Toshiba 140A Ultrasound Repair Case Study

Malfunction Phenomenon：

After the Toshiba 140A ultrasound machine is connected to the power supply, a fault code appears on the instrument monitor screen. Pressing other keys on the keyboard does not change the monitor screen, and the instrument cannot enter the normal working state.

Fault Analysis:

 The circuit boards that have malfunctioned include: CPU board, keyboard I/O interface board, PULSER board, and T/R control board, etc.

Troubleshooting: 

Remove the above suspected faulty boards from the motherboard one by one, turn on the instrument one by one and observe the display on the monitor screen.

This instrument has two PULSER boards. When the board numbered PM30-22554 (YWA2482×C) is removed from the instrument, and then the instrument power is turned on, the instrument undergoes self-test and initialization for a few seconds, and then an ultrasonic echo image appears on the monitor screen without any fault code. If this board is inserted back into the motherboard and the instrument is powered on, the fault phenomenon remains, indicating that this board is the faulty board.

This board has five sets of input power supplies, namely: VH, -10V, +12V, ±5V. When measuring the resistance between the +5V input terminal (that is, the insertion terminal where the board is connected to the motherboard) and the 20th pin (+5V power input pin) of the T1733 integrated circuit on the board, or other integrated circuits (such as HC240, HC164, and HC564, etc.) 5V power input pins with the multimeter ohmmeter ×1Ω gear, the resistance is infinite, indicating that the +5V power supply is not applied to these integrated circuits, causing these integrated circuits to not work. When the CPU in the instrument communicates with this board, it detects a fault, and the instrument stops working.

Following the +5V input terminal of this board, it was found that the +5V power supply is connected in series with the inductor (LCH15) and then connected to the +5V power input pin of the integrated circuit on the board. Careful observation revealed a small crack in the middle of the inductor (LCH15). Measurement with a multimeter ohmmeter showed that the inductor was open-circuited.

An inductor was replaced using a diameter of approximately 0.5mm knee-wrapped wire, wound into an outer diameter of approximately 3mm, 3-5 turns. At this time, measuring with a multimeter ×1Ω gear, the +5V power supply input terminal and all the +5V power input pins of the integrated circuits on the board were connected.

However, when measuring the DC resistance between +5V and ground with a multimeter ×1Ω gear, the resistance was approximately 1Ω, indicating a short circuit between +5V and ground. Since the +5V terminal is connected to dozens of integrated circuits and dozens of capacitors, any damaged component will cause a short circuit, and it is impossible to unsolder each component for measurement. The faulty component was quickly discovered through the pressurization method described below.

First, the +5V output terminal of an independent PC power supply is connected in series with a cement resistor of approximately 2Ω, then the other end of the cement resistor is connected to the input terminal of the inductor (LCH15), and a multimeter (in current mode) is connected in series with the +5V power supply output terminal. After turning on the PC power supply, the multimeter indicates a current of approximately 1A. Then, quickly touch each component on the board by hand. As a result, it was found that the 74AC04F integrated circuit and the surface-mount capacitor C651 had higher temperatures.

Turn off the PC power supply, unsolder this capacitor, measure the resistance between the two ends with a multimeter, which is approximately 1Ω, and then measure the resistance between +5V and ground with a multimeter, which is a few hundred ohms. This confirms that the capacitor is short-circuited, causing the inductor (LCH15) to burn out, and the board does not receive the +5V power supply, resulting in the above fault. After replacing the capacitor with the same model, insert the board into the motherboard, turn on the instrument, and the instrument resumes normal operation.

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