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My Receiver has lost Sensitivity!
By Kevin Custer  W3KKC
Maintained by Mike Morris WA6ILQ 		Print this Page

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 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 the 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 "whisker" problem as several satellites have been lost due to electronics failures that have been traced to whiskers. The first published reports of whiskers date back to the 1940s and 1950s and are from some tin alloys, zinc, cadmium, indium, antimony and silver among others. Whiskers are growths that are single crystal wide and up to 10mm long. Most whiskers fuse open anywhere from 10 to 50 milliamps... .

Click here for a comparison photo of a human hair versus a whisker.

Some of the pages below are focused on "tin whiskers". Note: Whiskers are NOT dendrites. The page below explaing the difference, among other things.

The NASA info page: https://nepp.nasa.gov/whisker/background/index.htm.

Here's the Wikipedia page on Whisker (metallurgy).

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.

Additional reading:

A full-page advertisement for Bell Telephone Labs in the December 1955 isue of Radio-Television News described tin whiskers......
A comment from the page maintainer: Whiskers were a known problem in the 1940s - long before the lead-free (Pb-Free) craze in the early 2000's... In a rush to reduce the presence of lead by a miniscule fraction in the European Union electronics industry they created a huge problem of tin whiskers growing out of lead-free solder joints. These whiskers caused short circuits between adjacent circuit elements: closely spaced connector pins, between leads on fine pitch integrated circuit packages, with high density surface mount circuit board layouts, etc. Military and aerospace engineers and scientists had fits initially trying to figure out what was going on. This BTL advertisement shows that the issue was known and dealt with half a century earlier. Keep that in mind when you see that Pb-Free or RoHS symbols proudly displayed by vendors.
The famous words of Jorge Agustín Nicolás Ruiz de Santayana y Borrás (aka George Santayana)... "Those who cannot remember the past are condemned to repeat it."
From Analog Devices Corp.:   Tin Whiskers Are Real and Complex   (Off-site pointer, opens in a new browser tab)

The FDA:   Tin Wiskers Problems, Causes, and Solutions   (Off-site pointer, opens in a new browser tab)
A large number of pacemakers from the same manufacturer failed with the same symptoms... The FDA issued a recall... Whiskers were found inside a component provided to the manufacturer.
Quotes:
"The manufacturer, however, failed to test the crystal components for proper material composition. It relied on its vendor to deliver proper components. Unfortunately, a bad batch of crystal components resulted in 80 percent of the affected devices having tin-plated crystal components."
"Whiskers can fully develop in minutes or take decades to form."
"High-compression pressure from bolts or screws will always produce whiskers in tin deposits."
"Two methods are currently used (for controlling whisker growth). The most common is to avoid using tin. Other metals or alloys of tin are used instead with solder (tin/lead) being the most popular. The other method is known as "reflow." After the tin is in place, the tin coated part is heated to a temperature above tin's melting point. This heating releases any stress that exists within the tin deposit."   FYI tin melts at 449.5°F or 231.9°C.

Electronic Design magazine: Don’t Let Tin Whiskers Destroy Your Design   (Off-site pointer, opens in a new browser tab)

InCompliance magazine: The Impact of Tin Whisker Formation on Vehicle Electronics   (Off-site pointer, opens in a new browser tab)

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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.