Floating Amplifier Driver and On/Off Switch (Service Sheet 3)
The On/Off Switch is opened for 15 ms when the Sharp Cutoff Filter is in and the frequency of the signal is changed to prevent the ringing in the filter from getting to the instrument’s output.
The unity-gain, inverting buffer amplifier formed by U403 and U404 isolates the Floating Output Amplifier from the Sine X/X Compensation. The combination of resistors R412, R413, and R421 form
a1 kR resistance which can handle the output current from the Audio Amplifier. U404 is a current booster stage. C401 bypasses U404 at high frequencies to stabilize the buffer at high frequencies. L401 and R416 compensate the high-frequency rolloff of the Floating Output Amplifier.
Floating Output Amplifier (Service Sheet 4)
The Floating Output Amplifier is a single-ended-to-differential converter. Conversion is accomplished by a precise combination of negative feedback, positive feedback, and cross-coupling which yields a symmetrical differentialoutput with high common-mode rejection and a well-definedoutput impedance of 50R. The complexity of the circuit makes detailed analysis difficult.
Output transistors Q401 through Q404 boost the current drive capability of differential amplifiers U401 and U402. Constant-current sources CR423 through CR426 provide bias current for diodes CR404 through CR407 and the bases of Q401 through Q404. CR423 through CR426 thermally compensate the base-emitter junctions of Q401 through Q404. Q405 through Q408 shut off Q401 through Q404 when the latter transistors start drawing too much current to protect the amplifier. Further protection is provided by CR408 through CR411 and CR419 through CR422. C425 equalizes the phase shift through the upper and lower portions of the amplifier for best high-frequency balance.
The +16 and -16V power supply regulators (Q300 and Q301) supply current to only the amplifier. This minimizes the effects of the large-current needs of the amplifier on the other circuits and minimizes distortion.
Overvoltage Protectionand Output Circuits (Service Sheet 4)
Between the Floating Output Amplifier and the output connectors are several circuits.
Balun T400 improves the balance of the Floating Output Amplifier at high frequencies where the amplifier’s balance has deteriorated.
The 48 dB Attenuator is a 500, pi-section attenuator. C505 compensates the high-frequency effects of the parasitic capacitance of the contacts of K8.
The Overvoltage Protection Circuits open the relays on the two output lines when a large voltage is inadvertently applied to an output connector. The circuit thus protects the instrument’s output circuits (especially the Floating Output Amplifier).Application of a large voltage on the output is sensed (after attenuation by voltage dividers R490 and R491 and R492 and R493) by comparators U400A through U400D which are referenced to voltages from the voltage dividers R495 and R497 and R494 and R496. Both positive and negative references are sensed on both output lines. When a comparator trips, K402A and K402B are opened via flip-flop U2B and drivers UlOG, U8D and U8E. U2B also turns on Q33 which is read by the controller to generate an error message on the instrument’s display. CR415 through CR418 protect the comparators. VR31 shifts the comparators’ outputs to a TTL level. C440 and C441 prevent arcing of the contacts of K402 when the relay is deactivating. C64, CR17, CR18, and R122 open K402 at instrument turn-on.
Components C409 through C413 and L402 prevent high-frequency parasitic signals on the high-output line from radiating from the instrument.
Relay K404A controls the float or ground state of the low-output line.
