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Details the issue of squealing on transmit and lists potential causes like high SWR and inadequate ground.
Provides four actionable steps to resolve the squealing issue, including checking SWR and adjusting components.
Instructs to check maximum modulation after changing resistor R126.
Step-by-step guide to measure and verify the radio's microphone sensitivity.
Details the issue of squealing on transmit and lists potential causes like high SWR and inadequate ground.
Provides four actionable steps to resolve the squealing issue, including checking SWR and adjusting components.
Instructs to check maximum modulation after changing resistor R126.
Step-by-step guide to measure and verify the radio's microphone sensitivity.
This technical service bulletin, numbered 10020 and dated June 3, 2003, from Cobra Electronics Corporation, Consumer Response Group, addresses a common issue with the 148GTL and 148NWST radio models: squealing on transmit. The bulletin provides a comprehensive guide for diagnosing and resolving this problem, focusing on the underlying causes and offering practical solutions.
The primary function of this bulletin is to equip technicians and users with the knowledge to troubleshoot and repair radios exhibiting the "squealing on transmit" symptom. This issue can significantly degrade the radio's performance, making clear communication difficult or impossible. By outlining potential causes and providing step-by-step actions, the bulletin aims to restore optimal functionality to the affected devices.
One of the key usage features highlighted in the troubleshooting process is the importance of proper antenna SWR (Standing Wave Ratio). The bulletin suggests checking the SWR of the antenna and ensuring it is below 3. A high SWR indicates an impedance mismatch between the radio and the antenna, which can lead to various performance issues, including the squealing phenomenon. Maintaining a low SWR is crucial for efficient power transfer and clear transmission.
Another critical usage feature addressed is the grounding of the radio. The bulletin recommends connecting a ground wire to a screw on the side of the radio to ground the chassis. Inadequate grounding can introduce noise and interference, contributing to the squealing sound. Proper grounding ensures a stable electrical reference, which is fundamental for reliable radio operation.
The bulletin also delves into the internal aspects of the radio's usage, specifically mentioning the tightening of screws around the edge of the circuit board. Over time, vibrations or normal wear and tear can cause these screws to loosen, potentially leading to intermittent electrical connections or physical instability of components, which might manifest as squealing. This simple maintenance step can often resolve such issues by ensuring all internal connections are secure.
A significant aspect of the radio's usage and a common cause of the squealing is the microphone sensitivity. The bulletin explains that if the radio still squeals after addressing SWR, grounding, and internal screw tightness, the microphone sensitivity might be too high. This implies that the radio's input stage is overly responsive to audio signals, leading to feedback or distortion during transmission. The bulletin provides a clear specification for microphone sensitivity: 1 to 2mV for 50% modulation with Soundtracker off, and 0.5 to 1mV for 50% modulation with Soundtracker on. This detailed specification helps in accurately assessing whether the microphone sensitivity is indeed the root cause.
The maintenance features outlined in the bulletin are practical and actionable. For instance, the recommendation to change R126 to a 12KΩ resistor if the microphone sensitivity is too high is a direct maintenance action. This involves a component-level repair, indicating that users or technicians need to be comfortable with opening the radio and performing minor soldering or component replacement. This type of maintenance ensures that the radio's internal circuitry is correctly calibrated for optimal performance.
Following the adjustment of R126, the bulletin emphasizes another crucial maintenance step: checking maximum modulation. This ensures that while the microphone sensitivity has been corrected, the radio is still capable of achieving its full modulation potential without over-modulating, which could lead to distorted transmissions. This step highlights the importance of re-calibration after any internal component changes.
The bulletin also includes a dedicated section on "Checking Microphone Sensitivity," which serves as a detailed maintenance procedure. This procedure involves several steps:
This detailed procedure for checking microphone sensitivity is a vital maintenance feature, providing a systematic way to diagnose and correct an internal electrical parameter. It empowers technicians to precisely tune the radio's audio input stage, ensuring clear and undistorted transmissions.
In summary, this technical service bulletin functions as a comprehensive troubleshooting and repair guide for specific Cobra radio models experiencing squealing on transmit. Its usage features emphasize the importance of external factors like antenna SWR and proper grounding, as well as internal factors like component security and microphone sensitivity. The maintenance features provide clear, actionable steps, including component replacement and detailed calibration procedures, ensuring that the radio can be restored to optimal performance for reliable communication. The bulletin's structured approach, from identifying potential causes to providing specific solutions and verification steps, makes it an invaluable resource for maintaining these radio devices.
General| Modes | AM/SSB |
|---|---|
| Channels | 40 |
| Power Output | 4 Watts AM, 12 Watts SSB |
| Microphone Type | Dynamic |
| SWR Calibration | Yes |
| Antenna Impedance | 50 Ohms |
| Power Supply | 13.8 VDC |
| Weight | 4.5 lbs |
| Frequency Range | 26.965 MHz to 27.405 MHz |
| Dimensions | 2.25 x 7.25 x 9.25 inches |
