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84 Year Old Radio Receiver Restoration! Stromberg-Carlson 145L

Mr Carlson's Lab・116 minutes read

The radio receiver from the 1930s is being meticulously restored to perform as it did back then, with a focus on replacing old capacitors and testing components for proper functionality to ensure optimal performance and longevity. The restoration process involves careful testing, alignment adjustments, and replacement of components like resistors and capacitors to enhance sensitivity, sound quality, and overall performance of the Stromberg Carlson 145 radio receiver, highly recommended for AM radio enthusiasts.

Insights

  • The radio receiver being restored is a high-tech device from the 1930s, with innovative features still relevant today, showcasing the enduring nature of its design principles.
  • Capacitors are a critical component in old radio receivers, often prone to failure, leading to potential damage and transformer issues if not replaced promptly, emphasizing the importance of meticulous restoration work.
  • Understanding schematics and reverting modifications to factory specifications are crucial steps in restoring radio chassis for optimal performance, ensuring that components are correctly replaced and aligned.
  • The alignment process, particularly in the IF section, is meticulous and time-consuming, requiring precise tuning of capacitors and transformers to enhance sensitivity and achieve optimal performance, ultimately improving sound quality and alignment accuracy.

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Recent questions

  • How does the restoration process of a 1930s radio receiver work?

    The restoration process of a 1930s radio receiver involves electrically restoring the device to function as it did in the 1930s. This includes replacing old capacitors, testing components like the variable IF, and reverting modifications to factory specifications. Careful testing and optimization of replaced components are crucial for optimal performance. Specific resistor values are essential for replacement, and components like resistors and capacitors are replaced with high-quality alternatives for longevity. The restoration process also involves testing resistors and capacitors for failures, ensuring proper functioning, and aligning the IF section for improved sensitivity and alignment.

  • What are the common issues with old radio receivers?

    The most common issue with old radio receivers is the failure of capacitors, which can lead to component damage and transformer failure. Old capacitors are prone to degradation and leakage, with some on the verge of catastrophic failure. Specific capacitors like mica mold capacitors can be leaky and cause issues in the unit. Components like resistors and capacitors may need replacement due to heating from failed parts. It is crucial to test and replace faulty capacitors promptly to prevent sparking and dangerous issues.

  • How are capacitors tested and replaced in a radio receiver?

    Capacitors in a radio receiver are tested for resistance, leakage, and functionality using specialized devices. Capacitors with resistance up to 40 billion ohms are considered functional, while those with significantly lower resistance may be faulty. Testing for leakage through moisture aggression is essential to detect potential issues. Replacement capacitors should match factory standards and be installed correctly to ensure longevity and optimal performance. Careful testing and meticulous work are required to avoid shortcuts like the j-hooking method during replacement.

  • What is the significance of tuning capacitors and transformers in the IF section?

    Tuning capacitors and transformers in the IF section of a radio receiver are crucial for achieving optimal alignment, sensitivity, and performance. Adjustments to these components can significantly impact signal strength, center frequency accuracy, and receiver sensitivity. Fine-tuning multiple capacitors and transformers in the IF section is essential for achieving proper alignment and sensitivity. The alignment process involves moving back and forth between capacitors to ensure precise tuning and proper alignment, resulting in improved sensitivity and overall performance of the radio receiver.

  • How is the audio cutoff adjusted in a radio receiver?

    The audio cutoff in a radio receiver is adjusted using a device like the Stanford Research SR780 to sweep the audio section and make necessary adjustments. The signal from the SR780 is fed to the wiper of the volume control, and the audio response is analyzed. By tweaking a control until a dip or null is achieved around 10 kHz on the analyzer, the audio cutoff after 10 kHz is adjusted. This adjustment ensures clear and full sound quality in the broadcast band, enhancing the listening experience, especially with music.

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Summary

00:00

1930s Radio Receiver Restoration: High-Tech Revival

  • The radio receiver being restored is from the 1930s and is high-tech for its time, with companies still using similar ideas today.
  • The radio has high-fidelity AM reception and is being electrically restored to perform like it did in the 1930s.
  • The radio is a communications receiver in a wooden case with intricate woodwork and original features like the gold grill cloth.
  • After restoration, the radio should function as it did in the 1930s, with all dials and indicators working properly.
  • The backside of the radio features an "acoustical labyrinth" design with cardboard pipes and an electromagnet for sound production.
  • The chassis of the radio is complex with many tubes, a power transformer, and audio output tubes like 6L6s.
  • The speaker in the radio is in excellent condition with a felt surround and adjustable components for optimal performance.
  • The underside of the radio chassis reveals old components like capacitors that need replacement due to potential damage from shorting out.
  • The most common issue with old radio receivers is the failure of capacitors, which can lead to component damage and transformer failure.
  • The restoration process involves removing and replacing old capacitors and testing components like the variable IF for proper functionality.

15:47

Capacitor Replacement and Testing for Radio Receivers

  • Newer style capacitors lack markings for outside foil, a device to locate it is available on Patreon.
  • Replacement capacitors should be installed correctly to match factory standards.
  • Replacement of capacitors is meticulous work, avoiding shortcuts like the j-hooking method.
  • Testing of resistors and capacitors is essential to identify failures and ensure proper functioning.
  • Modifications made over time on the radio receiver should be reverted to factory specifications for optimal performance.
  • Understanding schematics is crucial when working on radio chassis to decipher modifications.
  • Replacing components on the chassis requires careful testing and optimization for better performance.
  • Specific resistor values in the radio receiver are crucial for replacement and are marked for future reference.
  • The death capacitor across the AC line needs to be tested and replaced with modern safety-rated capacitors.
  • Components like resistors and capacitors are replaced with high-quality, long-life alternatives for longevity and optimal performance.

28:57

Troubleshooting capacitor issues in electronic unit

  • The unit being discussed has a capacitor tied off to the ground, despite not being explicitly shown in the diagram.
  • Some parts listed in the parts list are missing, like part number 175.
  • The capacitors in the unit serve as a 10 kilohertz filter to maintain high fidelity sound.
  • Specific capacitors labeled as "mica mold" are found to be leaky, undetectable by normal capacitor testers.
  • The mica mold capacitors, especially those with two leads, are causing issues in the unit.
  • Various components, like resistors and capacitors, have been replaced due to degradation and leakage.
  • The unit had been in use until recently, leading to heating in some components due to failed parts.
  • The original capacitors in the unit were on the verge of catastrophic failure.
  • A cautionary note is given about replacing specific capacitors in the oscillator and RF sections.
  • A custom-designed low-voltage capacitor tester is highlighted as a safer and more sensitive alternative to standard testers.

43:18

Capacitors and Resistors Testing Results Revealed

  • Capacitors tested beyond 40 billion ohms without issues
  • Mica capacitors tested for 3900 picofarads, showing 3.87 nano farads
  • Larger value capacitor tested for 0.1 microfarads, leaking at 6 volts
  • Resistance of capacitors tested, one showing 10 meg ohms, indicating a dead short
  • Capacitors tested for resistance, ranging from 3 to 500 mega ohms
  • Capacitor tester used to detect leakage through moisture aggression
  • Outside foil tester used to mark capacitors for correct placement
  • Resistors decoded using color bands, with examples of incorrect values found
  • Replacement of resistors and capacitors shown on the bottom side of the chassis
  • Original line cord in the radio confirmed as flawless, not a curtain burner type

59:12

Repairing Stromberg Carlson Radios: Tips and Tricks

  • Stromberg Carlson radios made in Canada are known for being leaky.
  • Mica capacitors are acceptable, while paper and foil caps are not recommended.
  • Filter caps have been replaced with modern Rubicon capacitors, which are highly effective.
  • A resistor on the back of the radio often fails, lacking ratings in the parts list.
  • A piece of thermally conductive silicone is placed under the resistor to enhance thermal conductivity and prevent damage.
  • The silicone acts as a cushion, ensuring proper thermal conductivity across the resistor body.
  • The lack of heat sink compound is compensated by the heat transfer silicon underneath the resistor.
  • The resistor prevents holes from burning in the chassis due to excessive heat.
  • Cleaning the band switches and spraying the switches with contact cleaner is the next step.
  • Checking and replacing a resistor near the eye tube is crucial for proper sensitivity and functionality.

01:15:53

Optimizing IF Alignment for Improved Sensitivity

  • The RF signal runs through the RF amplifier and coils into a line leading to a capacitor for signal feeding from a tracking generator of a spectrum analyzer.
  • To ensure successful alignment, lightly couple the spectrum analyzer into the circuit to avoid affecting the device under test.
  • Heavy coupling with sensitive components like tubes can lead to alignment errors, necessitating extremely light coupling for accurate results.
  • Protection devices like boxes with diodes and capacitors safeguard the spectrum analyzer's input and output from high voltages, crucial for preventing damage.
  • Adjusting the fidelity control on the radio receiver widens the signal peak, aiding in alignment adjustments.
  • Setting the spectrum analyzer to a span of 20 kHz around the center frequency of 465 kHz helps in visualizing and adjusting the IF alignment.
  • Tuning capacitors in the IF section can significantly impact sensitivity and alignment, with adjustments improving signal strength and center frequency accuracy.
  • Fine-tuning multiple capacitors and transformers in the IF section is essential for achieving optimal alignment and sensitivity in the radio receiver.
  • Tuning capacitors and transformers in the IF section can lead to improved receiver sensitivity and alignment, enhancing the overall performance of the radio receiver.
  • After tuning capacitors and transformers in the IF section, the receiver should exhibit improved sensitivity and alignment, resulting in a more even spread on the IF display.

01:30:50

"Precision Tuning for Optimal Radio Reception"

  • Alignment process takes about an hour to an hour and a half to perfect everything.
  • Tuning involves moving back and forth between capacitors to ensure proper alignment.
  • Achieving the stage curtain effect involves tuning two capacitors to create double humps evenly.
  • Tuning the IF section in the Stromberg Carlson radio takes about an hour and a half.
  • Sensitivity of the IF section is crucial, with even slight adjustments causing significant pattern changes.
  • Maintaining the double hump symmetry is essential for proper sound quality.
  • Narrowing the IF bandwidth requires precise tuning to ensure even rising and falling of the humps.
  • IF bandwidth is measured at 4.16 kHz in the narrowest position and 21.04 kHz in the widest position.
  • The radio receiver's IF section is designed to be incredibly close to ideal specifications.
  • Antenna and RF alignment are performed, with the oscillator adjustments deemed unnecessary due to the receiver's accuracy.

01:48:20

Restored Stromberg Carlson 145 Radio Receiver Review

  • The eye tube is brand new and requires some time to come to life, providing a brighter and more sensitive display.
  • To align the other bands, rotate the band switch and tune each band starting with band X, adjusting the cans for alignment.
  • The oscillator section is well-aligned, except for the 10 kHz audio cutoff, which needs adjustment using a Stanford Research SR780.
  • The SR780 is connected to the radio receiver to sweep the audio section and adjust the audio cutoff after 10 kHz.
  • The adjustment involves feeding the signal from the SR780 to the wiper of the volume control and analyzing the audio response.
  • The adjustment is made by tweaking a control until a dip or null is achieved around 10 kHz on the analyzer.
  • The radio is reassembled, and the broadcast band is tested, showcasing clear and full sound quality.
  • The radio's fidelity control enhances the listening experience, especially with music, providing a full and clear sound.
  • The radio's performance across different bands, including the beacon band and shortwave broadcast, demonstrates its versatility and quality.
  • The restoration of the Stromberg Carlson 145 radio receiver is highly recommended for AM radio enthusiasts due to its exceptional sound quality and performance.

02:08:00

Capacitor Safety and Testing Guidelines

  • Shorting both sides of a capacitor can cause a fuse to blow, while shorting one side can connect the line cord directly to the chassis, creating a safety hazard.
  • X1 Y2 rated capacitors are recommended replacements for safety, designed to fail in a safe manner.
  • A sensitive device can measure resistance in capacitors up to 40 billion ohms, detecting any leakage or degradation.
  • Testing a new capacitor shows a resistance up to 40 billion ohms, indicating its functionality.
  • Testing a faulty capacitor reveals a resistance just below 20 mega ohms, significantly lower than the standard DMM reading.
  • One capacitor is on the verge of shorting out, potentially causing sparking and dangerous issues if connected to a line cord, highlighting the importance of replacing faulty components promptly.
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