[ARC5] T-17 mike with a SW-217 PTT repair

[J Mcvey]

I bread boarded the mike circuit that you posted.The mike used had to be a low Z carbon. I tried it with my weak carbon cartridge and it did wonders for it.There is not enough gain, impedance or voltage for an electret type. It didn't work at all.
For the carbon, I used a 2.2k from collector to base, with no other parts except the mike which is bypassed with a .01 uf cap.The circuit is very sensitive to the resistance of the element used, so I think it's a case by case cut and try situation.Thermal stability is probably very much in question too, but the exact q point is not too critical in this application as long as it is not too far on one extreme of the other.  I got it to sit between 6 and 12 volts with 25V feeding the BC456, depending on where the resistance of the mike "landed" . This one could sit anywhere between 200 to 1K . very unstable...but it works!It was able to produce as much as 4Vpp at normal speech levels, while the "naked" cartridge could do about a 1 Vppat loud levels.

I have designed a fairly simple circuit for the electet types. It uses a fet op amp with a gain of 200 and emitter follower.I will publish it to the list once I have optimized it and have done some 'real world" testing of the prototype. 
It can be built for less than $10 and is very stable.
 

    On Friday, March 31, 2017 9:53 PM, Jim Haynes <jhhaynes at earthlink.net> wrote:
 

 
In case it will help, attached is the schematic of an electret replacement
for a carbon T-1 element.  I guess in a T-17 replacement you could leave
out the diodes since the polarity of DC is always (?) the same.

This schematic came from disassembling a T-1 replacement obtained from
a telephone supplier, sandman.com


   
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