The SG165 is a versatile calibration instrument designed for comprehensive testing and adjustment of various electronic signals and circuits. It offers a range of functions for signal generation, modulation, and measurement, making it suitable for technicians and engineers working with audio, radio frequency (RF), and intermediate frequency (IF) systems.

Function Description

The core functionality of the SG165 revolves around its ability to generate precise signals and provide a platform for their measurement and adjustment. It can produce a 400 Hz sine wave, which is a fundamental signal for audio and modulation testing. This sine wave can be configured for various outputs, including a speaker load, and can be used to assess separation in stereo systems. The device also supports the generation of a 19 KHz pilot tone, crucial for FM stereo broadcasting and reception testing.

For modulation capabilities, the SG165 offers both FM and AM modulation. The FM modulation can be set to IHF MPX (Institute of High Fidelity Multiplex), a standard for FM stereo broadcasting, allowing for accurate testing of FM receivers. AM modulation is also available for testing AM radio systems. The device includes RF tuning capabilities, with a fully counter-clockwise (CCW) setting for specific adjustments.

Signal output levels can be controlled and monitored through the Microvolt Output function, with a range selector (X1000) and an adjustment knob for fine-tuning. This allows for precise control over the signal strength delivered to the device under test. The SG165 also features a Marker Height adjustment and an AM/FM IF Rocker, both set to mid-scale for initial calibration, indicating internal signal path adjustments.

The SG165 is equipped with a sweep function, specifically a 10.7 MHz sweep with markers, which is essential for aligning IF stages in FM receivers. This sweep signal, when viewed on an oscilloscope, helps visualize the frequency response and bandwidth of the IF amplifier. The markers provide reference points for accurate alignment.

Usage Features

Operating the SG165 involves a series of steps for setting up and performing various calibration tasks. The device requires external equipment for comprehensive testing, including a scope (SC61), a frequency counter (FC71), and an RF voltmeter (Boonton).

For power supply adjustments, the user needs to measure the voltage on the positive lead of C105 and then adjust R113 to achieve an equal negative voltage at the negative lead of C106. This ensures proper power rail balance within the device.

When generating a 400 Hz sine wave, the user connects a scope to the "all signal output" and adjusts R375 to achieve the desired 400 Hz frequency. The expected level is around 30 VPP (Volts Peak-to-Peak) with a direct lead. The device can also switch to a 400 Hz square wave, which should maintain the same frequency and amplitude as the sine wave, allowing for waveform integrity checks.

The SG165 supports an "extra crystal" function, where the output should be absent unless a crystal is inserted into the SG165 crystal holder, indicating a specific mode of operation for external crystal testing. For 67 KHz adjustments, the device switches to "SCA 67 KHz" and L307 is adjusted to achieve the 67 KHz frequency.

MPX signal generation is a key feature for FM stereo testing. The user can turn on left and right 400 Hz signals to observe multiplexed sine waves. R250 is adjusted for a straight baseline, and turning on both left 400 Hz and the pilot 19 KHz (at various percentages like 5% or 10%) allows for observation of modulated sine waves, crucial for assessing stereo separation and pilot tone injection. The FM output modulation can be switched between STD. MPX. and IHF MPX, affecting the output level and allowing for testing against different standards.

For 10.7 MHz sweep and markers, the user connects an oscilloscope to a specific point (near TR305) and adjusts L305 for a 100 KHz output. Further adjustments involve connecting scope probes to molex Pins 2 and 3 of the 300 board, using the vector function on the scope, and adjusting R103 for a perfect circle, indicating proper phase and amplitude balance. The sweep signal itself, with its three markers, is then observed on the scope, allowing for visual alignment of IF stages.

RF level adjustments involve connecting the "all signals output" to an RF voltmeter. R391 is adjusted for 100 mV when in 10.7 MHz IF mode. Similar checks are performed for 10.7 MHz XTAL controlled mode. For AM RF, R392 is adjusted for 100 mV, and levels are checked across different AM dial frequencies using X1000 and X10 ranges. For FM RF, R360 is adjusted for 10 mV with the microvolt output at X1000, and levels are checked across FM frequencies using X1000 and X100 ranges.

Frequency calibration is a detailed process. For AM RF, the user connects the output to a frequency counter (FC71) with a 1 Meg load. L302 is adjusted for 500 KHz, and C305A for 1600 KHz, with a check at 1000 KHz. For 10.7 MHz IF, L301 is adjusted for 10.7 MHz, and the rocker switch is rotated to observe frequency variation. XTAL controlled mode is checked for 10.7 MHz. Similar adjustments are made for 455 KHz IF (L303) and 262 KHz IF (L304), with the rocker switch used to observe frequency adjustments. AM modulation is also checked on the scope at 30%. For FM RF, L308 is adjusted for 88 MHz and C341A for 108 MHz, with a check at 98 MHz.

The meter load test involves connecting the signal output to a radio test setup using specific leads. The 19 KHz pilot is set to 10%, and the SG165 dial is tuned to match the radio's frequency until the pilot light comes on. Left and right 400 Hz signals are turned on to observe meter deflections, and the power range is adjusted (10w, 100w) to observe corresponding decreases in readings, indicating proper power scaling. The separation test involves specific manipulations of the left 400 Hz signal to observe its effect on the meters. Finally, the pilot 19 KHz is switched to 5% and then 0% to observe the pilot light's behavior, confirming its functionality.

Maintenance Features

The manual provides a structured approach to calibration, which inherently serves as a maintenance guide. By following the detailed adjustment procedures, users can ensure the SG165 maintains its accuracy and performance over time. The step-by-step instructions for adjusting various potentiometers (e.g., R113, R375, R250, R391, R392, R360, R103) and inductors (e.g., L307, L305, L302, L301, L303, L304, L308) allow for fine-tuning of the device's internal circuits.

The "NOTE: One calibration may affect the other somewhat" serves as a crucial maintenance tip, reminding the user that adjustments are interconnected and may require iterative fine-tuning to achieve optimal overall calibration. This highlights the importance of re-checking previous adjustments after making subsequent ones.

The requirement for external test equipment (scope, frequency counter, RF voltmeter) is also a maintenance feature, as these tools are essential for verifying the SG165's output and ensuring its proper operation. Regular use of these external instruments during calibration procedures helps in identifying any deviations or malfunctions within the SG165 itself.

While the manual doesn't explicitly detail component replacement or repair, the comprehensive calibration steps provide a diagnostic framework. If the device cannot be calibrated to the specified parameters, it indicates a potential internal fault that would require further troubleshooting and repair by a qualified technician. The detailed settings and expected outputs serve as reference points for diagnosing issues.

Sencore SG-165 - Page 6

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