| Electronics > Repair |
| Need help with a Metcal MX500 |
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| jbryan2:
--- Quote from: kff on January 18, 2015, 07:29:03 am ---Trying to fix a Metcal MX500. Q4 dies as soon as the unit is switched on, with all three terminals shorting to each other (2-6 ohm resistance). I've double checked all capacitors, transistors, diodes, and now inductors on the board, and everything looks ok. All capacitors that could have any effect on Q4 have been checked out of circuit. With Q4 removed from the board, the voltages on Q4's gate and drain look ok (checked with an oscilloscope), nothing that would exceed the +/-20V gate/source and 200V drain/source ratings of IRF640N. Some questions: 1) In what scenario would an N-channel MOSFET fail in such a way that all terminals are shorted to each other? Overvoltage on the gate? 2) Is it common for capacitors to test ok at lower voltages (as tested with a multimeter), but fail at higher voltages? What would be a good way to test capacitors that normally operate at ~50V and 13.56Mhz? 3) What other components typically fail when Q4 dies? 4) Any other tips on how I should proceed? Many thanks! --- End quote --- I would suspect excessive current since it's a 100V FET. Make sure Q4 is not shorted to the heatsink (look at the notes at the end of the parts list). Pull T2 and check the inductances and that the windings are isolated. Work downstream from there. |
| kff:
After rewinding T2, there is still no heat produced even though the green light is on, but at least Q4 doesn't blow anymore. Something else must be broken somewhere. One thing that I noticed is that my unit has IRF640N for Q4 instead of IRF530N. This is a newer model made by Oki, serial number > 120,000. Is it possible that Oki switched to IRF640N recently, or do I have the wrong MOSFET in my circuit? Another difference is that R10 is ~17K instead of 4.7K. |
| wraper:
Probably they had reliability issues and switched to beefier mosfet. Unless someone repaired it before you, unlikely that it is wrong. |
| jbryan2:
--- Quote from: kff on February 01, 2015, 01:27:43 am ---One thing that I noticed is that my unit has IRF640N for Q4 instead of IRF530N. --- End quote --- Big difference in the transfer characteristics. Three amps versus 20 amps at Vgs=5V. |
| kff:
--- Quote from: jbryan2 on February 01, 2015, 03:09:44 pm ---Big difference in the transfer characteristics. Three amps versus 20 amps at Vgs=5V. --- End quote --- Thanks for noticing this. IRF640N is probably wrong then, I will go ahead and order some IRF530Ns. The unit was repaired by someone else before me, and given that Q4 was blowing due to a short in T2, it is not surprising they decided to go for a higher rated transistor. |
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