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Bendix turn and slip indicator (3934-1AG-C1-1)
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I like instruments with gyroscopes and the turn and slip indicator is de cheapest and simplest gyro device. And since I was placing an order with several other avionics instruments, I added this Bendix turn and slip indicator to the virtual shopping basket. The price was right and this seems a nice instrument to add to the collection.
 description: Electrc Turn and Slip indicator
manufacturer name: Bendix
manufacturer part number: 3934-1AG-C1-1
serial number: 005513
other code: FAA-TSO-C3A AS395
FAA-TSO-C3A AS395 meaning
 I wondered what the code 'FAA-TSO-C3A AS395' meant. So I searched for the explanation. I found a document in the national archives titled 'Federal Register of the United States' Volume 29 / Number 224 that was issued on tuesday 17 november 1964. There's a part of the Federal Aviation Agency (FAA) in there. And I found on page 15.323 the explanation. This part of the document is about the Rules and Regulation 'title 14' for Aeronautics And Space part 37. And paragraph 37.133 C3a is about Turn And Bank Indicators:
'(c)Specific instructions
(1) Marking
In addition to the identification information required in the referenced specification, each turn-and-bank indicator shall be permanently marked with the Technical Standard Order designation 'FAA TSO C3' to identify the turn and bank indicator as meeting the requirements of this order in accordance with the manufacturer's statement of conformance outlined below. The Administrator accepts this identification as evidence that the established minimum safety requirements for turn and bank indicators have been met.'
So; this device complies to the rules and regulations and this is confirmed by the marking 'FAA-TSO-C3A AS395'.
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Source: Federal Register, Volume 29, Number 224, Title 14, Paragraph 37
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Purchase
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After a while since the order was placed, the package with several instruments was received all the way from United Kingdom. Unfortunately a display module was damaged due to transport and also the glass of this instrument was shattered. The damage was a result of packing the instruments with insufficient care unfortunately. The shattered glass can be seen on the image below... The good thing is that the instrument itself is in working order and that the glass can be replaced though.
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dial illumination
The device was marked that the electrical power needed was 115V at 400Hz. But there were no connection details nor can a schematic be found. Therefore I reverse engineered the instrument and the drawing is shown below. Luckily the device is rather simple. There are three incandescent lightbulbs connected in parallel. If one incandescent lightbulbs fail due to wear, there are two other light sources left. The needed voltage is 5V and in my case the current draw was 510 in total for three incandescent lightbulbs. The power consumption is approximately 2,5 Watts.
warning indicator
There's a warning indicator installed to indicate that there's (no) electrical power. Under normal operating conditions the 'off' marker is hidden since the flag solenoid is powered. When 115VAC 400Hz electrical power is absent, a white 'off' marker is visible to indicate there's no electrical power and the indicator is not in normal operating mode.
gyroscope power
The 'heart' of the instrument is a electrical powered gyroscope. A one phase 115VAC 400Hz is fed to the gyroscope motor. Since the motor is a three phase motor, a phase shift is needed to get the motor working. A simple way to create a phase shift is feeding the source power trough a capacitor to obtain the phase shift. Instead of using a three phase 400Hz source, it's much easier to use a single phase and a phase shift capacitor. The capacitor value is 0,136uF. This is a rather odd capacitance value since the frequency and motor properties combined result in this optimal value. The result is an exact 120 phae shifted signal to obtain the maximum motor speed.
schematic
The reverse engineered schematic of the device is shown below.
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As mentioned the glass window failed during transport. Recreating a new window of glass is a hassle and not necessary. I decided to lathe a new window of a piece of acrylic plastic. The piece of plastic is clamped between two supports on the lathe and the desired diameter is obtained by removing excess material using a lathing tool. The result is shown below. Later on the plastic is polished to remove the dirt rings from the window.
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