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| TTi EX354T Power supply - CRACKING! |
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| weejag:
I have spent days trying to get a TTi EX354T Triple Power supply working. I bought it with an obviously blown NTC (negative temperature coefficient) thermistor (RT1). I discovered that the main 'boost' Power MOSFET (Q5) was open circuit, and that D54 boost diode was also blown. Despite many hours of searching I have found no other faulty components (although I have really only focused on the primary side of the main transformer) . However, when I apply power loud 'cracks' are heard at a frequency of about 1-2Hz. It sounds like a capacitor discharging a high voltage but I can not find the cause. I have done most of the testing with the board not connected to the front panel but when it is then one circular dot/led lights up on each section of the display in time with the crack. I only persist for a couple of seconds then cut the power as I am afraid that other components are likely to be adversely affected. This makes it difficult to get any more clues. Has anyone else seen this problem? I would be grateful for some guidance as I have just about reached the limits of my abilities. Thanks. Regards, weejag |
| TomKatt:
Have you taken any voltage measurements? Or looked with a scope? I follow pretty basic troubleshooting, checking levels and ripple first. I also wonder if you’d see anything that aligned with the sound you hear. |
| Martin72:
Hi, --- Quote --- I discovered that the main 'boost' Power MOSFET (Q5) was open circuit, and that D54 boost diode was also blown. --- End quote --- How did you find out about Q5, you can't test a MOS-Fet like a transistor. In fact, IC8, Q5 and D54 are components of the PFC circuit. This makes it somewhat difficult to pinpoint the fault, as everything is interdependent. But there is the circuit diagram, I'll look over it again later, maybe I'll come up with something. |
| nctnico:
I'd start by applying power to the auxilary supply voltage which feeds the primary part using a PSU. That way you can measure signals in the primary part WITHOUT connecting it to mains. The next step is to put a low voltage like 30V DC to the high voltage DC and see if something appears at the secondary side. This power supply looks like it is a forward converter with Q4 and Q7 driving the transformer. The use of magamps at the secondary side is a clear sign. I think the PSU is in hick-up mode. Either there is a short / overload at one of the secondary output voltages or an overvoltage condition. Q6 and Q10 seem to form some kind of undervoltage detection circuit. PS: I'd fire the person who drew the schematic 8) |
| nctnico:
--- Quote from: Martin72 on July 19, 2024, 11:03:22 pm ---Hi, --- Quote --- I discovered that the main 'boost' Power MOSFET (Q5) was open circuit, and that D54 boost diode was also blown. --- End quote --- How did you find out about Q5, you can't test a MOS-Fet like a transistor. --- End quote --- You can but differently. A MOSFET has an internal body diode which you should be able to measure between drain & source. The gate is OK if it has no continuity to source and drain. |
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