The bridge output is fed through an attenuator to a 

 pair of cascaded voltage amplifier stages . The output of 

 this second voltage amplifier is transformer coupled in 

 push-pull into the modulator grids. The carrier is 

 injected into these grids in parallel. The phase sense is 

 recovered by this action,, The signal is then fed to a 

 demodulator and voltage amplifier, then to the Sanborn 

 driver amplifier. 



The injected carrier is taken from the driver amplifier 

 plate and fed to the injection point by means of a cathode 

 follower„ 



In order to detect bridge balance, a device to indicate 

 bridge signal voltage is necessary. In this unit, this 

 function is accomplished by removing the injected carrier 

 from the modulator and disabling one side of the demodulator, 

 leaving the other side as a straight amplifier. This removes 

 the phase -sensitivity of the demodulator, and any signal 

 present drives the recorder in one direction. It is then 

 possible to balance the bridge, using the minimum recorder 

 deflection as an indication of bridge balance. 



In order that the unit may be used with two or more 

 gages by means of a multichannel balancing unit (for example, 

 TMB Type 286-2A, shown in the circuit section of this manual), 

 a switch is installed to disconnect the internal bridge 

 balancing system. This allows the components in the multi- 

 channel balancing unit to complete the bridge and supply the 

 balancing function, see PART I, BALANCING UNIT, page 4„ 



The front panel GAGE connector is paralleled by pins 

 6 and 9 of connector J-204 on the rear apron of the driver 

 amplifier (9 is ground). Thus, it is possible to bring the 

 gage cables into the rear of the unit rather than the front. 



When two or more carrier systems are used close to one 

 another, the carrier oscillators may beat with each other 

 and produce a cyclic variation in the output signal at the 

 difference frequency of the individual oscillators. 

 Provision has been made in the Type 286-1A for synchronizing 

 the oscillators to avoid this undesirable effect. 



The positioning of the recording stylus is controllable 

 from the front panel of the Type 286-1A. 



ADJUSTMENT 



The following adjustments are made as part of the 

 construction and should remain fixed short of a major change 

 in one of the associated components. 



3 



