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First check the power supply circuit, the output voltage of the 7805 regulated output voltage is less than 0.5V, +8V and non regulated +1OV voltage normal. The input voltage of the 7805 input voltage is about 2V and the instability is not stable. The AC voltage of the rectifier diode is 11V normal. It is clear that the rectifier diode circuit is damaged. The machine also uses 4 1N4007 diodes for bridge type rectification, 4 of the rectifier diode, but no damage, positive and negative resistance are normal. Trial 4 1N5408 diode will be replaced by the original diode, the results of +5V voltage recovery normal. Test machine, fault elimination.7MQservice manual download site
When one or several of the poor performance of the rectifier diode, sometimes there will be a variety of output voltage is normal and produce a variety of difficult to judge the fault, error correction ability, mosaic serious, etc.. In this case, the use of the use of the meter to detect the diode is often found no fault, it is best to use the replacement method test.
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