[CW] Fixing Trimm Headphones

[Richard Knoppow]

      I have a small collection of headphones. One pair is a set of 
Trimm Commercial phones made for RCA Radiomarine with the Radiomarine 
name molded into them.  At some point one phone began to be louder than 
the other. I tried the usual voodoo of switching around diaphragms and 
caps with no success.  I decided something had happened to one magnet 
although its hard to think of what it could have been.  Well, I pulled 
them out again and tried something else that got them working.  The 
magnet stucture of these and the Trimm Featherweight phones is simple. 
The pole pieces and magnets are separate pieces held together by a sort 
of clamp made of a crescent of bakelite which, in turn, is held in place 
by three nuts.  Under the pole pieces is a piece of thick paper which is 
evidently used to adjust the clearance of the pole tips from the 
diaphragms.  In this set the paper is 0.01 inches thick. I tested both 
spacers to make sure they were the same, they were.  What I discovered 
is that the pole and magnet assembly must be carefully seated so that 
the pole tips are even and not tilted. The three nuts holding the 
assembly together must be pretty tight.  I had originally tried putting 
thicker spacers in to put the pole tips closer to the diaphragm. This 
does make the phones louder but the distortion gets worse. The originals 
are evidently close to being optimum. I replaced them but in the process 
of doing all this found that the mechanical alignment was very 
important.  I had tried something like this before but I don't remember 
adjusting the tightness of the clamping nuts or noticing the pole tips 
might be slightly tilted.  In any case the two capsules now match in 
loudness and resonance frequency.
     FWIW these are specials. The impedance is about 8K ohms.  The 
standard Commercial phones are about 15k. I have another pair of similar 
phones but made for McKay with their name on it.  The main difference is 
that the clamp screws for the leads is internal on the Radiomarine 
phones (as it is in standard Commercial and Featherweight phones) but 
those on the McKay phones are on the outside. Presumably so that the 
cord can be replaced without opening up the capsules.
     The Commercial and these specials have salt shaker type 
perforations in the caps while the Featherweight has a single large 
hole.  Presumably the damping of the diaphragm is somewhat greater with 
the salt shaker perforations although they may have been intended to 
give some protection to the diaphragm (from sticking a pencil in or 
something of the sort).  Trimm advertised the Commercial as having 
higher quality audio for broadcast monitoring and similar applications 
but I find no difference in the sound, both have the sharp resonance at 
about 1Khz typical of magnetic type headphones.  Both are more 
comfortable for copying CW than the larger and heavier type phones.
     I have been able to find out very little about Trimm and have never 
found a catalog on line although they advertised one.  From print 
advertising the Featherweight model appears to have originated about 
1935. At one time I thought they were just cheap phones but they were 
not and the Commercial model was as expensive as Western Electric phones.
      I hope this will be of some interest here.  I like to pass along 
repair tricks but where does one discuss old fashioned headphones?
-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL