I returned to the faulty power supply about which I posted several days ago. Using the same DC power supply, which measured 7.8 VDC at the tip with no load, I once again applied power to the board, which was out of the case, unattached at P3 (to the disk drive) and free of the RF shield.
The last time I ran it I noticed some of the components, including the transformer, got very hot. This time that symptom did not occur. I checked the voltage at various points on the board; I found 7.6 VDC at the on-off switch, for example, and 6+ at the VCC and GND pins (8 and 5) of the MB3761. However, there was no voltage at the OUT or hysteresis pins of either the A or B channel. Next I looked at the capacitors. I couldn't test them in-circuit except to check for resistance. Most were in the low-K-Ohms. None were of infinite resistance or zero resistance and where there were two rated the same their resistances seemed similar so I moved on to checking resistors. One resistor was out-of-spec, low. A few were in spec. At least 11 were of infinite resistance, including the voltage dividers at the input to the MB3761. I may have contributed to their demise with my previous test of some days ago. This is a DC-to-DC converter and uses an LC resonant circuit to up the voltage. It is a form of switching supply, in other words, is it not? Could I have fried it when I applied power without the load of the disk drive attached? Too much heat comes from too much current, possibly, so I believe there is another component that is shorted and that short contributed to the failure of the (tiny) resistors. Which component I do not yet know. Please feel free to add your suggestions or set me straight on what I have concluded so far. Thanks for your replies. -CH-
