burned contacts on its edge connector.  This happened because the two mounting screws were not tightened down in the transmitter where the module resides.  Those screws, near the top of the module, are the RF ground.  If the module cannot find a proper ground there, it will attempt to use the power connector at the bottom and will burn the contacts on the board and in the mating socket in the transmitter.  
When we run into module damage like this, we clean the fiberglass board and then use a two-part epoxy to build up the circuit card to its original dimensions.  Then we add adhesive-backed copper foil.  That copper is then soldered to some of the original traces on the card to give a good low-resistance path for power.


The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   Ham WMH      

     Questions?  Email Mark Persons:  teki@mwpersons.com      

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page last edited 09/03/2016