The Siemens 6SE70 series inverter's operation control panel PMU LCD display is not showing, resulting in a "black screen" issue. This problem can be caused by multiple factors, and the following are several cases that have been diagnosed and resolved.
(1) In one case, the PMU LCD displayed a black screen. Upon checking the bottom board, it was found that the gate protection chip resistor (24Ω) had changed from its normal value of 500kΩ to 500kΩ, indicating damage. Further checks revealed no voltage on pin 20 of N2, while pin 1 showed 11.3V and pin 3 of N3 was at 4V, with pin 2 at 3.3V. The resistance between pin 1 and 3 of the N3 block MC340 was measured at 9kΩ instead of the expected 500kΩ. After replacing the N3 block and the 24Ω SMD resistor, the system was tested and operated normally. The main power supply failure was traced back to the V34 power supply control electrode, where the 24Ω resistor played a key role. When this resistor’s value changes significantly, it can cause the LCD to go black.
(2) Another instance involved a similar black screen. Testing the bottom board showed that the K4 relay coil's parallel freewheeling diode V20 and the series diode V16 were shorted. The N7 power block L7824 was also damaged, and the resistance of pin 1 to ground on the N4 UC3844AN was 500Ω instead of the normal 15kΩ. Replacing the two diodes, the N4 UC3844AN, and the N7 block restored normal operation.
(3) In a third case, the LCD remained dark. Checking the bottom board revealed a burned-out 7.5kΩ resistor between pins 4-8 of the N4 UC3844AN. Additionally, the R133 resistor (gate current limiter for V34 FET K2225) had changed from 100Ω to 720kΩ. After replacing these components, the system functioned properly.
(4) In another situation, the black screen occurred due to a faulty gate protection resistor (24Ω), which had increased to 430kΩ. Also, a 100Ω resistor connected in parallel with V58 on the secondary winding of T6 had changed to 10MΩ. Replacing the 24Ω and 100Ω resistors resolved the issue.
(5) A more severe case involved blown fuses F1 and F2 (25A). Measuring the IGBT module output between U and V phases showed a short circuit with a resistance of 11Ω instead of the normal 210kΩ. Several trigger board connections also showed low resistance, indicating a fault. Replacing the IGBT module and trigger boards restored normal operation.
(6) In a different case, the N4 (UC3844) PWM block was checked, and the external oscillation resistance at pin 4 was found to have increased from 7.5kΩ to 420kΩ. Replacing this component restored normal function.
(7) Another instance involved burned-out resistors R133 (100Ω and 24Ω) connected to the V34 FET. The overcurrent protection resistor on the N4 block had also changed from 100Ω to 400kΩ. Replacing these components brought the system back to normal.
(8) In a more complex scenario, multiple resistors across the switching power supply showed abnormal values. For example, the 4th and 8th pins of the N4 PWM block had an oscillation resistance of 420kΩ instead of 7.5kΩ. The N1 TL084 reference voltage block also had a resistor that changed from 47Ω to 544kΩ. After replacing all faulty resistors, the system operated correctly.
(9) One case involved a burned-out gate current-limiting resistor (2000Ω) for V34 (K2255), along with base resistors for V28 (5C) triode. A 1000Ω resistor connected to the N3 MC340 block was also burned out. Replacing these components restored full functionality.
(10) In another case, the V34 FET and drain resistor R400 (10Ω) were burned out. After replacing them and reinserting the CUVC board, the inverter booted up with a "008" code, which was cleared after parameter re-entry, and the system resumed normal operation.
(11) A black screen was observed, and upon powering on, a loud buzzing sound was heard from the switching power supply. The voltage at the 20-pin output of N2 was slightly lower than normal. After replacing the current detection board A1, the inverter displayed "F029," which was resolved by replacing the current transducer T4.
(12) In a final case, the internal relay K3 would engage and disengage rapidly, causing the main contactor to cycle. The resistance between pins 4 and 6 of the current detection board A1 was 2140Ω instead of the normal 3200Ω. After replacing the board, the system operated normally.
(13) In the last case, the gate current-limiting resistor for the V2 (IRF520) FET had changed from 10Ω to 590kΩ, and further testing showed it had completely burned out. Replacing the resistor restored normal operation.
In summary, the most common causes of the PMU LCD "black screen" in Siemens 6SE70 inverters include damaged resistors, failed power supply blocks, and issues with the IGBT modules or their associated circuits. Regular inspection and timely replacement of faulty components can prevent such failures.
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