HP 204B - User Manual

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Overview

Function Description
Important Technical Specifications
Usage Features
Maintenance Features
Operation
Principles of Operation
Maintenance Procedures
Replaceable Parts

This document is an operating and service manual for the Hewlett-Packard Model 204B Oscillator, identified by serials prefixed 416-. It was printed in June 1964 and covers general information, installation, operation, principles of operation, maintenance, and replaceable parts.

Function Description:

The Model 204B Oscillator is a transistorized, battery-powered instrument designed to produce sine-wave signals across a frequency range of 5 cps to 560 kc. It delivers an output of 10 millivolts (2.5 vrms) into 600 ohms, with 5 vrms open circuit, and maintains a continuously variable bridged "T" attenuator with at least 40-db range. The oscillator's output is isolated from power line ground and instrument chassis. The 600-ohm output impedance of the oscillator makes it compatible with transmission lines and many distribution systems. When battery-powered, the oscillator is useful for isolated applications. The Model 203A AC Voltmeter, also by Hewlett-Packard, is described as an ideal companion instrument for the Model 204B.

Important Technical Specifications:

Frequency Range: 5 cps to 560 kc in 5 ranges, vernier control.
Dial Accuracy: ±3%.
Frequency Response: ±3%, with rated load.
Output Impedance: 600 ohms.
Output: 10 millivolts (2.5 vrms) into 600 ohms, 5 vrms open circuit, continuously variable bridged "T" attenuator with at least 40-db range.
Distortion: Less than 1%.
Noise: Less than 0.05%, ac or battery operated.
Power Source: 4 batteries at 6.75 volts each, 7-ma drain, life at least 300 hours.
Dimensions: Module 6-3/32 in. high, 5-1/8 in. wide, 8 in. deep.
Net Weight: 6 lb.
Equipment Available: 204B-11A AC Supply Kit for field installation, 204B-11C Rechargeable Battery Supply Kit for field installation, 204B-64B Battery Kit for field installation.

Usage Features:

The Model 204B can be powered by three types of power supplies: a battery pack (204B-64B), an AC operated power supply (204B-11A), or a rechargeable battery power supply (204B-11C). These power supplies are available in kit form and can be easily interchanged. The instrument uses a two-section, eight-digit serial number (000-00000). The first three digits of the serial number indicate differences between the instrument and the manual description.

The front panel controls include a FREQ (frequency) dial with a vernier for fine adjustment, a RANGE switch (X1, X10, X100, X1K, X10K, OFF), and an AMPLITUDE control. The output is provided via two terminals, and there's a cabinet ground terminal. The RANGE switch in the OFF position inactivates the oscillator. The indicator glows in all positions except OFF. Frequency selection is achieved by multiplying the desired range dial calibration by the RANGE switch position. The AMPLITUDE control sets the output power level.

For battery-powered operation, the instrument uses four 6.75-volt Mallory mercury batteries. For AC-powered operation, the Model 204B requires 115 or 230 volts AC, 50 to 1000 cps. The rechargeable battery power supply uses four 6.25-volt nickel-cadmium batteries.

Maintenance Features:

The manual provides detailed instructions for inspection, rack/bench installation, combining cases, adapter frames, and repacking for shipment.

Operation:

General Operating Considerations: To prevent damage to transistors, always load the oscillator output with a minimum of 200 picofarads when using a capacitor across the output. Do not exceed 25 volts peak-to-peak across the output.
Operation with Rechargeable Batteries: The rechargeable battery power supply allows for continuous operation. When the RANGE switch is turned OFF, power is removed from the oscillator, and the battery charger is connected. The charger will continue to operate as long as the power cord is connected to a power source. The resistor placed across the output of the power supply acts as a load to maintain charge through the battery. A diode in the output circuit prevents battery discharge when the oscillator is not connected to a power source.
Recharging Nickel-Cadmium Batteries: The batteries should be fully discharged when the maximum oscillator output drops below 0.5 volt rms into 600 ohms. They are charged at a constant current of 11 milliamperes for at least 30 hours. The batteries are designed for a life cycle of 225 milliampere-hours capacity.
Mercury Battery Replacement: The manual details how to remove the top cover, wing nuts, battery clamps, and old batteries, then install new ones, ensuring correct polarity.

Principles of Operation:

The Model 204B consists of an RC bridge oscillator circuit, a peak detector circuit, and an output attenuator (bridged-T type). The RC bridge oscillator circuit consists of an RC frequency-selective network and a resistive voltage divider network. The frequency-selective network is similar to one leg of a Wien bridge. The RC bridge provides positive feedback to the amplifier, determining the frequency of oscillation. The resistive voltage divider network supplies negative feedback to the amplifier. The peak detector changes the RC bridge oscillator output voltage and changes the division ratio of the resistive voltage divider network, thereby changing the amount of negative feedback. The peak detector with the divider network maintains the RC oscillator output at a constant level.

Maintenance Procedures:

Required Test Equipment: An oscilloscope, distortion analyzer, AC voltmeter, DC voltmeter, clip-on milliammeter, and frequency counter are needed for maintenance.
Performance Checks: Includes dial accuracy, frequency response and output voltage, and distortion checks.
Repair: Instructions for cover removal, servicing etched circuit boards, and troubleshooting.
Adjustment and Calibration: AC power supply checks, battery charger current adjustment, DC balance adjustment, distortion adjustment, frequency calibration adjustments, frequency potentiometer replacement, cam cable replacement, and range switch assembly replacement.
Power Supply Maintenance: Details for battery-pack power supply, AC power supply, and rechargeable battery power supply. This includes instructions for replacing rechargeable batteries.

Replaceable Parts:

The manual includes an index by reference designator and a list of replaceable parts with stock numbers, descriptions, manufacturer part numbers, and total quantity (TQ). It also includes manual backdating changes for instruments with different serial prefixes, detailing specific component changes and corrections to the manual.