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My Receiver has lost Sensitivity!

By Kevin Custer  W3KKC
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Concept:
Some models of the GE MASTR II, Exec II, MVP and Delta receiver front-end helical resonator castings lose tuning and sensitivity

Problem 1; RF Conductivity - Tuning Screws:  In some MASTR II (style) castings, the helical tuning threads may have lost their ability for good RF electrical contact with the casting.  Why? Only the surface of the casting is conductive (RF wise).  If the locking nuts on top of the casting have severely chewed into the casting metal under the surface, the tuning screws may have lost RF conductivity.

Solution to Problem 1:   Remove the original locking nut with the teeth. Remove the tuning screw and lubricate it with electrical contact grease like those sold under the name of Penatrox, Noalox, etc. Replace the tuning screw.   Place a split lock washer *not a star washer with teeth* over the threaded tuning screw. Place a a stainless steel hex nut on the adjustment screw and secure it to provide just enough tension to keep it from moving.  The use of a wrench or pliers to hold the lock nut may be needed while readjusting the helical screw.


Problem 2, Growing Tin Whiskers:   Receiver front end castings with a shiny appearance, made before approximately 1983 *may* exhibit the "tin whisker" problem:

The cause of the sensistivity loss is that tiny tin whiskers growing inside casting walls may short out the coil or tuning capacitor.   Frequently the whiskers are difficult to to see without magnification, or light at an angle that causes it to reflect off teh whisker.   Disassembling the casting and use of a movable light source and possible a magnifying glass will reveal their existence if they are present.   The whiskers may appear to be silvery or a slightly greenish in color.   They are created *possibly* because of a reaction of the plating metals with the atmosphere.   The picture below was taken of a 1977 GE MASTR II VHF helical casting.

Solution to Problem 2: Adjust the helical tuning screws all the way to the top (screws sticking out of casting).   With a cleaning spray that leaves no residue (NOT lubricated tuner cleaner, or WD40, use chemical 111 or the like), wash the inside of the helical resonator cavity.   A small brush or Q-Tip type cotton swab may be helpful.   After thoroughly drying the interior of the helical casting, coat the interior cavity walls, coil and tuning capacitor with a coat or two of clear lacquer or enamel spray.   Krylon or other brands will work.   This last step is to try and seal the surface from interaction with the atmosphere.   Two thinner coats are more effective than one thicker coat.   If the inside of the cover of the casting is shiny and shows whiskers, treat it the same way.

CAUTIONS:

  1.  Chemical 111 is a very nasty solvent, so handle it as directed.   111 is found in solvents like soldering Flux Remover.
  2.  Be careful not to damage the tuning capacitor when adjusting it all the way to the top of the casting.   Do Not force the adjustment.
Notes:  Not all shiny castings have the whisker problem.   Few reports of problems stem from early production, before 1983 in MASTR II *type* equipment.   Also, castings made after July 1994 are now being manufactured again with a shiny finish but they are not using the same process and plating recipe that caused the earlier problems.   The problem-prone dull gray casting finish was used for many years.

NASA and other satellite operators are very concerned about the "tin whisker" problem as several satellites have been lost due to electronics failures that have been traced to whiskers.   Search the web for "tin whiskers".

Here are two NASA videos:     (no, your speakers aren't broken, they are silent videos)

Tin Whiskers on Tin Electro Deposit, Beryllium Copper Circuit Card Retainers   a 1 minute 16 second video formatted as a 4.94 MB WMV file

Optical Inspection Techniques for Metal Whiskers: Electromagnetic Relay Header and Ternminals   a 3 minute 19 second video formatted as a 12.1 MB WMV file
This video shows tin whiskers on the case and terminals of a hermetically sealed multipole relay.


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Picture of the whisker growing was provided by Tracy Hooker KA5ECS.
Original source has asked us to remain anonymous
HTML Copyright © March 2001 by Kevin Custer W3KKC
NASA videos provided by Gary McDuffie AGØN

All Rights Reserved, including that of paper and web publication elsewhere.