Sencore POWERITE PR57

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Summary

Servicing Hot Chassis and Performing the Leakage Test

What is the Leakage Test?

Explains the safety check to prevent customer shocks from exposed metal parts on AC-powered devices.

Why Dont More Servicers Perform the Leakage Test?

Highlights that technicians avoid the test due to complexity, setup time, and required parts.

Leakage Current Cold Check

Details how to measure resistance between AC plug and exposed metal parts when the unit is unplugged.

Leakage Current Hot Check

Explains measuring voltage across a resistor connected to exposed metal and earth ground while the unit is powered.

Why You Should Perform the Leakage Test

Why Should You Perform the Leakage Test?

Emphasizes that even one leaky chassis can lead to costly damage, lawsuits, and customer harm.

Causes of Leakage

Shorted Antenna Bypass Capacitors

Explains how shorted bypass capacitors can expose antenna terminals to AC voltage.

Improperly Installed Tuner

Discusses how incorrect tuner installation can tie the AC line to exposed metal parts.

Conductive Knobs

Warns that metal or chrome-plated knobs can conduct current from a hot chassis.

Defective Isolation Transformers

Notes that leakage in isolation transformers can cause the entire chassis to become hot.

AC Bypass Capacitors

Describes how failed AC bypass capacitors can create a direct connection between the AC line and chassis.

Bent Rabbit Ears

Explains how bent rabbit ears can touch a hot chassis, energizing the antenna terminals.

Improper Installation of Parts

Mentions forgetting insulating parts like "fish paper" can create a hazard.

Foreign Objects Touching the AC Line

Warns about wires or solder falling onto AC line components.

A Broken Safety Ground

Indicates a broken ground can cause controls to "float" at line voltage.

Using Long Metal Screws

States long screws can penetrate mounting tabs and touch the metal chassis.

Adding An Earphone

Highlights the shock hazard of connecting earphones to un-isolated sets.

Foreign Objects

Mentions metal objects like coins or hairpins falling inside the set can cause a shock hazard.

Connecting an External Speaker

Warns about wiring accessories to sets without isolation transformers.

The PR57 Leakage Test

The PR57 Leakage Test: Its Fast and Simple

Explains the PR57 simplifies the test by referencing all circuits to its isolated output.

Performing the Leakage Test with the PR57

Provides a step-by-step guide on how to conduct the leakage test using the PR57.

Leakage Limits and Calibration

How Much Leakage is Bad?

Discusses the effects of electrical shock and safe leakage current limits.

Measuring Leakage at Lower Levels

Explains measuring very low leakage (10-100 microamps) for medical or hospital equipment.

Checking the Calibration of the Leakage Test

Details how to verify the PR57's leakage test calibration using its built-in resistor.

Testing Leakage on Three-Wire Units

Explains how the PR57 test works on units with grounded AC cords, maintaining chassis at earth potential.

Testing for Line Cord and Extension Cord Leakage

Covers testing line and extension cords for leakage, which can cause fires.

Summary

Servicing Hot Chassis and Performing the Leakage Test

What is the Leakage Test?

Explains the safety check to prevent customer shocks from exposed metal parts on AC-powered devices.

Why Dont More Servicers Perform the Leakage Test?

Highlights that technicians avoid the test due to complexity, setup time, and required parts.

Leakage Current Cold Check

Details how to measure resistance between AC plug and exposed metal parts when the unit is unplugged.

Leakage Current Hot Check

Explains measuring voltage across a resistor connected to exposed metal and earth ground while the unit is powered.

Why You Should Perform the Leakage Test

Why Should You Perform the Leakage Test?

Emphasizes that even one leaky chassis can lead to costly damage, lawsuits, and customer harm.

Causes of Leakage

Shorted Antenna Bypass Capacitors

Explains how shorted bypass capacitors can expose antenna terminals to AC voltage.

Improperly Installed Tuner

Discusses how incorrect tuner installation can tie the AC line to exposed metal parts.

Conductive Knobs

Warns that metal or chrome-plated knobs can conduct current from a hot chassis.

Defective Isolation Transformers

Notes that leakage in isolation transformers can cause the entire chassis to become hot.

AC Bypass Capacitors

Describes how failed AC bypass capacitors can create a direct connection between the AC line and chassis.

Bent Rabbit Ears

Explains how bent rabbit ears can touch a hot chassis, energizing the antenna terminals.

Improper Installation of Parts

Mentions forgetting insulating parts like "fish paper" can create a hazard.

Foreign Objects Touching the AC Line

Warns about wires or solder falling onto AC line components.

A Broken Safety Ground

Indicates a broken ground can cause controls to "float" at line voltage.

Using Long Metal Screws

States long screws can penetrate mounting tabs and touch the metal chassis.

Adding An Earphone

Highlights the shock hazard of connecting earphones to un-isolated sets.

Foreign Objects

Mentions metal objects like coins or hairpins falling inside the set can cause a shock hazard.

Connecting an External Speaker

Warns about wiring accessories to sets without isolation transformers.

The PR57 Leakage Test

The PR57 Leakage Test: Its Fast and Simple

Explains the PR57 simplifies the test by referencing all circuits to its isolated output.

Performing the Leakage Test with the PR57

Provides a step-by-step guide on how to conduct the leakage test using the PR57.

Leakage Limits and Calibration

How Much Leakage is Bad?

Discusses the effects of electrical shock and safe leakage current limits.

Measuring Leakage at Lower Levels

Explains measuring very low leakage (10-100 microamps) for medical or hospital equipment.

Checking the Calibration of the Leakage Test

Details how to verify the PR57's leakage test calibration using its built-in resistor.

Testing Leakage on Three-Wire Units

Explains how the PR57 test works on units with grounded AC cords, maintaining chassis at earth potential.

Testing for Line Cord and Extension Cord Leakage

Covers testing line and extension cords for leakage, which can cause fires.

The Sencore PR57 "POWERITE" is a safety analyzer and variable isolation transformer designed for electronic servicing, particularly for performing leakage tests on AC-operated devices. Its primary function is to quickly and accurately check for AC leakage between the AC line and exposed metal parts of a device, ensuring safety for both the user and the customer.

Function Description:

The PR57 facilitates a crucial safety leakage test, which is mandated by manufacturers in their service literature to meet legal obligations for delivering safe products. This test identifies potential electrical shock hazards caused by current leakage from internal circuits to accessible metallic parts of an electronic device. The device is especially important for servicing "hot chassis" units, which lack an isolation transformer and thus present a higher risk of shock.

The leakage test can be performed in two main ways:

Leakage Current Cold Check: This involves measuring resistance with an ohmmeter between a jumpered AC plug and exposed metallic parts of the receiver while the unit is unplugged. For parts with a return path to the chassis, the resistance should be between 240 kΩ and 5.2 MΩ. For parts without a return path, the resistance should be infinite.
Leakage Current Hot Check: This involves plugging the AC cord directly into an AC outlet (without an isolation transformer) and using a specific test circuit (a 1.5 kΩ, 10W resistor in parallel with a 0.15 µF capacitor) connected between exposed metallic parts and a good earth ground (e.g., a water pipe). An AC voltmeter (with 1000 ohms/volt or more sensitivity) then measures the potential across the resistor. The voltage at any point should not exceed 0.75 volts RMS, corresponding to a leakage current not exceeding 1/2 milliamps. The PR57 simplifies this hot check significantly.

The PR57's internal circuitry references all leakage test paths to its isolated output, allowing the test to be performed while the unit under test is still connected to the isolated output. This reduces the risk of shock during testing and eliminates the need to move the AC line cord to a non-isolated outlet. It also negates the requirement for an external earth ground reference, making it possible to perform the test anywhere, even in homes without grounded outlets.

The device also functions as a variable isolation transformer, providing an isolated AC output for safe servicing.

Important Technical Specifications:

Leakage Current Limits:
- Hot Check (Traditional Method): Potential across resistor should not exceed 0.75 volts RMS. Leakage current must not exceed 1/2 milliamps.
- UL Allowable Limit (since 1972): 500 microamps.
- UL Allowable Limit (before 1972): 750 microamps.
- Hospital/Clinic Equipment: Less than 100 microamps, or even 10 microamps, requiring higher resolution measurement (e.g., with a Sencore DVM37 or DVM56A in series with the Safety Leakage Probe).
Leakage Test Circuit Components (Traditional Hot Check): 1.5 kΩ, 10W resistor in parallel with a 0.15 µF capacitor.
AC Voltmeter Sensitivity (Traditional Hot Check): 1000 ohms/volt or more.
PR57 Leakage Meter Scale: Calibrated directly in microamps.
Safety Leakage Probe: Includes a current-limiting resistor in series with the test circuit, which is bypassed when the button on the probe is depressed. This resistor prevents the meter from pegging when contacting a high-leakage point.
Calibration Resistor: The Safety Leakage Probe itself contains a calibration resistor for checking the PR57's leakage scale calibration. When plugged into the ISOLATED OUTPUT socket and the HI SIDE LEAKAGE pushbutton is depressed, the meter should read 780 microamps without the probe button pushed.
AC VOLTS Output: Adjustable to 117 volts for uniform test results.
AC Output Current: 0-3 Amps Continuous, 3-4 Amps Intermittent Use.
AC Output Voltage: 0-140 VAC.
Wattage Output: 175W (0-1.5A), 470W (0-4A).

Usage Features:

Fast and Simple Leakage Test: The PR57 significantly simplifies the leakage test compared to traditional methods, which often involve complex setups with external components and earth grounds.
Integrated Safety Leakage Probe: The probe is designed for easy and comprehensive testing of all user-accessible exposed metal parts (screw heads, antennas, control shafts, handles, etc.). The probe's pushbutton allows for reading the true leakage current.
HI SIDE and LO SIDE Leakage Buttons: These buttons allow testing for leakage paths to both sides of the AC line. A full-scale reading with the button "out" indicates a direct connection to one side of the AC line.
Isolated Output: The PR57 provides an isolated AC output, allowing the unit under test to be powered safely during the leakage test.
Three-Wire Unit Compatibility: The PR57's leakage test works identically for both two-wire and three-wire (grounded) AC line cord units. The ground connector on the ISOLATED OUTPUT jack is connected to earth ground through the PR57's power cord, ensuring the unit under test's chassis is maintained at earth potential without affecting the leakage test.
Testing Line Cord and Extension Cord Leakage: While the PR57 focuses on device leakage, the manual suggests using a Z-Meter to measure leakage between conductors of line cords and extension cords, which should be absolutely zero when unplugged.

Maintenance Features:

Self-Calibration Check: The PR57's leakage test calibration can be verified at any time using the built-in calibration resistor in the Safety Leakage Probe. This ensures the accuracy of readings obtained during tests.
Troubleshooting Leakage Causes: The manual provides a comprehensive list of common causes of leakage, including:
- Shorted antenna bypass capacitors
- Improperly installed tuners
- Conductive knobs
- Defective isolation transformers
- AC bypass capacitors (especially older wax-coated paper types)
- Bent rabbit ears (touching hot chassis)
- Improper installation of parts (e.g., missing insulating "fish paper")
- Foreign objects touching the AC line
- Broken safety ground
- Using long metal screws
- Adding an earphone to a non-isolated jack

By understanding these potential causes, technicians can more effectively diagnose and repair leakage issues. The PR57 helps technicians meet their legal and ethical obligations to deliver safe products, protecting both customers and their businesses from potential shock hazards and lawsuits.