Maybe someone here is familiar with the preamp circuit in this unit.. if so,
I'd be glad to hear about it.

Model A152B
Labeled as Fairchild unit, but I've also seen references to L3
Communications.
P/n 93-A152-20 93A152
Model A152B
Primarily installed in commercial aircraft

Manufacture was about 1990, although in service until 2003 or possibly still
utilized. Newer digital cockpit voice recorder systems are replacing the
older units.

Contents of the small enclosure are condenser microphone, mic preamp board,
headset jack, Test switch, Erase switch, test meter and a 41-pin connector.

The overall connectivity to the unit is a bit weird, definitely unlike
typical audio equipment, in that the mic, preamp input and outputs, headset
jack, meter and switch connections are all routed to pins in the 41-pin
connector.
This means that the mic, meter and switches, headset jack etc, don't connect
to the preamp internally.

I realize that this control unit was part of a larger system with a lot of
interconnections between cockpit, radio, intercom stations and a recording
unit placed far back in the airframe.

It just seems odd to me that the weak mic signal would be sent out of the
box instead of passing thru the preamp first, as the preamp output would be
stronger to send to a remote location.

I have a reasonably good idea of the connections to the preamp board, but a
schematic would be great to have.
Unfortunately, the circuit details don't appear to be available from any
source that I could find, although there are numerous repair/reconditioning
services, so some servicing/testing info is available (possibly from the OEM
or reverse-engineered circuit details).

The preamp circuit isn't extremely complicated or large.. the board measures
about 2" x 3.5" and the components are thru-hole devices which compliment 6
transistors (one 2N2483 and five 2N915).

If I understand the info that I was able to find, the preamp board includes
a tone oscillator for the test mode (pressing Test switch generates a tone,
verified by headset speakers), and the additional transistors make up the
preamp circuit.

The headset jack, BTW, doesn't include a mic input, just a mono source for
headset speaker(s).

There is a 6.2V zener diode in the power supply string that I believe is the
DC voltage applied to the condenser mic, although it too, is connected to
the 41-pin connector, not internally.

What I'd like to do is connect the mic to the preamp and possibly modify the
power requirement from 18VDC to 12V (resistor(s) change).

If my deductions are correct, the 18V supply first passes thru a small
1N4735A diode which indicates that the preamp circuit current draw is fairly
low (continuous max current rating is only about 200mA for this diode, if
the specs that I found are correct, but this diode could be replaced).

Having the test meter indicate a good audio signal level, and a functioning
test tone would be desireable.. also a monitor signal at the headset jack
might also be handy.
The test meter is marked FS DC MA 233.. so it wasn't likely normally be
powered by the preamp circuit.

The scale of the test meter is 0-10, with 7.5-10 being in the Good range.
The test meter may require an opamp or other circuit for it to be integrated
into the preamp.

http://www.seaerospace.com/loral/a152.htm

Power: 18VDC

AGC: Fast reacting 30-dB range-low distortion, true ac attenuation
Audio Input Impedence: 20K ohms
Audio Input Level: 3 millivolts

Output Impedence: 300 ohms
Output Level: 500 millivolts

Audio Gain: Pin selected 6,12,24 dB steps

It appears that the 3 gain levels are available on 3 separate pins of the
41-pin connector).
(no selector switch or other means to select gain, which is probably set by
external means, or possibly wired into the connector of different models of
aircraft).

No internal lamps for panel illumination.

--
Cheers,
WB
..............

Wild_Bill wrote:

Maybe someone here is familiar with the preamp circuit in this unit.. if so,
I'd be glad to hear about it.

Model A152B
Labeled as Fairchild unit, but I've also seen references to L3
Communications.
P/n 93-A152-20 93A152
Model A152B
Primarily installed in commercial aircraft


L3-Communciations bought up a lot of small companies and divisions of
large corporations in the mid '90s. Good luck getting anything from
them. They also own Cincinnati Electronics and Microdyne.


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