Repair of the oscilloscope C1-94. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This article assumes the use of the factory scheme of the device.

Many specialists, and especially radio amateurs, are well aware of the C1-94 oscilloscope (Fig. 1). The oscilloscope, with its rather good technical characteristics, has very small dimensions and weight, as well as a relatively low cost. Thanks to this, the model immediately gained popularity among specialists involved in mobile repair of various electronic equipment, which does not require a very wide bandwidth of input signals and the presence of two channels for simultaneous measurements. Currently, a fairly large number of such oscilloscopes are in operation.

In this regard, this article is intended for specialists who need to repair and configure the S1-94 oscilloscope. The oscilloscope has a block diagram common for devices of this class (Fig. 2). It contains a vertical deflection channel (VDO), a horizontal deflection channel (HTO), a calibrator, an electron beam indicator with a high-voltage power supply, and a low-voltage power supply.

The CVO consists of a switchable input divider, a preamplifier, a delay line, and a final amplifier. It is designed to amplify the signal in the frequency range of 0 ... 10 MHz to the level required to obtain a given vertical deviation coefficient (10 mV / div ... 5 V / div in steps of 1-2-5), with minimum amplitude- frequency and phase-frequency distortions.

The CCG includes a timing amplifier, a timing trigger, a trigger circuit, a sweep generator, a blocking circuit, and a sweep amplifier. It is designed to provide linear beam deflection with a specified sweep factor from 0,1 µs/div to 50 ms/div in 1-2-5 steps.

The calibrator generates a signal to calibrate the instrument in terms of amplitude and time.

The CRT assembly consists of a cathode ray tube (CRT), a CRT power circuit, and a backlight circuit.

The low-voltage source is designed to supply all functional devices with voltages of +24 V and ±12 V.

Consider the operation of the oscilloscope at the circuit level.

The investigated signal through the input connector Ш1 and the push-button switch V1-1 ("Open / Closed input") is fed to the input switchable divider on the elements R3 ... R6, R11, C2, C4 ... C8. The input divider circuit ensures that the input impedance is constant regardless of the position of the vertical sensitivity switch B1 ("V/DIV."). Divider capacitors provide frequency compensation of the divider over the entire frequency band.

From the output of the divider, the signal under study is fed to the input of the KVO preamplifier (block U1). A source follower for a variable input signal is assembled on a field-effect transistor T1-U1. For direct current, this stage provides symmetry of the operating mode for subsequent stages of the amplifier. The divider on resistors R1-Y1, Ya5-U1 provides an input impedance of the amplifier equal to 1 MΩ. Diode D1-U1 and zener diode D2-U1 provide input protection against overloads.

Rice. 1. Oscilloscope S1-94 (a - front view, b - rear view)

The two-stage preamplifier is made on transistors T2-U1 ... T5-U1 with a common negative feedback (OOS) through R19-Y1, R20-Y1, R2-Y1, R3-Y1, C2-U1, Rl, C1, which allows get an amplifier with the required bandwidth, which practically does not change with a step change in the stage gain by two and five times. Changing the gain is carried out by changing the resistance between the emitters of transistors UT2-U1, VT3-U1 by switching resistors R3-y 1, R16-yi and Rl in parallel with the resistor R16-yi. The amplifier is balanced by changing the potential of the base of the transistor TZ-U1 with a resistor R9-yi, which is brought out under the slot. The beam is shifted vertically by resistor R2 by changing the base potentials of transistors T4-U1, T5-U1 in antiphase. The correcting chain R2-yi, C2-U1, C1 performs frequency correction of the gain depending on the position of switch B1.1.

To eliminate parasitic connections in the power circuits, the preamplifier is powered through the R42-U1, S10-U1, R25-yi, C3-U1 filter from a -12 V source and through the R30-yi, S7-U1, R27-yi, S4-U1 filter from +12 V source.

To delay the signal relative to the start of the sweep, a delay line L31 is introduced, which is the load of the amplifying stage on transistors T7-U1, T8-U1. The output of the delay line is included in the base circuits of the transistors of the final stage, assembled on transistors T9-U1, T10-U1, T1-U2, T2-U2. This inclusion of the delay line ensures its coordination with the cascades of the preliminary and final amplifiers. Frequency correction of the gain is performed by the chain R35-yi, C9-U1, and in the final amplifier stage - by the chain C11-U1, R46-yi, C12-U1. Correction of the calibrated values ​​of the deviation coefficient during operation and change of the CRT is carried out by the resistor R39-yi, brought out under the slot. The final amplifier is assembled on transistors T1-U2, T2-U2 according to a common base circuit with a resistive load R11-Y2 ... R14-Y2, which allows you to achieve the required bandwidth of the entire vertical deflection channel. From the collector loads, the signal is fed to the vertical deflecting plates of the CRT.

Rice. 2. Structural diagram of the oscilloscope S1-94

The signal under study from the KVO preamplifier circuit through the emitter follower cascade on the T6-U1 transistor and switch V1.2 is also fed to the input of the KGO synchronization amplifier for synchronous triggering of the sweep circuit.

The synchronization channel (US block) is designed to start the sweep generator synchronously with the input signal to obtain a still image on the CRT screen. The channel consists of an input emitter follower on a T8-UZ transistor, a differential amplification stage on T9-UZ, T12-UZ transistors, and a synchronization trigger on T15-UZ, T18-UZ transistors, which is an asymmetric emitter-coupled trigger with an emitter follower at the input to the transistor T13-U2.

The D8-UZ diode is included in the base circuit of the T6-UZ transistor, which protects the synchronization circuit from overloads. From the emitter follower, the clock signal is fed to the differential amplification stage. The differential stage switches (B1-3) the polarity of the synchronizing signal and amplifies it to a value sufficient to trigger the synchronization trigger. From the output of the differential amplifier, the clock signal is fed through the emitter follower to the input of the synchronization trigger. A signal normalized in amplitude and shape is removed from the collector of the T18-UZ transistor, which, through the decoupling emitter follower on the T20-UZ transistor and the differentiating circuit S28-UZ, Ya56-U3, controls the operation of the trigger circuit.

To increase the synchronization stability, the synchronization amplifier, together with the synchronization trigger, is powered by a separate 5 V voltage regulator on a T19-UZ transistor.

The differentiated signal is fed to the trigger circuit, which, together with the sweep generator and the blocking circuit, provides the formation of a linearly changing sawtooth voltage in standby and self-oscillating modes.

The trigger circuit is an asymmetric emitter-coupled trigger on transistors T22-UZ, T23-UZ, T25-UZ with an emitter follower at the input on the T23-UZ transistor. The initial state of the trigger circuit: the T22-UZ transistor is open, the T25-UZ transistor is open. The potential to which the C32-UZ capacitor is charged is determined by the collector potential of the T25-UZ transistor and is approximately 8 V. The D12-UZ diode is open. With the arrival of a negative pulse at the T22-UZ base, the trigger circuit is inverted, and the negative drop on the T25-UZ collector locks the D12-UZ diode. The trigger circuit is disconnected from the sweep generator. The formation of the forward stroke of the sweep begins. The sweep generator is in standby mode (switch B1-4 in the "WAITING" position). When the amplitude of the sawtooth voltage reaches about 7 V, the trigger circuit through the blocking circuit, transistors T26-UZ, T27-UZ returns to its original state. The recovery process begins, during which the time-setting capacitor C32-UZ is charged to the initial potential. During recovery, the blocking circuit maintains the trigger circuit in its original state, preventing the synchronization pulses from transferring it to another state, that is, it delays the start of the sweep by the time necessary to restore the sweep generator in standby mode and automatically starts the sweep in self-oscillating mode. In the self-oscillatory mode, the sweep generator operates in the "AWT" position of the switch B1-4, and the launch and disruption of the operation of the trigger circuit - from the blocking circuit by changing its mode.

As a sweep generator, a circuit for discharging a time-setting capacitor through a current stabilizer was chosen. The amplitude of the linearly changing sawtooth voltage generated by the sweep generator is approximately 7 V. The time-setting capacitor C32-UZ during recovery is quickly charged through the T28-UZ transistor and the D12-UZ diode. During the working stroke, the D12-UZ diode is locked by the control voltage of the trigger circuit, disconnecting the timing capacitor circuit from the trigger circuit. The capacitor is discharged through the T29-UZ transistor, which is connected according to the current stabilizer circuit. The discharge rate of the time-setting capacitor (and, consequently, the value of the sweep factor) is determined by the current value of the T29-UZ transistor and changes when the time-setting resistances R12 ... R19, R22 ... R24 are switched in the emitter circuit using switches B2-1 and B2- 2 ("TIME/DIV."). The sweep speed range has 18 fixed values. A change in the sweep factor by a factor of 1000 is provided by switching the time-setting capacitors C32-UZ, S35-UZ with the switch Bl-5 ("mS / mS").

Setting the sweep coefficients with a given accuracy is carried out by the capacitor C3З-UZ in the "mS" range, and in the "mS" range - by the trimming resistor R58-y3, by changing the mode of the emitter follower (transistor T24-UZ), which supplies the timing resistors. The blocking circuit is an emitter detector based on a T27-UZ transistor, connected according to a common emitter circuit, and on R68-y3, S34-UZ elements. A sawtooth voltage is supplied to the input of the blocking circuit from the divider R71-y3, R72-y3 at the source of the transistor TZO-UZ. During the working stroke of the sweep, the capacitance of the S34-UZ detector is charged synchronously with the sweep voltage. During the recovery of the sweep generator, the transistor T27-UZ is closed, and the time constant of the emitter circuit of the detector R68-y3, C34-UZ maintains the control circuit in its original state.

The standby sweep mode is provided by locking the emitter follower on the T26-UZ switch V1-4 ("WAITING / AUTO."). In the self-oscillating mode, the emitter follower is in a linear mode of operation. The time constant of the blocking circuit is changed in steps by switch B2-1 and coarsely by B1-5. From the sweep generator, the sawtooth voltage through the source follower on the TZO-UZ transistor is fed to the sweep amplifier. The repeater uses a field-effect transistor to increase the linearity of the sawtooth voltage and eliminate the influence of the input current of the sweep amplifier. The sweep amplifier amplifies the sawtooth voltage to a value that provides a given sweep ratio. The amplifier is made as a two-stage, differential, cascode circuit on transistors TZZ-UZ, T34-UZ, TZ-U2, T4-U2 with a current generator on the transistor T35-UZ in the emitter circuit. The frequency correction of the gain is carried out by the capacitor C36-UZ. To improve the accuracy of time measurements, the CVO of the device provides for a sweep stretch, which is provided by changing the gain of the sweep amplifier by connecting resistors Y75-U3, R80-UZ in parallel when contacts 1 and 2 ("Stretching") of the ShZ connector are closed.

Table 1. Modes of active elements for direct current

The amplified sweep voltage is removed from the collectors of the transistors ТЗ-У2, Т4-У2 and fed to the horizontally deflecting plates of the CRT.

The synchronization level is changed by changing the potential of the base of the T8-UZ transistor by the resistor R8 ("LEVEL"), displayed on the front panel of the device.

The beam is shifted horizontally by changing the base voltage of the T32-UZ transistor with resistor R20, which is also displayed on the front panel of the device.

The oscilloscope has the ability to supply an external synchronization signal through socket 3 ("Output X") of the ShZ connector to the T32-UZ emitter follower. In addition, a sawtooth voltage output of about 4 V is provided from the emitter of the TZZ-UZ transistor to slot 1 ("Output N") of the ShZ connector.

The high-voltage converter (block U31) is designed to power the CRT with all the necessary voltages. It is assembled on transistors T1-U31, T2-U31, transformer Tpl and is powered by stabilized +12V and -12V sources, which allows you to have stable CRT supply voltages when the mains voltage changes. The supply voltage of the CRT cathode -2000 V is removed from the secondary winding of the transformer through the doubling circuit D1-U31, D5-U31, S7-U31, S8-U31. The supply voltage of the CRT modulator is also removed from the other secondary winding of the transformer through the multiplication circuit D2-U31, DZ-U31, D4-U31, C3-U31, S4-U31, S5-U31. To reduce the influence of the converter on the power sources, an emitter follower ТЗ-У31 was used.

The CRT filament is powered from a separate winding of the Tpl transformer. The supply voltage of the first anode of the CRT is removed from the resistor Ya10-U31 ("FOCUSING"). The brightness of the CRT beam is controlled by the resistor R18-Y31 ("BRIGHTNESS"). Both resistors are brought to the front panel of the oscilloscope. The supply voltage of the second anode of the CRT is removed from the resistor Ya19-U2 (brought out under the slot).

The backlight circuit in the oscilloscope is a symmetrical trigger, powered from a separate 30 V source relative to the -2000 V cathode power source, and is made on transistors T4-U31, T6-U31. The trigger is triggered by a positive pulse taken from the emitter of the transistor T23-UZ of the trigger circuit. The initial state of the backlight trigger T4-U31 is open, T6-U31 is closed. A positive edge of the pulse from the trigger circuit switches the backlight trigger to another state, a negative one returns it to its original state. As a result, a positive pulse with an amplitude of 6 V is formed on the T31-U17 collector, equal in duration to the duration of the forward sweep. This positive pulse is applied to the CRT modulator to illuminate the forward sweep.

The oscilloscope has the simplest amplitude and time calibrator, which is made on the T7-UZ transistor and is an amplifier circuit in the limiting mode. The input of the circuit receives a sinusoidal signal with the frequency of the power supply. Rectangular pulses are taken from the collector of the T7-UZ transistor with the same frequency and amplitude of 11,4 ... 11,8 V, which are fed to the input divider KVO in position 3 of switch B1. In this case, the sensitivity of the oscilloscope is set to 2 V / div, and the calibration pulses should occupy five divisions of the vertical scale of the oscilloscope. Timebase calibration is performed in position 2 of switch B2 and position "mS" of switch B1-5.
The voltages of the sources 100 V and 200 V are not stabilized and are taken from the secondary winding of the power transformer Tpl through the doubling circuit DS2-UZ, S26-UZ, S27-UZ. The source voltages of +12 V and -12 V are stabilized and are obtained from a stabilized 24 V source. The 24 V stabilizer is made on transistors T14-UZ, T16-UZ, T17-UZ. The voltage at the input of the stabilizer is removed from the secondary winding of the transformer Tpl through the diode bridge DS1-UZ. The adjustment of the stabilized voltage of 24 V is carried out by the Y37-U3 resistor, brought out under the slot. To obtain sources of +12 V and -12 V, an emitter follower T10-UZ is included in the circuit, the base of which is powered by a resistor R24-y3, which adjusts the +12 V source.

When carrying out repairs and subsequent tuning of the oscilloscope, first of all, it is necessary to check the modes of active elements for direct current for compliance with their values ​​\u1b\uXNUMXbgiven in Table. XNUMX. If the checked parameter does not fit within the allowable limits, it is necessary to check the serviceability of the corresponding active element, and if it is serviceable, the "strapping" elements in this cascade. When replacing the active element with a similar one, it may be necessary to adjust the operation mode of the cascade (if there is an appropriate tuning element), but in most cases this is not necessary, because. the cascades are covered by negative feedback, and therefore the spread of the parameters of the active elements does not affect the normal operation of the device.

In the event of malfunctions associated with the operation of the cathode ray tube (poor focusing, insufficient beam brightness, etc.), it is necessary to check the compliance of the voltages at the CRT terminals with the values ​​\u2b\uXNUMXbgiven in Table. XNUMX. If the measured values ​​do not correspond to the table values, it is necessary to check the serviceability of the nodes responsible for the generation of these voltages (high voltage source, output channels of the KVO and KTO, etc.). If the voltages supplied to the CRT are within the permissible range, then the problem is in the tube itself, and it must be replaced.

Table 2. DC CRT Modes

Notes:

1. Checking the modes given in table. 2 (except contacts 1 and 14) is made relative to the instrument case.
2. Checking the modes on contacts 1 and 14 of the CRT is carried out relative to the cathode potential (-2000 V).
3. Operating modes may differ from those indicated in Table. 1 and 2 by ±20%.

Author: Zakharychev E.V., design engineer; Publication: cxem.net

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