S-Line Trimmers

 

The trimmers in the RF section of S-Line equipment have a tendency to become gummed up if subjected to contamination over long periods of time.  These trimmers may be cleaned and restored to good working order if you have a little patience.  The band switch shaft  and the shield can(s) must be removed to access the rear of the trimmers.  The spring loaded solder tab to the center of the trimmer may be removed by sliding it away from the center pin with a pair of long nose pliers.  The center pin may then be pushed which will cause the trimmer to come apart.  There are 3 pieces that are removable.  The main ceramic tuner, the other ceramic disk and a neoprene washer.  Normally if the trimmers do not seem to tune when rotated, they probably need cleaning.  Dirty/stuck trimmers will have the main tuner and the disk stuck together.  These must be carefully separated or the thin disk will break.  An Exacto knife can be used to pry them apart if one is careful.  Don’t cut yourself either!  The main tuner and the disk have smooth surfaces which need to be cleaned and dried prior to reassembly.  There are several ways to clean the disk and the tuner — a soft pencil eraser or  a good liquid cleaner that will leave no residue (carbon tetrachloride is good but environmentally unfriendly).   Alcohol will leave a residue so don’t use it.  The solution mentioned above also tends to leave a residue unless extraordinary care is taken to clean and dry after it’s use.  When pieces are clean, reverse the process and reassemble the capacitor.  Put the neoprene washer on first with the opening placed so as to allow the contact tabs to make contact with the disk.  The plating on the thin disk must contact  those tabs.  Finally, the center pin of the tuner of the trimmer may be inserted and the solder tab replaced.  When replacing the solder tab be careful and patient so as not to break the tab at the bend.  The tab will snap on when properly placed and does not require more than a little effort .  Don’t force it! After reassembly, turn the trimmer and make sure that it turns freely and smoothly.  It should feel “dry” as you tune.  These trimmers are getting very expensive so the effort is worth it.  Replace the shield cans being sure to replace the RF gasket material between them and the chassis,  I have seen several radios where these have been lost and the transmitters are more than just a little spooky.  A replacement gasket can be fabricated from small (1/4” wide) tin or silver plated copper braid.   Of course if you have “real RF shielding material” so much the better.  When you replace the switch shaft, again work carefully and slowly and do not force.  Watch in the holes in the cans as you work to make sure nothing is in the way etc.  To ensure the switch is properly aligned, tighten a set screw and check the 14.2 MHz position.  When you look in the holes with the front panel away from you, the switch should be making contact with both tabs visible on the right side of the opening.  Marking the switch shafts and coupling prior to removal will assist in getting this correct.