![]() So in the end all I needed to replace was the VSB3200 diode. The gate capacitance is charged by the momentary touch to the Gate and the D-S conducts for a few seconds until the Gate charge dissipates. I found that if I add a 1.5V AA battery in series, giving about 4V total, when I touch the meter to the Gate in step 3 then the D-S conducts correctly. I have learned that some power MOSFETs, particularly the high voltage types like the 6N62K3 and NDF04N60ZG, need even more than 2.5V on the gate to turn them on. I owe you an apology Swake, the MOSFET is not dead. Mine is still working but if you damage it I would try a 200Kohm NTC thermistor or you could try a resistor of about 150K-220K but you wouldn't have any thermal protection so it's at your own risk! The thermistor markings are invisible under the green epoxy but as best as I can tell it is possibly a 200Kohm NTC device. I'm not sure yet how I will reassemble this, possibly with heatsink compound and/or epoxy. ![]() ![]() I was able to gently lever the side of Q3 until they both cracked away from the heatsink but then had to carefully cut the thermistor free from the dead FET with a hacksaw blade! I made sure to cut into the FET body so that the thermistor was undamaged. Q3 and TH3 were glued together and to the aluminium heatsink with a hard green "epoxy" glue. Once they were removed from the board I was able to test Q3 to confirm it had failed. The pictures I have attached show the board with the heatsink, Q3 and the thermistor TH3 removed. I have replaced D5 with a second hand MBR5200 (5amp Schottky instead of the original 3amp unit) that I had lying around and I have some 6N62K3 MOSFETs on order on eBay, 10 units for less than USD$15 delivered. Likely the D5 diode failed and took out Q3. I removed it from the board and used a couple of multimeters in diode test mode to verify it. All the electrolytics have been tested for capacitance and ESR, most of the diodes and several of the SMD transistors seem to check out OK too. As far as I can tell all the other components are OK. The board looked flawless, no signs of heat or failed components, but testing found that the main diode D5 (VSB3200) on the secondary side is short circuit and the main MOSFET Q3 (6N62K3) is open circuit except for the drain-source reverse diode. ![]() With the low voltage stage dead there is nothing to discharge the cap so BE CAREFUL! I used a 100K ohm resistor and some leads to discharge the cap and then had a closer look. Firstly a warning The box is quite easy to open, just 2 Torx screws hidden under the corners of the label, but a check with the meter showed over 300 volts still on the main capacitor. There are other units 0957-2259 0957-2479 0957-2271, these seem to be able to supply 32V at 1500mA but they don't have the 12V sleep mode so may be totally different internally and I don't know if they would work correctly with my Photosmart 7510. BTW I suspect that the 0957-2305 is a similar, possibly identical, unit. The mode is controlled by a signal fed back from the printer on a third wire. This is a slightly unusual power supply in that it has 2 modes, 32 volts at up to 1094mA output and a "sleep mode" of 12 volts up to 250mA. I have found and ordered one on ebay but decided to have a look inside and see whether it was worth repairing. I soon tracked the problem to the 0957-2304 power pack, the green indicator light was off and a quick check with the multimeter showed 0 volts output. After years of mostly trouble free operation, I tried to turn on my HP Photosmart 7510 printer/scanner/copier a few days ago and found it totally dead. ![]()
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