The multimeter VC86E cannot be connected to the computer? Disassemble the machine to find out whether the fault is caused by the poor soldering of the components

In the article "Oscilloscope Special Topic" on the resolution and DC voltage measurement accuracy of oscilloscopes, it is mentioned that the accuracy of voltage measurement using an oscilloscope is much lower than that of a multimeter. In fact, this is related to the application range of the oscilloscope and the internal ADC characteristics. Oscilloscopes require real-time performance, as high bandwidth and operating speed as possible, so they use Flash ADCs, while multimeters use integral ADCs to improve measurement resolution and reduce the impact of noise, power supply clutter, etc.

If our test requirements are relatively accurate and not demanding on reality, we can read the test data of the multimeter and convert it into a waveform. Therefore, some more advanced multimeters will provide some communication interfaces to communicate with the computer host computer and transmit the values ​​measured by the multimeter to the host computer. For example, today's case 86E has this function. As shown in the figure below, the multimeter communicates with the computer through the USB serial port. Although it is called communication, in fact, the multimeter only has the function of sending data and cannot be controlled by the host computer.

For example, we can use a multimeter to measure the ambient temperature and the forward voltage of a diode. Because the ambient temperature changes slowly, and we want to measure the diode forward voltage very accurately. Under this application condition, reading the value of the multimeter can accurately know the relationship between the diode forward voltage and the environment.

  What happened in this case was that a colleague said the multimeter could not be connected to the computer. No other special information was given.

After getting the multimeter, we must first reproduce the phenomenon, which is the first step in analyzing and solving the problem. By plugging in the USB cable and pressing the RS232 button of the multimeter, I figured out the problem phenomenon.

1. The computer can be connected to the multimeter because the serial port can be recognized in the device manager. (Compared to a normal multimeter, even if it is not turned on, it can recognize the serial port as long as the USB cable is inserted.) This means that the USB-to-serial chip inside the multimeter is probably good and has not been damaged by static electricity, surges or other external overstress.

2. The RS232 button on the multimeter can be pressed, but there is no response (there is no RS232 icon on the display interface). This button on the multimeter should mainly trigger and let the MCU output data through the serial port, and prevent the multimeter from automatically shutting down. Because if it is shut down, it will not be able to measure. (In order to increase the use time of the multimeter, handheld multimeters normally have an automatic shutdown function).

Analyzing the above problems, it is possible that the key is malfunctioning, for example, the key gold finger is dirty and cannot be turned on. You need to disassemble the machine. The status of the whole machine is as follows:

Lock the battery cover screws and battery, remove the four screws of the back cover, and disassemble the back cover.

Remove the back cover, and you can see the silicone plugs of the board and serial port. There are 11 screws on the board, some of which are used to fix the PCB, and some are used to fix the LCD bracket and the turntable bracket.

After removing the mainboard, I actually feel that the internal structure design is quite complicated. The buttons also have special key board structures, acrylic LCD panels, and so on.

I checked the gold finger and conductive contacts of the RS232 button and found nothing wrong. But I saw a diode with abnormal soldering next to the button. As shown below, it is obvious that one of the pins of the diode has deviated from the pad. I moved it with tweezers and found that it was not connected to the pad.

The diode characteristics are normal after testing, so we directly re-solder it, install the battery, and press the RS232 button. The multimeter screen shows that the button is pressed. We try to connect to the computer, and the computer receives the corresponding hexadecimal data, and the repair is completed.