Some models of the GE Mastr II, Exec ii, MVP and Delta receiver front-end helical resonator castings lose tuning and sensitivity. In some mastr II (style) castings, the helical tuning threads may have lost their ability for good RF electrical contact with the casting. This is because 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.

One solution is to 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. then 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.

Receiver front end castings with a shiny appearance, made before approximately 1983 *may* exhibit the "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, but disassembling the casting will reveal their existence if they are present. The whiskers may appear to be 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.

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. If the bottom cover is shiny and shows whiskers, treat it the same way.

Precautions:

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 years. Here's a couple of examples of the problem. The picture on the right is enlargable.

 

 

 

 

 

 

 

 

 

 

 

NASA and other satelite operators are very concerned about the "tin whisker" problem as several satelites have been lost due to electronics failures that have been traced to whiskers. Explain MORE

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