While we are "cutting" into the Mitrek radio, some research was performed on the intermittent problem of failing sensitivity of the radio. This plagued the Westlink in the year of 2006, until enough persistence corrected the problem. As you can see the RF coils are in a "cavity" with a window for RF coupling. Also note each coil (except for the first and last, for I/O) are mounted mechanically and electrically only in ONE spot; the bottom of the coil has a lead going down to the bottom plate, where it's soldered only at that point. The top of the coil (form) snaps into the top (inside) of the cavity section, with it's four plastic tabs. One needs to be aware of this, in the event excessive heat is applied to the front end section. If you have to access the cavities, remove all the Pozi-drive #2 screws, unsolder all of the ground tabs (going to the main board) and unsolder and peel up the bottom plate. You can identify another option for the receiver; the holes in the right picture shows no pre-amp option in this radio.

Note: Pozi drive is similar to cross-head but designed not to slip, or cam out. It does not have the rounded corners that the Phillips screw drive has. Phillips screwdrivers will usually work in Pozi drive screws, but Pozi drive screwdrivers are likely to slip or tear out the screw head when used in Phillips screws. Heads are marked with crossed, single lines at 45 degrees to the cross recess, for identification.

The metallurgy definition does not seem to address this phenomena. More information on this subject can be found by clicking here.

After checking out the inside, now, this is not likely. About the only control for performance is making sure each coil (bottom) lead has a good solder connection to this bottom plate. Also verify all the screws are tight to make a good ground connection to the entire front end chassis. It was believed (at first) there was a Metallurgic problem of "whiskers" growing on the inside of these cavities (similar to the Mstr-II G.E. radios) and touching the coils. More information on this subject can be found by clicking here.

The filter coils for both the transmitter and receiver sections are in the casting. Sometimes this coil's leads get broken, or in the form of a cold solder joint. Re soldered the bottom of the casting will correct this condition, however you need a large amount of heat to quickly melt the area, but not damage surrounding components or let the coils heat up to much by slow heat caused by insufficient heat from an iron. With two people, you can use two Weller D-550 guns, the third hand being for feeding the solder. Another way is to find a large iron, say in the 150-300 watt range. The Author found this type for about $25. Be careful limit the time the iron is plugged in as you can see it gets red hot in the heater section.

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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