ENGINEERING RESEARCH INSTITUTE UNIVERSITY OF MICHIGAN ANN ARBOR OPERATION MANUAL FOR THE AIR COMP MOD 4 ELECTRONIC DIFFERENTIAL ANALYZER By R: M. Ho'we. Assistant Professor- f Aeronaitical Engineering Project 2002 U. S. Navy Department, Office of Naval Research Contract N6 onr 23223 AIR-4 March 1, 195,3

TABLE OF CONTENTS Page 1. General Description of Computer 1 2. Operational Amplifiers and Associated Circuits 4 2. 1 Theory of D-C Operational Amplifiers 4 2. 2 Air Comp Mod 4 Drift-Stabilized Amplifiers 17 2. 3 Description of Amplifier Panels 20 2. 4 Circuits for Obtaining Non-Integral Resistor Values 27 2. 5 Amplifier Relay-Rack Circuits 27 3. Variable Coefficient Selector 29 4. Power Supplies 37 5. Recording Assembly 47 i

LIST OF FIGURES Figure Page 1 Front View of Computer 2 2 Recording Galvanometer Cart 3 3 Rear View of Computer 5 4 Operational Amplifier 6 5 Operational Amplifier for Summation 6 6 Operational Amplifier for Integration 6 7 Drift-Stabilized DC Amplifier 10 8 DC Amplifier for AIR COMP Model 4 Computer 11 9 Drift Stabilizer for AIR COMP Model 4 Amplifiers 12 10 DC Amplifier and Drift Stabilizer 13 11 Bottom View of DC Amplifier 13 12 Open-Loop Frequency Response of AIR COMP 18 Model 4 DC Amplifier 14 13 Maximum Amplifier Output Voltage as a Function of Load Resistance 19 14 Front View of Amplifier Patch Panel 21 15 Rear View of Amplifier Patch Panel 22 16 Internal View of Amplifier Patch Panel 23 17 Circuit for Amplifier Patch Panel 24 18 Circuit for Initial-Condition and Hold Relay Operation 26 19 Circuit for Obtaining Non-Integral Resistor Values 26 20 Wiring Diagram of Amplifier Rack 28 21 Step Approximation to Continuous Function 30 22 Schematic Circuit for Variable Coefficient Selector 30 23 Binary Resistor Plug-In Circuit 33 24 Stepping-Relay Panel 34 25 Stepper Control Chassis 36 26 Power Supply for +300 Volt Regulator 38 27 Power Supply for -190 and -350 Volt Regulators 39 28 Power Supply for +100 and -100 Volt Regulators 40 29 Regulators for +100, -100 and -350 Volt Supplies 41 ii

Figure Page 30 Regulators for +300 and -190 Volt Supplies 42 31 Power Supply Distribution Panel 44 32 28 Volt DC Supply 45 33 Schematic for Cable Connections 46 34 Power Supply for Recorder Amplifiers 48 35 Amplifier Schematic for the Recorder Drive 49 36 Amplifier Can Circuit and Input-Feedback Can Circuit 51 37 Frequency Response of Uncompensated and Compensated Recorder 52 iii

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN OPERATION MAXIUAL FOR AIR COMIP 1iCD 4 ELECTRONIC DIFFERENIfTIAL ANALYZER 1. General Description of Computer The Air Comp Mod 4 Computer is an electronic differential analyzer designed and built for the Office of Naval Research by the Department of Aeronautical Engineering, University of Michigan. The computer is capable of solving linear ordinary differential equations up to sixth order and having variable coefficients. The physical equipment making up the computer consists of the following units: a) Relay Rack No. 1: 10 drift-stabilized operational amplifiers, including 6 integrators. b) Relay Rack No. 2: 17-digit, 25-step, variable coefficient selector. c) Relay Rack No. 3: d-c power supplies, including electronically regulated supplies of +300, +100, -100, -190, and -350 volts, and an unregulated 28-volt d-c supply. d) Recording galvenometer cart, including a Sanborn Model 60 1300 2-channel recorder, and 2 electronic amplifiers. Photographs of the above units are shown in Figures 1 and 2. Detailed descriptions, circuit diagrams, and theory of operation of the various units are presented in the following sections. Power inputs required for the Air Comp Mod 4 Electronic Differential Analyzer are 110-120 volts, 60 cycle ac (about 1500 watts),: and 12 volts dc regulated at 15 amps. It is recommended that the 60-cycle input also be regulated.

Figure 1. Front View of Computer. 2

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ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN 2. Operational Arrplifiers and Associated Circuits PRelay rack No. 1 consists of ten drift stabilized operational amplifiers and associated circuits. A front view of the rack is shown in Figure 1; a rear view is shown in Figare 3. As is evident from the photographs, the ten operational amplifiers plug into the backs of the three amplifier panels, each panel having provision for four maplifiers. The amplifier panels furnish the operational arplifiers with the various voltages required for amplifier operation. Also located on the back of the amplifier panels are reset and hold relays. On the front of the panels are the patch boards into which the computer components can be plugged The circuits of the amplifier panel are described in Section 2.3. Two panels containing 64 toggle switches each can also be seen in Figure 1. These panels are used to provide any desired resistance up to 16 megohms in 0.001 megohm steps. They are discussed in detail in Section 2.4. At the bottom of relay rack No. 1 is the panel containing the relays and patch connections for the variable coefficient selector. Circuits and descriptions are given in Section 3. 2.1 Theory of D-C Operational Amplifiers a) Derivation of Fundamental Equations. The basic unit of the electronic differential analyzer is the operational amplifier, shown schematically in Figure 4. It consists of a d-c amplifier having a gain -RL, an input impedance Zj, and a feedback impedance Zf. The input voltage to the d-c amplifier proper is E' and its output voltage is eo. If we neglect any current into the d-c amplifier (it is usually less than 10-10 amps), then the currents through input and feedback impedances must be equal. Thus by ohms law 4 -

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e —. |>'DC A'P ^ e E-l, E1 GAIN=LLeo Figure 4. Operational Amplifier. Ra iR _R IsR e- -i>~ - ic ec Figure 5. Operational Amplifier for Summation. Figure 6. Operational Amplifier for Integration. 6

,- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN el- E' - e ( ) Zi Zf where ei is the input voltage to the operational amplifier. But by definition 4EI = - e so that we can solve equation (1) for eo, obtaining 0O I | Zi eo =...... ei (2) Zf 1 + - ('.1.+ —- ) Zi For the amplifier gain>>l+Zf/Zi, equation (2) reduces to Zf eO - - - e. (3) Ziwhich is the fundamental equation governing the behavior of the operational amplifier. For Zi and Z resistors, we can multiply any voltage ei by a constant K by making the ratio of feedback to input resistance equal to K. The output voltage eo will be - Kei, as required, except for a sign reversal. By employing several input resistors Ra, Rb, and Rc with a single feedback resistor Rf, the operational amplifier can be used to suma input voltages ea, eb, and ec. Thus in Figure 5 currents ia + ib + i c if and by ohms law ea -e' eb - e' e t _ ea- +'A b ^A. -~' = -S —^ (4) Ra Rb Bc Bf If we neglect Et as small compared with ea, eb,'e or eo (E' - eo/ where,L is very large), then equation (4) can be solved for eo. Thus Rf Rf Rf eo = - ea - - -ec (5) Ra Rb Rc

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN The output voltage eo is the sum of the three input voltages ea, eb, and ec each multiplied respectively by the ratio of feedback to input resistance. Hence the operational amplifier can be used to multiply by constants and sum voltages. Finally, consider the operational amplifier shown in Figure 6. Here the input impedance is a resistor R and the feedback a capacitor C. Again neglecting any current into the amplifier proper and assuming that E' is negligibly small compared with ei or eo, we have ei _ 1 i = R and e i dt from which o --- edt (6) B RC Thus the output voltage is proportional to the time integral of the input voltage. The constant of proportionality is 1/RC, and RC is known as the time constant of the integrator. The manner in which operational amplifiers used for multiplication by a constant, summation, and integration can be combined to solve linear ordinary differential equations is described elsewhere.' b) Stability Considerations. We have seen that the behavior of the operational amplifier is described by equation (2), which can be rewritten as Zf eo — ei (7) 1+ -+ f This is the usual form for the equation describing a feedback control system.4 In general, the amplifier gain v and impedances Zj and Zf are not constants, but include various time derivatives. Hence, equation (7) is really an equation

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN of motion for the system. We can determine whether our closed-loop system will be stable by finding out whether it has any-characteristic roots with positive real parts. But the characteristic roots will be the roots of the denominator in equation (7). To insure that these roots never have a positive real part, it is necessary that the gain versus frequency characteristic of the d-c-amplifier does not exhibit a slope more negative than -12 db per frequency octave until the amplifier gain is well below unity (0 db). This will insure stable feedback ajnplifier operation for all possible combinations of input resistors and feedback resistors or capacitors. For example, the d-c gain p. of the Air Comp Mod 4 amplifiers is about 90 db (30,000). It is necessary to add capacitors to the d-c amplifier circuit so that at high. frequencies the gain does not fall off faster than 12 db per frequency octave. In practice, a conservative fall-off of 6 db per octave is utilized, as can be seen in Figure 12. c) Drift-Stabilized D-C kiaplifiers A d-c amplifier must be balanced so that with zero input the output voltage is zero. When a d-c amplifier circuit has been properly designed, this balance can usually be achieved by slight changes in the operating conditions of the first stage of vacuum-tube amplification. Once the amplifier has been balanced, subsequent changes in heater voltage, B voltages, ambient temperature, etc., may cause the amplifier to drift off of balance so that zero voltage input no longer gives zero voltage output. In order to reduce such zero drifts, a drift-free a-c amplifier can be added to the d-c amplifier, as shown in Figure 7. The additional a-c amplifier consists of a sychronous vibrator or chopper which converts the d-c input voltage E' to an a-c signal (60 cycles in the Air Comp Mod 4 amplifier) the magnitude of which is proportional to el. This a-c signal is sent through an a-c amplifier and reconverted to d-c by

,_ Za& W. w Main DC AMP i el ----- ^' GAIN = U -,. E GAIN=G ll i!,t Drift-free DC Amplifer Figure 7. Drift-Stabilized DC Amplifier. 10

,~ ~I, ~,~~ Sv IZSW-EI P; 0W40W o E 2MEG IOK J 4 MEG +0 4 4 CHASSS00G 5N /L/XFD +28vS 5lob I o BL( * 5 SIo END 5691_ _ 6 5691A U O U12SN7 ~-350 9 PURPLEj~ 9 0oe INPUT o LT.BLUE _ _ d|| POLYSTYRENEOW 12vOUTPUT 120 DK. BLUE L 15AUTO INW —-- 1> ___ ____ ___ ____ ___ ____ ___ ____ ___ ____ ___ ___SOCK CHASSIS GND I 0 BLACK 2 Figure 8. DC Amplifier for AIR COMP Model 4 3 GRANE - F I FIL 51 - Vl RE DRAYN +300v 40-( —- BLA *+'0 0 ^-^ ^ —------ ^ -0 SI P T END SIG GNDG110-W~iT —AJJ ------- 1 —J --------- 1 —J-J --— I 0 T SOCRED OUTPUT 12o DK.BLUE ( I Figure 8. DC Amplifier for AIR COMP Model 4 Computer.

o o 0 0 N. OF, 12AXDIMEG 200 K I MEG O.FDIOK K i 12X7 <, FDC 2 o ORANGE 0 0 i ] X 9 BOTTOM VIEW IN.25u.FD w GS D W, X OCTAL 0. PLUGS F 4L o NE51 30 POWER GND 40S <l SIGGND 50 T9 0. AUTO OUT 6 DK Bv 60 INC 70 CHASSIS ( —1-CONVERTER o +300v 80 N1 - 0 POLYSTYRENE AUTO IN 0 LT. BLUE Figure 9. Drift Stabilizer for AIR COMP Model 4 Amplifiers.

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20 0 I0 —10 100 1KC IOKC OOKC FREQUENCY (CPS) Figure 12. Open-Loop Frequency Response of AIR COMP Model 4 DC Amplifier.

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN means of another pair of contacts on the same synchronous vibrator. The net result is a drift-free d-c amplifier, the output of which is supplied to an additional input E' to the main d-c amrplifier. The way in which this driftfree amplifier improves the performance of the operational iamplifier is described below. Asswume that the unbalance in thie iain d-c amplifier in Figure 7 is eB volts referred to input. Then the output eo is given by eo - - J (e + "l + eB) (8) where E' and E' are ecqually effective inputs to the am.plifier, and where A is the main d-c amplifier gain. Let the gain of the drift-free lamplifier be G, and assume that this amplifier has an offset equal to eC volts when referred to input. (This offset may result from chopper contact voltage or stray pickup in the a-c:amplifier at the chopper frequency.) Then the output'' of the drift-free amplifier is Et G (E' + eC) If wre neglect any current flow into the main d-c wamplifier or the auxiliary, drift-free amplifier, then equation (1) gives us the relationship between the input voltage ei to the operational amplifier and the output voltage eo. Combining equations (1), (8), and (9), we have Zeo 1 -[- (l >)( -e G Z( eo ~ -+ - eC) (10) + 1 ( Zf LZi Zi 1 +G 1 + G 1+ (- + -).L (1+G) Zi The effective gain of the amqplifier before feedback has evidently been increased from a. to a (1 + G). Since the auxiliary drift-free amplifier cannot pass frequencies higher than one-lalf the vibrator frequency (60/2 = 30 cps for the Air Comp Mod 4 amlplifier), it is necessary to use low-pass filters on both input 15

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN and output sides of this amplifier. Thus the gain of the drift-free amplifier mrEay be considerable at d-c and very low frequencies, but falls off rapidly at higher frequencies. At high frequencies the effective gain of the amplifier before feedback is p., while at low frequencies it is i( 1+ G). Consider equation (10) when the operational amplifier input ei equals zero. If 4(1 + G)>> 1+ + /Zi and if G>>1, then Zf eB eo (o): - (1+ I)(. +eC) (11) Zi 1+ G This equation represents the voltage unbalance in the output resulting from voltage unbalance eB in the main d-c amplifier and unbalance eC in the driftfree aplDlifier. Clearly the introduction of the auxiliary chopper amplifier has reduced the unbalance eB in the imain d-c amplifier by a factor 1 t G, where G is the gain of the chopper amplifier. It is also evident that there is no reason for having G larger than required to make eB/(l + G)<eC, since no matter how large G is lmde, we are still left with an unbalance of (1 + Zf/Zi)eC. In practice eC can be held to 10-4 volts or less, whereas eB depends upon the regulation of the power supply voltages and the constancy of the envirolmental conditions. For the Air Comp Mod 4 amplifiers |J 30,000 and G 4-00. As a scmple calculation of the zero offset resulting from the auxiliary amplifier unbalance ec, assurme that Zf = Z- 1 megohm and that eC = 10 volts. Then from equation (11), if evBil + G)<<eC, the zero offset of the operational amplifier with a gain of unity is 2 x 10-4 volts. This is a typical figure for the Air Cormp Mod 4 ampLlifiers. 16

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN 2.2 Air Co;p Mod 4 Drift-Stabilized iAmplifiers The circuit diagram for the Air Comp llo 4 d-c amplifier is shown in Figure 8, and the diagrams of the drift-stabilizing amplifier is shown in Figzure 9. Photographs of the clmplifiers are shown in Figures 10 and 11. The d-c:amplifier employs three stages of amplification. The first s:tage has ito inputs, one on each of the tw7ro grids of a 5691 twin triode (eqcuivalent of 6SL7). The input at the first grid is the input proper to the samllifier. The input at the second grid is the automatic or dirft-stabilizing input, and is connected to ground when the amplifier is operating on manual balance. The second and third stages of amplification are obtained with a second 5691 twin triode. Finally, a 12S117 cathode follower can be switched into the output to provide a high-power output if desired. Note that the two 6-volt 5691 filaments are connected in series, whereas the 12SN7 filament is connected in parallel with the 12-volt filatment supply. Since the 5691 tubes require 0.6-amp filament current while their 6SL7 counterparts require only 0.3 amps, it is not possible -to substitute a 6SL7 for one of the 5691 tubes without adding anr appropriate resistance in parallel with the filamient of the 6SL7. A special polystyrene-dielectric plug with silver-pl-ated contacts is utilized at the frond end of the d-c ampwlifier to supply input power and input and output connections (see Figures 10 and 11). An octal socket and an additional single contact with polystyrene insulation is used at the rear end of the d-c amplifier to connect it with the drift-stabilizing amplifier. Two DPDT utoggle switches at the rear of the d-c anmplifier provide high- or low-power output by switching in or out the 12SN7 cathode follower. A third toggle switch provides a control for manual or automatic-balance operation. 17.

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN A 24-volt d-c relay mounted underneath and at the rear of the amplifier chassis is used to check the d-c balance of the amplifier. When the relay is energized, it disconnects the external input and feedback impedances and applies a feedback-input ratio of 1000:1 in order to give a sensitive balance indication. In addition, when energized it connects the amplifier output to a special output balance terninal for convenience in metering the amplifier output in the balance condition. Balance itself is adjusted by means of the 40,000 ohm pot (course control) with the control shaft on the top of the ampliflier chassis, or by means of the 5000-o0hm1 pot (fine control) with the control shaft coming out the polystyrene plug on the front end of the amplifier. This latter control shaft protrudes through the front panel on the computer rack and allows a fine-balance adjustment to be nmade from the front of the computer. When the relay is in the balance position (1000:1 gain), the output balance voltage is 1000 times the unbalance referred to input (hence 500 times the unbalance with a 1:1 input-feedback ratio). The output balance voltage should nomnally not exceed 0.2 volts with the Air Comp Mod 4 amplifiers on "auto!atic" balance. A gain versus frequency plot for the amplifier is shoim in Figure 12. Maximumn output voltage as a function of load resistance appears in Figure 13. The drift-stabilizing amplifier used for automatic-balance operation employs a Leeds and Northrup Std. 3338-1 Converter as a synchronous vibrator. The 24-volt 60-cycle input is fed in through a shielded lead at the top of the converter. The first stage of a-c amplification is a 6SH7 pentode while the second stage of amplification is provided by the first half of a 12A X-7 miniature twin triode. The second half of the 12A X 7 is utilized to operate an IE-51 neon pilot light on the front panel at the top of the computer relay rack. The IE-51 fires whenever the input voltage to the d-c amplifier 18

240 -- 200 ___ 160 -y- —.... -— _-"- --— /' -SAT 40 -- I I I ___ —_ - - _ - - _ _ _ —-- /,. 8K 10K IOOK I MEG LOAD RESISTANCE Figure 13. Maximum Amplifier Output Voltage as a Function of Load Resistance. 0000f m ///~~~~~~~~~~~~~0 /~~~~~~~~~~~ op-~~~~~~~ O'l II I / K IOK I OO.////~ OA RESITANC Fiue1/Mxmu/mlfe/utu otg s ucino Loa Reitne

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN proper (W' in Figure 7) exceeds about t 0.07 volts. This indicates when the d-c amplifier output is saturated, for as soon as the output saturates, the input E' to the d-c amplifier begins to increase by the same amount as the input voltage E1. Recovery of the drift-stabilizing amplifier to overloads of this type will be rather slow due to the dielectric absorption in the input filter condensers. Thus it may take a minute or more for the operational amplifier to return to optiimum balance after an overload when automatic balance is being used. Note in Figures 10 and 11 that the 12AX7 tube and associated components are contained in altuninum cans. This provides excellent electrostatic shielding and allows easy replacement of thisportion of the drift-stabilizing amplifier. 2.3 Description of Amplifier Panels The d-c amplifiers described in the previous section plug into the amplifier panel shown in Figures 14-16 (four amplifiers per panel). The wiring diagram for the panel is shown in Figure 17. The back side of the panel provides sockets into which the four d-c amplifiers plug. In addition, if the two inner amplifiers are to be used as integrators, feedback capacitors for these two amplifiers are plugged into the back of the panel. A 20-pin plug on the back of the panel receives power voltages and relay-control voltages from the comlputer rack and connects the amplifier outputs and saturation-indicating voltages to the computer rack. The front of the amplifier panel provides sockets for plugging in input and feedback resistors. The insulating material is polystyrene. A push-button to energize the balance relay is provided for each amplifier on the front of the panel. Initial condition controls are also located on the front of the panel above the two inner amplifiers.

DO Figure 14. Front View of Amplifier Patch Panel.

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4 AMPLIFIER PANEL 12 COND. PLUG AN - 3102-28-16P AN CONNECTOR I POWER GND. 7 OVERLOAD SIGNAL I IH IR WSNI HM F Q? TL VU IP A OVERLOAD#3 L OUTPUT#4 2 - 12 VOLTS FI1L. 8 OUTPUT BALANCE IIIIIIIIIIIIIIIIIII B OVERLOAD #4 M +12 VOLTS FIL. 3 +12 VOLTS FIL. 9 -350 VOLTS C -12 VOLT FIL. N -350VOLTS 4 +300 VOLTS 10 INPUT D -190 VOLTS P -IOOVOLTS 5 +28 VOLTS II SIGNAL GND. E +100 VOLTS Q POWER GND. 9 -I90VOLTS _ 12 OUTPUT ______ F +300 VOLTS R OUTPUT BALANCE G +28 VOLTS S OUTPUT #2 H OUTPUT*I T OUTPUT#3 J OVERLOAD #I U HOLD RELAY K OVERLOAD *2 V RESET RELAY GI MFDF IMf- +D. MFDP 122010 2MFD0,220I b b 1 I I I I I 1 13 2 b I RE SE HOLDj^ j3 2 L 1 b L 6 5 4 6 5 4 -4 6 5 4 9 8 7 9 8 7 9 8 7 9 12 II 10 12 I 10 12 II 10 12 I10 OUTT S G IN SOT IN -r — - -- OUTi S<G IN* OUTi SiG SIGNAL 6ND. __ ^. ^!I'OF^^0 -I OFF j+^ 2020 SIGNAL-GNDF ~POT 10 f1O 100 ~ O 500K 500K 500K500K Figure 17. Circuit for Amplifier Patch Panel.

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN The function of the initial-condition and hold relays should probably be described in some detail. In order to solve differential equations with the electronic differential analyzer, it is necessary to impose initial conditions on each of the integrating cnmplifiers. The initial conditions are obtained with the reset relay by disconnecting the external input resistors and by connecting equal input and feedback resistors to the amplifier (see Figure 18). The negative of the initial condition voltage is then-applied to the input resistor, and the feedback capacitor charges to the desired voltage. anen the analyzer solution is begun, the 1:1 input and feedback resistors are disconnected from the am-polifier input and the external input resistors are reconnected. The reset relay is energized when initial conditions are applied and is de-energized when the solution begins. Actually a single DPDT relay is utilized to impose initial conditions on both integrating amplifiers. When the "hold" relay is energized, it disconnects the input resistors to the integrating amplifier and "holds" the voltage stored on the feedback capacitor at that instant (see Figure 18). In this way the differential analyzer solution can be stopped at any time and resumed at a later time by de-energizing the hold relay. When a solution is being held, the output voltages of the integrators will drift slightly because of leakage or grid current into the feed-10 back capacitor. For a current of 10 amps a 1-microfarad integrator will drift 0.0001 volt per second when held. In order to avoid undesirable transients when the relays are opened or closed, a 2-microfarad capacitor in series with 240 ohms is connected in parallel with the reset and hold relays. Note that the relays are operative only when both integrating condensers are plugged into the back of the amplifier panel. 25

500K 500K INITIAL COND. VOLTAGE I TO EXTERNAL INPUT RESISTOR _ -r C a,iAMPLIFER OUT PUT RESET HOLD RELAY RELAY Figure 18. Circuit for Initial-Condition and Hold Relay Operation. IM O.IM 10K \K 2M 0.2 20K 2K 2 S.2 D. 4M.4M 40K 4K 4'-^. 04<...0.4 i JC04<^ 8M 0.8M 80K 8K 8 0..o08.oo 0 0 + RFigure 19. Circuit for Obtaining NonIntegral Resistor Values. 26

- ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN 2.4 Circuits for Obtaining Ion-Integral Resistor Values The external co_1mputer components required for solving differential equations include resistors. A number of integral-valued resistors frequently required (such as 0.1, 0.5, 1 megohm, etc.) are provided as plug-in units. In the solution of a dif-ferential equation having non-integrOl coefficients, however, it is necessary to provide appropriate non-integral resistors for use as input or feedback impedances. Two panels containing 64 toggle swritches each are utilized for this purpose (see Figure 1). A given panel actually consists of 4 groups of 4 x 4 toggle switches. Each 4 x 4 group allows any resistance between 0.001 megolmn and 16 megolrms to be built-up in 0.001-raegomil steps. The circuit is shown in Figure 19. The desired resistance is obtained by short-circuiting the appropriate resistors. The first colurm contains 0.001, 0.002, 0.004, and 0.008 miegolns in series; the second column 0.01, 0.02, 0.04, and 0.08 megohmss in series, etc. Thus the first column is used to set the 0.001-umegolhm digit, tlhe second for the 0.01 mlegohm digit, the third for the 0.1-imegohlum digit, and the fourth to set megohms. Resistors employed are accurate to better than 1 per cent, but for high-precision computing it is necessary to set the resistor values by means of a calibrating circuit such as a Wheatstone bridge.2 The desired resistance is then patched into the appropriate amplifier by means of a connection from the two output sockets. 2.5 Amplifier Relay-Rack Circuits The amplifier panels which hold the operational amplifiers and computing components are in turn supported by Relay-Rack No. 1 (see Figure 1). The circuit diagram for the relay rack is shoan in Figure 20. Power and control voltage inputs to the rack are obtained through a plug at the rack base. Beside this input plug is an output socket which provides connections to the outputs of each of the operational amplifiers. __________________________?_ 27

13 OUT BAL. NE 51 OVERLOAD INDICATORS 500-0- 500/AMPS 012R L Oil AMPLIFIER LAMP I —---- - 72 5t w~ o81 0 7 1 — 3-o4-,-| o OFF | oY / SWICH OUT. BAL. Z 9 10 II 12 (FOR 1O09 EXTERNAL 40K WW S J I r I I METER) + W wO ~oO ~ ~~f~~ o[3 o STEP O0 "r FUNCTION J I o 2 OVERLOAD AMPLIFIER r 5 5 3 INDICATORS OUTPUTS 4 AA <r I o —---: ( J. 2 0B I k S 2 AMPLIFIER 3 o C To 3 OUTPUTS 4 LoD5 4J 5 o-E B POWER SWITCH R Ro OUT BAL. 6 o F F- +300 7 - O 5 Do -190 8 H N 350 9 o-L _Co-3 10 oK M [+ 2vFIL II o — Q POWER GND 12 0 — ID —-- --- --- r-100 cn v —-----—,E +100 c\Ji —---— Vo REST RELAY cr AMPLIFIERS N^, —--------— ^-oz~5-U HOLD RELAYOe r')" C ) I I I I 1z I I,TO AMPLIFIERS 0 m I II 9-12 o o 0) U-) > z o o cq) J I AN -3102-28-2P oool o 0 I- (AT RACK BASE) 1^> Z OO W SL-J + I I -L c I + + I 3: 0 Q. Figure 20. Wiring Diagram of Amplifier Rack. 28

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN At the top of the rack (in the rear) is a terainal strip frorm whiich power and control voltages are distributed to the aiplifier panels. Als. o at the top of the rack are the co.muter control circuits, a Ldlifier-overloai lilghts and meter circuits (see FiL.ire 11). A pilot-ligih indictes;heon the +2b-volt power is on. The three -posit ion HOLD -OPiEATE -PDESET switch is the main control switch operating the computer. The aplifier-ctor itch selectos which amplifier output is read by the meter. * The OUTPUT BALANCE term.inal pro-v ides a connection to the outut of any amplifier Then t1;e balnce button is pressed. (This terminal might be used if a l.more sensitive balance indica.tion than provided by the panel meter is recuired.) The STEP-FUICTIOiN controls provide a convenient source of a constant voltare for ampulifier inputs. The B) PO'IER,sitch is used to turn-off the B voltages while a problem is being set up. Th-ere is a one-toone correspondence between the location of the overload lilghts and thle actual amplifiers wrhich are overloaded. 3. Variable Coefficient Selector -In order to solve linear differential e cuations with variLable coefficients the variable coefficient selector shown in Figure 1 is used. Essentially it is a device which changes the value of one or more resistors in accordance with a prescribed function of time f (t). The resistors a.re in,urn used as feedback or input impedances in operational amplifiers, so that the gain of those amplifiers varies as f (t) or l/f (t). Instead of varying the resistors continuously with time they are varied in discrete steps, thus forming a stepwise continuous approximation to f (t) (see Figure 21). *Armaplifiers are nuimbered. 1 through 12, beginning at the upper lef-hand corner of the rack. Number 12 corresponds to the right-hand amplifier in the lowest bank of four. Positi9n 13 connects the meter to the output balance terminal. 29

--- CONTINUOUS FUNCTION STEP APPROXIMATION TIME -4 Figure 21. Step Approximation to Continuous Function. --- R -R 32K 16K 8K 4K 2K I K I II R6 4~R R, 6 R5 CR 4 $3 R 2.R 6 GANG STEPPING RELAY (4 STEPS) I )4 3 STEP I, STEP 2 ~ BANK. OF TOGGLE STEP 3 SWITCHS STEP 4...e Figure?22. Schematic Circuit for Variable Coefficient Selector. 30

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN After each interval of tire At the resistance is switched to a new value. The resistance of each step approximation is chosen so that at the end of the interval the area under the step curve eqcuals that under the continuous curve. The rosistance for each step is obtained by building it up from a set of binary resistors connected in series. Consider the set of binary resistors shoRn at the top of Figure 22. Evidently the total resistance R can be rmde anything from zero ohmls to 63k ohmns in 1k-ohm steps by short-circuitin; the appropriate binary resistors with their respective relays. The relays for each binary resistor are in turn controlled by the rows of toggle switches at the bottoim of Figure 3. Finally, a stepping relay selects just which row of toggle switches controls the binary-resistor relays. As the stepping relay moves from one rotary position to the next everyAt seconds, it switches control from one row of toggle switches to the next. By properly positioning the toggle switches ahead of time, 06any desired set of binary resistors can be switched in on each In the exple shown in Figure 22 the first row of toggle switches is up down, up, dolm, down, up, from left to rirht. Wlhen the stepping relay is on step 1, therefore, the binary relays short-circuit the 16k-, 4k-, and 2k-ollhm resistors, leaving the total resistance R for step 1 as R = 32 + 8 + 1 = 41 ohms. For step 2 the toggle switches are up, upI down, down, up, down, so th:at when the stepping relay rotates to step 2, the resistance R 32 + 16 2 = 50 k oh:is. As the stepping relay proceeds to step 3 and step 4 the resistance R chan;es in a.ccordance writh the positions of the toggle swit3ches in rows 3 and 4, respectively. Evidently, any step wise continuous function of resistance such as that slhown in Figure 21 can be obtained by properly positioning] ahead 31 -

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN of time the toggle switches at each step (i.e., in each row). Note that the sarne set of binary resistors is used over and over again. For the purposes of illustration the circuit shown in Figure 22 has provision for only 6 digits and 4 steps in time. The actual equipment for the air comp. mod 4 computer has provision for 17 digits and 25 steps. The binary resistors are plug-in units, so that the 17 available digits can be divided between two or more functions. If a function f (t) changes sign, a digit may be used to switch in an inverting amlplifier at the proper time. The stepping relay is driven by a synchronous contactor. The variable coefficient selector consists of two parts: (L) the relay rack containing the 17 x 25 toggle sritches, two 25 step 9 pole stepping relays, and the relay control circuits, and (2) the panel containing the 17 d-c 24v relays and the binary resistor plug-in sockets (one pair for each relay). The circuit for the latter is shown in Figure 23. Pow.er from the toggle-switch comes in through an 18-conductor cable. The 24 volt dc relays are energized or not energized depending on the position of the corresponding toggle switch on a particular step. When a relay is energized, the connection across the binary resistor is opened, and its resistance is added to the total series resistance corresponding to f (t). When the relay is not energized, the binary resistor is short-circuited. Note that two resistors can be controlled by each relay, since they are double pole relays. Thus two similar time varying resistances f (t) can be set up with a single set of relays and toggle switches. It is only necessary to plug the binary resistors into the proper digital sockets and add patch-cord jumpers to connect the resistors in series. The circuit diagram of -the stepping-relay panel is shown in Figure 24. The stepping-relays are interconnected to the stepper-control chassis through a.3 Z

T GND Af s 17 - 24 v ~ XI RELAYS o o o o o o R o 16 Q o15 Po14'; No3: So t P o 14 0 0 0 a. Mo 12 ( L O - 0 0 - Loll K KolO 7G07 I 2 0 0 0 0 0 0 17 z Fo 6 <Eo Do04 Co 3 BO 2 Aorc Figure 23. Binary Resistor Plug-In Circuit.

6 COND JONES PLUG (FROM STEPPER CONTROL GND CHASSIS) 25 25 40 —-eno0 —-o -d r''0 S G ND I T — |1 I 4 0 COL. 17,2 BRING1 0 5-C3 9 > — oBRIDGING 2V 15 O Q START CO NTROL > 1 i 140 ST EP 0 ---- RELAY |o N # — / 2 1| 26 120M I1-25EXT I 47 26 QCONTACTS 0- 0 I T I N 2 Figu-COL. 2'2 PK z _____ [ jsa~i * -- ^ -2 2 BRIDGIN 9 4 o ~' STEP I STEPPER VOLTAGE ROg o8 H ^Tcn 1# I6 472 70 G 26 1-25 EXT. 2560F 0 CONT ACTS I D —-. 6 F COL. I I, 50 E 0 -------— p___ 2 -^ 2BRIDGING 40D INITIAL COND. CONTROL 2 I30C +28v 26 tloo~f 2 NON-BRIDGING 6v 0.25A 25 3 BULBS STEP-INDICATOR LAMPS Figure 24. Stepping-Relay Panel.

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN 6-conductor Jones plug. Fromil this plug comles the 28 volt d-c pulses which activate the stepping relays (pin 4). In standby operation the relays are on step 26. In this position they apply 28 volts to the initial-condition-relay circuit (pin 5), thus energizing all reset relays in the rzain uolplifier rack. When the first 28 volt pulse is appliel throull pin 4, the relays go to step 1, which releases the ini-tial -condition-relay circuit and starts the problem solutin in the imain amplifier rack. The relays continue to step around to step 26, at lhich tille the input pulses through pin 4 cease and the initial-condition-relay voltage on pin 5 is restored. In addition, the external set of contacts on the stepping relay asre used to prevent the start of a solution if the two relays are not synchronized (both on step 26). Note in Figure 24 that the relay co-llnmon for each digi- has a pilot a1mp in series with it before going to the 18 conductor output plug to the relay panel. When the pilot lamp lih-ts (at the top of the toggle switch rack) it means that power has been a-pplied to the relay for that digit, and that the corresponding binary resistor has been switched in. Itote also that a set of stepping relay contacts is used to light the pilot l:laps dotwn the side of the toggle-switch rack,; thus providing an indication of which step the relays are on. The stepping-relay control circuit is shown in Figure 25. Main power into the stepping relay raclk is 28 volts d-c from the power supply rack. It is controlled by a powrer switch on the front panel at the bottom. To begin a solution (start the stepping relays), the panel start switch is turned on. This closes the start control relay, which, if both stepping relays are on step 26, closes the solution relay. The starter relay is held closed, even after the panel start switch is releasedl, until step 26 is reached aCgain, at which time the starter relays drop out. While it is closed, the starter relay allows 28 volt 35

FUSE POWER +28 /^ _,. SWITCH 6 COND. 28v POWER JONES SOCKET AN-3102 -12S -3P 6v PILOT (TO STEPPNG RELAYS)'^UT GND ________ ^_~O A LAMP (TO STEPPING RELAYS) ( ON FRONT) I 2 PANEL / START EXTERNAL A SR START TROLT CONTROL PANEL RELAY RELAY AN-3102-12S-3S START SWITCHW B F [ N~ L to - r -- 03 INPUT oB 0 SWITCH A 200Z 01 PULSE RELAY MANUAL 2. Sp3 p FD INITIAL MANUAL _r CONDITION STEP-PING RELAY PUSH BUTTON CONTRO SWITCH O o TO INITIAL oB CONDITION,) %RELAYS AN-3102-14S -5S E Figure 25. Stepper Control Chassis.

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN pulses from the pulse relay to reach the coils of the stepping relays, thus causing them to step. The pulse relay operates at all times when the main 28 volt power is on and the cam-driven microswitch is operating. Finally, the initial-condition control relay is closed. when the steppers are on step 26, but opens (releasing the reset relays) when the steppers are on positions 1 through 25. A manual stepping push-button switch on the front panel allows the steppers to be cycled manually step by step. Also, an external-start-control cable for remote starts of the solutions can be plugged into a socket on the front panel. After the stepping relays start their 26-step cycle the starting switch should be opened so that the relays stop on step 26. 4. Power Supplies The power-supply rack contains electronically regulated d-c B supplies of +300, +100, -100, -190, and -350 volts, and an unregulated +28 volt relay supply. The rectifier and filter circuits for the B voltages are on separate chassis (Figures 26-28) from the regulator chassis (Figures 29 and 30). The rectifier and filter chassis receives 115-volt 60-cycle line power from line cords from the distribution panel. Cables from the rectifier and filter chassis to the appropriate regulator chassis connect the unregulated B voltages and the 6.3 volt regulator-tube filament voltages to the regulator chassis. The rectifier, filter, and regulator circuits are entirely conventional. Note that the voltage standard for the +100 volt supplies is a 90-volt B battery. This battery should last a time equivalent to its shelf life. The -190 supply has the least regulation of any of the supplies, but is the least critical in affecting anplifier balance. The +300 and -350 supplies may drift the order of 1 volt after -warn-up, but with drift stabilizers on the operational amplifiers this will only, —------ 37

5V ~3A 2A o 1030 VLT 300MA i 005 U 9 0 5<5U4G lw4G 4G oS 5V 3A 5h 600MA 5h600MA 2A 0 A 1030 CT Mh ____ 0 o300MA o I 1 00 I 0 NFO _. IONFO 9K 400V 400V 6 20W 5U4G 5U4G 0 I6A Figure 26 Poe Spl fo 00 0 6.3/IOA Figure 26. Power Supply for + 300 Volt Regulator.

N N-< -< ~ ~O- O-I' o -) < G3 O<,,^' - -- - * —N <N-< 03 < O< IC Figure 27. Power Supply for -190 and -350 Volt _~T: A >z / 1 I ^^rf^c~0'(^-^ -- — 8uFD ELECT. J^ \ >'| 600v.1~t._ II Iq+ ---- SI 1 ELCT w O <7>> 1 ----- ||64000v — 0: 16/uFD ELECT. I II 600v o~ Io,16/FD EILE C T 75K IW N N 15.K 23W 5K lo AN - 3102A -18-8S AN-3102-18-20S Figure 27. Power Supply for -190 and -350 Volt Regulators.

5V 5A 5h 250MA 5h 250MA 15K lo5wK 550VLT 275 MA 2AMP 0' —*- V V5U4GT T o I ^>~^ IOMFD IOMFD 400 V 400 V 170V 6lo~~.3~~ \ —----------------- * ----- *-AN3102-1820S 6A 0 AC LINE 5V 55 2AMP 550VLT 275MA 15 IK IOW 0U4GT IOMFD IOMFD 0 0-4^ 400V 400V V 5h 250 MA 5h 250 MA 170V <0^3" -------- r — -O AN 3102~18 20S 6A0 Figure 28. Power Supply for + 100 and -100 Volt Regulators.

10 0.5A 6 ii I | cD& TCft 200 1K c _a $, I2 C\1 (Wu 0 oAe' ci 6Y6T o 6.3v, 6A <lq I +100 It 12 I I B-: < ~ < W 0.25' 10 w 0 2 Z K 00.25 III I I Ii~ o',~ vI 90vI + MEG 4' 0 D —+ 100 6' 0 E -— 00 20 -' C — 350 500K 0 C —350 o N, A \ NC B - I 6Y6~ I 0 z - * -Fg. 2 R a s ~1 0 -100 <*?~~~~~ <LY' &_L j~g <^A541'W low I 1: 20 IK -I MEG 0 >A50> —- 90v 0 L-iI^ I I 6Y ^ P2AC7 Ia~<rn I! I -200 u~~~3 0 I 0 Figure 29. Regulatorsfor 100,R-100, + 002 101 ad 35 Rglaor AR OP OD41D 41

6AS7 600 3/4A. I NA +350/, -O6AS7 I BGND GN0 0 D63vFIL e= —XIOA to 0 ~~Z 0:z < z~o g 6AS7 1N r+ 300 v ___ i 5W 00 0 m NA oC-'' I.SMEG 0 H+300 o Z E GND 10K -F 0.25 VR-105 o 0600~ i, Ah,) w5I 6A7 6 -,0 -- I [ 2 —-7S GUARDIAN - Ay 3/4A SERIES 00 600 B 6.3v FIL 200 6AS7 6.3v FIL L) z F 6AS7 A 0 AIR 75COMP MOD4 EDA 500-K to - Figure 30. Regulators for +300 and -190 Volt Supplies. 42

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN cause a negligible zero drift. The +100-volt supplies should hold to within 0.1 volt of their set value. Capacities of the various supplies are noted below +300 600 rLm +100 275 ma -100 275 1ma -190 560 rma -350 50 ma + 28 4 amps On the regulator chassis there are a-c 115-volt relays which connect the unregulated B voltages to the plates of the regulator tubes. This connection is not made until the filanents in the control tubes have had a chance to warm up. This is governed by a time delay tube in the power supply distribution panel (Figure 31), which closes the circuit to the 115-volt relays only after the start switch on the front panel of the power supply has been turned on. When the timedelay tube closes, the ready light goes on, and when the operate push button is depressed, the 115-volt relays close and the B-supply voltages are turned on. Note that each B-supply voltage has a d-c relay across its unregulated supply. Unless all these relays (and hence all the B supplies) are on, the 115-volt a-c relays drop out and turn all the B voltages off. This is to prevent damage to the computer if one of the B supplies goes out. The operate push buttonby-passes this safety feature in order to close the 115-volt relays initially. The distribution panel (Figure 31) receives power cables from the regulator chassis, +12-volt filament source, and-+j28-volt relay supply (Figure 32) and distributes them through the main power cable to the amplifier relay rack. It also distributes +28 volts to tile stepping-relay panel and receives 43

POWER TO AMP. RACK +300 oH F -190 oF. —-BO - GNDE A a0 GND oO.) c E f% I < ~ i- +o G 0 o B D 0C Da::o C z L 0-5MA M 0 Z (- V Is > G-N00D 5-OD H RE l-'^^o-.o'^_-ig 31. Power S Drt Pan o44^'^^"3 OUT OUT 0 _B^ (D P0^~ E<o ~'^^4- POER T0 A 3 4 HHRESET RELAY OUT B ~z~o OB 60"' TO RESET RELAY ~TO CT. o - INITIAL CONDITION RELAYo IGND-130 o c+3000 30 0 ~0~ muj=) -0 C, I cn 0 I CONTROL I_____ OI (G R OOK 0-5MA + 0-50 cj a- A 115vAC 115vAC aVOLTS ~L Na + 0 — 3 OU ~O~J 00 2 OUT OUT 8 0 GND na'n,1 _ 0 I POWER TO U- >:) z_ B CO (/ <ID: STEPPERS c\_ 10 A TIME DELAY AIRCOMP MOD 4 EDA AC FigureN 31. Power Supply Distribution Panel..L. 60fv I44 START START READY I 1 5v AC R L. SW. P.L. POWER TO RESET RELAY TO RECT. x~j CHASSIS o~ o~ INITIAL CONDITION RELAY I,~ E POWE_,FROM + GN- +.2 -20 z qt PN-D ST EPPS350 o +3000 o350 0 0 OFF: OFF OFF HOLD RELAY 0 —5 OFFo 0 0 OOFF CONTROLO-5MA t 0-50 DC VOLTS AIRCOMP MOD 4 EDA Figure 31. Power Supply Distribution Panel. 44

RECT. TRANSFORMER 18-CT-18 SEC 12A + SEL. RECT.,r_ --- ^,*x 36v 6A AC IN I A i \ 28v DC OUT lo ig \/ 4AMPS C 28 LINE PLUG -+ GNDT,. 50v ~ < Figure 32. 28 Volt DC Supply. 45

POWER POWER TO 24v~~~~~~~~~~~~~~~2 0 FIL. ~ 115v 0 60-AIN 0 TRANSORMER,115v 60I^ 5 5<OWR POWER A qB INPUT 26v 10 STEPPERS ~OES 6 COND. QJONES! W ~24v -— 5- AMPLIFIER&ATRANSFORMER 115v F 3 hA FeOR f C C60 oneto RESET RELAY 28v TO CHOPPERS VOLTAGE FROM STEPPER STEPPERS POWER TO 115v AMPLIFIER 60 ~ Figure 33. Schematic for Cable Connections.

ENGINEERING RESEARCH INSTITUTE - UNIVERSITY OF MICHIGAN the +28 volt initial-condition voltage from the stepping-relay panel. Voltmleters are provided on the front of the panel to ionitor the various voltages. The hold-relay socket on the back of the panel is designed so that when a connection is made between the terminals, the hold relays are energized. 5. Recording Assembly A recording galvanometer cart, including a Sanborn Model 60 1300 2-channel recorder and two electronic amplifiers is provided for the Air Comp. Mod. 4 Electronic Differential Analyzer. The regular Sanborn instruction ranual should be consulted for a description of the actual recording galvanometer. The two-channel electronic drive for the recorder consists of a power supply and two d-c amplifiers. The circuit for the porer supply is shown in Figure 34; it is a conventional circuit with VR regulated d-c output voltages of +300, +75, -195, and -345, and unregulated d-c output voltages of about +200 and -200 volts. Power input is provided through a separate 115-volt 60-cycle plug. rTwo 12-pin Jones sockets are provided for cable connection to the respective amplifier channels, and a 10-pin Jones socket is provided for cable connection to the galvanometer assembly. The circuit diagram for the d-c amplifier is showm in Figure 35. It actually consists of two operational amlAlifiers. The first amplifier is used to provide various amplifications of the input signal, depending on the amount of feedback given by the selector switch. The output of the first amplifier feeds into the second amplifier, which has a 6AQ5 cathode follower as its final output stage in order to drive the 3000 ohm recorder coil. This second alplifier has input and feedback irpedance networks which compensate for the falloff withl frequency of the galvanomleter output and give the amplifier a d-c gain of about 2. The amplifier-galvanometer combination has a frequency response flat within _____________ ~47. __________________________

x TO CATHODE 6.-3v 66X5 #I 5h 150MA +200 + 200 2A -L,, S| F 7 |I — 5 I ) -20030 +300~ 12 COND. 750 VCT 5]L FD_.. 5pF D | + 75 1 D. |l50 MAi - " ~ ---- ~,,9 JONES. /ND2 - TO - 1454 AMPLIFIER, I| 1u_ ~-345,08 CHANNEL 6.3v FIL z —0 5 A OUT 12 TO CATHODE 6.3v 6X 5 #2 5v 0_______. ^kkL)200 2A 1 200 — 0 0 0-<o<F I 30 6- 12 COND. 115v 750 VCT 75 JONES, ~60" 150~MA o TO 02 AMPLIFIER, -175 7 CHANNEL 8 B 5h 15OMA -200 z 6.3v 12.___ Z VR 150, ) g6.5v _ FD 20/LFD 1 i.5A" --: VR75 ~ *5 - 5/.LFD 6X5 VR 75(' 7 2A T _ _ _ K -(F xiM? VR105^ + 8' — 0 8 10 COND. I 6 5 15,FD I VR90 - TO 2 FD — 3 GALVENO-' F -195 1 \4^Y VR150 METER 5h 70MA 5k IOW 45 Figure 34. Power Supply for Recorder Amplifiers. 48

I-L<~ *.^< BAL.POT lp 99 X 127COND 2MEG 3 V KNOB r ---- JONES /dItPLIFIER 2f — ^'^CAN 20P5 + 200 0 —45 O 1 U- 120 INPUT -345 lOK2( o 2 INPUT^ AL. 75 0 (SREaAL.POT 0 +300 3 ODl DRIVER) 0 -145 20AO 200 1. 4 +300 ---- AMPLIFIER 7 63v FIL ATTENUATOR i 5691 5:C00 -345 SW$ITCH IOyFI 3 -o 8 oS~~~wlTCH~~ i0 <-l 0 +75 0 0 )-o 9 IMEG N4 c 0 I —---- VAr^ /PUTJ I i-L~ 2 |I~ ----—,f F~EEDBACK —. _______ LS ______ I^ ~NETWORK N'>C)~ ~CA \y CAL.M S 2 200 0 o 5 100o c0 0 50 —'I —------— 4 ^ +300 _ o SMALL Ui, 0 ON tio QFF 0 BIAS CONTROLS -195 Figure 35. Amplifier Schematic for the Recorder Drive. 49

ENGINEERING RESEARCH INSTITUTE * UNIVERSITY OF MICHIGAN 0.1 dcl' to about 75 cycles per second, and is down 1 db at 100 cycles per second (see Figure 37). Without the comecnsation the galvanometer alone is down 1 db at 30 cycles per second when tlhe driving impedance is adjusted for a galvanomlete dcuaping ratio of 0.7. An additional input is provided to the d-c aimplifier for a bias control. The unregulated B+ and B- voltages for the 6A2Q5 are designed so that positive and negative saturated outputs correspond approximately to full scale recording arM deflections. Hence it should not be possible to daimae the galvanomieter through too big a signal input. The circuit for the plug-in portina of the d-c aip lifiers is sho,.wn in Figure 36. After severa l miinutes wanr-up ti he following procedure should be used to adjust the recorder. Witih no input turn the attenuator control to off. Here the amp lifier gain is a maximuwl with zero input, so that the d-c balance control hias max,:imui sensitivity. The balance knob should be adjusted until the recorder arm is about on center. This adjusts the balance of the first amplifier. The second armp1lifier (with 6AQ5 output) has a screrw-driven balance adjustmlent!Jwhich norma 1ll' does not need to be changed. After the balance adjustment turn the attenuator control to 200. Here the amplifier gaLin is a mlinirmjmna and negligible zero offset is assured. The bias control should now be adjusted until the recorder arm reads zero deflection. The bias compensates for any unbalance either in the second amplifier or in the Calvanomreter suspension. Iext the sensitivity control is varied with the attenuator on "CAL"i until the desired recorder anrm deflection for 75 volts input and 200 attenuation is reached. Finally the attenuator is switched to the desired attenuation and the voltage to be recorded is plugged in. Zero drift of the amplifier should be negligible on all but the loTwest several attenuations. However, some hysteresis (about 0.5 Im) mlay be noted in the recording galvanometer. 50

I ~LOuu FD 400K I0-F 25 400K + 300J L2.5 I I | o I L I 1 IN 250K 800K IQOQ T *1000 IN 250K _ IFD 10 3K C 1/2 5691 (,> 1/2 5691 -345 8 o —------ -% W I MEG GND II -I -- o OUT 200q FD 7 0 9 0 — 6.3v FIL 10 -- 685K IN. 270K 0.0047, FD 6 o AMP IN ci OUT 32K GND 6.4K 2.56K 0.502 LFD Figure 36. Amplifier Can Circuit and InputFeedback Can Circuit. 51

r -2 rco'd o ---— Recorder With Feedback CC^~' ~~Amplifier Compensation CD -10 - v I --- Recorder With No Compensation. C). 0 =:,C =p0.707 (Best for Minimum a) CD Response Time to Stop Input) c 1 ____ —14' 0 (1) 0\ (0 CZ -20_ 10 20 30 40 50 70 100 200 300 Frequency (cps)