[ARC5] More on Bashless T-17 Microphone Resurrection

David Stinson arc5 at ix.netcom.com
Wed Jun 1 02:34:52 EDT 2016


So work continues on "resurrecting" 
stone-deaf WWII T-17 microphones with little "booster" 
circuits.  I have 6 or 7 T-17s.  One 1942 contract
stock T-17 will adequately modulate a WWII radio
 (yes; I did double-check and it is actually stock, LOL). 
The others are deaf as a post.  Others have had luck
whacking them around to "wake them up," but 
I've beaten, bounced, bashed, baked and battered T-17
elements for years. If I've ever gotten one to work 
properly that way, I don't remember it.  

So far, I've been working-over mikes with missing 
or unrecoverable elements (except the candle stick), 
using at first an electret element and later using 
a simple transistor "current booster" to amplify
the currents through the weak carbon elements.
Time to turn to a "stock" T-17 element 
and see what can be done.

The subject T-17 is a stock 1944 contract with the 
black plastic case.  Used with the BC-669, held right
to my lips and me saying "Foooooooore," this was the
modulation waveform, which is typical of most stock 
T-17 mics today:
https://goo.gl/photos/ZeYyD6tjVKTvcDNn9

We've investigated and tested two very simple 
circuits for "boosting" weak carbon elements 
and both have worked well.   
See my previous posts in this thread.  Elements that
measure low resistance, 60-250 Ohms or so, 
should be installed in the Emitter lead of the NPN
transistor.  Those that measure high resistance-
2K Ohms or more- work best installed in the
Base lead where they can be a large fraction
of the total bias voltage divider resistance.

I removed and tested the stock T-17 element 
with an Ohmmeter. It read around 1K- too much
to go into the Emitter lead so I installed it in the 
Base lead.  Here is the circuit used to "boost"
this element:
https://goo.gl/photos/NSqmQrmimRvV9ySx5

Important:  Note that the circuit above is almost
the same one we used back on the 29th to boost
the large "candle stick microphone" element.
But there is an important difference.
The carbon element in the Base lead
is part of the resistive voltage divider coming from
the Collector, bringing a positive bias voltage to 
turn the transistor "on."  The circuit works because
the element acts as a variable resistor, varying that
bias current and thus controlling the larger current flowing
in the Collector-Emitter circuit, much like the small
Grid current in a Triode controls the larger Plate current.

Because the condition of these weak carbon elements
varies widely, the resistor "R" must be selected for 
the individual element and for the transistor you use.
The difference can be dramatic.  
For instance- the resistor for the "candle stick" mike
which used a lower-gain 2N2222 transistor (beta 
tested 70) was 4.7K Ohms.   This T-17 element 
and a transistor with a beta tested at 110 required
a resistor of 220K Ohms.   The idea is to use as 
high a value as possible and strike a balance between
providing adequate Base current and loading-down 
the microphone bias voltage provided by the radio.
The easiest way to do it is to use a pot and tweak for
best response and output.

With the little booster helping the original T-17
element, this is what the "Fooooore" modulation
envelope looks like now:
https://goo.gl/photos/rUnhMAEixfyAx4Gp8

As with the earlier work, I prototyped the booster on
a board and mounted it behind the T-17 element:
https://goo.gl/photos/w6QUp3vy3C5rwZEdA

Boosting the carbon elements has been going
so well, I've decided to set the electret subber
aside for now.  I get excellent audio reports 
and I was never a fan of electrets anyway- 
too noisy.

As I mentioned in a previous part of this 
thread- the transistors I'm using are 
unmarked NPNs that test with a beta of
about 110.  If anyone can suggest a common
number transistor with at least this beta
for this circuit I'd be grateful.

GL OM ES 73 DE Dave AB5S



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