[ARC5] T-17 Metal 1942 or Plastic 1944 for 1949 GRC-9?

[Richard Knoppow]

    A little more about the T-17.  I took a closer look, mine also says 
SW-217 on the switch.  I opened it up. The front cap is held in by three 
screws which are covered over with plastic or hard wax. You can scrape 
it out and get the screws out.  The cap and capsule are an assembly, the 
capsule is not a separate part.  When you look at the back you will see 
that there is a center part held in place by two clamp screws. A third 
screw holds part of a terminal arrangement that holds the small 
capacitor in place.  The rear terminal fits into this and is held in 
place by a nut. In mine all were painted over with heavy lock paint.  If 
you remove the screws and clamps the center part of the microphone will 
come out. However, be careful to hold the assembly with the front facing 
up because the part that comes out the back is the cup with the carbon 
granules in it. They are held in by felt washers on both sides. When 
assembled the felt meets and seals the carbon in. Mine reads very high 
resistance, about 12K where a good carbon element should read perhaps a 
couple of hundred ohms.  Its hard to know what has happened.  I did not 
try to improve it but put it back together again.  I may open it again, 
take the carbon out, and carefully clean the contact surfaces to see if 
that reduces the series resistance. Next step is to roast the carbon to 
drive out moisture. I will try the cleaning first. I think I still have 
a couple of old telephone capsules that I can get carbon from.
     I don't know if all T-17s are made the same way. This one was made 
by Universal Microphone.  There is no way to substitute another kind of 
capsule in it without a lot of surgery.  I have had limited success with 
replacing carbon in carbon microphones but its not a sure cure.
    Anyway, I thought the construction might be interesting to others. 
I would like to hear from anyone else who has opened one up to find out 
if they are all the same. I think I said this one had three small holes 
in a line, in fact they are in a triangle. It would also be interesting 
to know where the design originated.
     FWIW, mine also has a Signal Corps inspection stamp (if that is 
what it is) in the familiar orange/red ink on the top. SC and a number 
that looks like 1083A.

On 9/14/2016 11:41 AM, Richard Knoppow wrote:
>      My T-17 is about like yours, its plastic and made by Universal
> Microphone.  The holes do not have to be large to admit sound. Also, the
> small holes are part of the acoustic network of the microphone, they
> provide some damping to the diaphragm. Evidently more than one design of
> carbon element was used in the T-17 so the cap design may reflect this.
> The RS-38 has the same arrangement, three small holes in a line.
>     It would be interesting to know where the design originated. Several
> companies made hand held carbon mics for radio use by the mid 1930s,
> perhaps the T-17 was adopted from one of them.  The general design
> suggests the advanced engineering of Western Electric.
>
>
> On 9/14/2016 6:47 AM, Don Merz via ARC5 wrote:
>> The metal 1942 one I have has a pattern of just 3 pin-sized holes. Hard
>> to see how that helps much. It was made by WE. The black plastic 1944
>> T-17-B version has seven much larger holes at the mouthpiece.
>>
>> I also have 3 more of these in bad shape. Two metal ones are dated 1935
>> (!) and 1939, MFR not identified. They I have a 1944-issue T-17-D
>> version in olive drab plastic.
>>
>> The metal ones ALL have the 3 tiny pin holes in the mouthpiece
>> regardless of date. Both plastic ones have the 7 larger mouthpiece holes.
>>
>> 73 Don merz N3RHT
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL