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 small1N4735A 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.
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 the41-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 .............