Trimphone.

[GG]

Something I just discovered that may be useful for folks who are restoring Trimphones.

The acoustic design of the underside of the phone is such that the warbler is noticeably louder when the phone is sitting on a solid surface, rather than sitting on an open surface such as a wrought-iron mesh table or on top of some cloth or whatever.  Apparently there is some kind of acoustic resonance principle involved, or acoustic back-pressure from the solid surface the phone is sitting on. 

You can try this for yourself by setting the Trimphone ring at low volume, and then calling it and listening to the difference in loudness of ring when the phone is sitting on a solid surface vs. if you pick the whole thing up and hold it a few inches off the table top. 

So if you are working on a Trimphone and the tone caller sounds weak, it may be because you have the housing off (thereby changing the acoustics of the interior cavity of the phone) or because you have it sitting on a soft pad on your workbench.  Putting the housing back on and putting the phone on a solid surface will solve that. 

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As it turns out, the USA version of the Ericofon, with its tone caller, uses the same pitch as the Trimphone.  So if you have one of each on your desk, the chirping will sound the same.  But the Ericofon gets the necessary ringing volume because the tone comes out of the earpiece, which is sticking up like a periscope where the sound path is directly into the room. 

Two different approaches to the same issue, producing a convergent result.