Otrona Attache power supply woes...

Hi Josh Getting closer. Pin 9 should be almost zero. The two transistors ( Q17, Q18 ) form a latch. Normally this latch is not tripped and Q17 is on while Q18 is off. Two sources of signals can trip the latch. One is an over voltage on the 5 volt line and the other is an over current in the switcher transistor. The over current is detected by the optical coupler ( D1 in the switcher board ). All the levels on U5 look as expected under the current conditions. There are a number of possible reasons that the Q17,Q18 latch has fired. Not all are normal operation of this circuit. 1. D18 may be shorted and any voltage on the output trips it 2. Replacing the bad U4 has caused the voltage adjustment to be too high, causing the circuit to trip. 3. Q18 may be shorted. 4. Q17 may be open. 5. Something else We know that U4 was bad so it may have been the only bad part. This makes #2 a likely possibility. Try adjusting the pot R39 such that the wiper is closest to R38 ( maximum resistance ). Cycle the power switch. Normally I wouldn't suggest adjusting any pots in a non-working circuit but this one should be harmless. Don't worry about remembering the location on this pot as it would have needed some adjustment anyway, with the replacement of U4. Do not fiddle with any of the other adjustments! Dwight _________________________________________________________________ See how Windows Mobile brings your life together?at home, work, or on the go. http://clk.atdmt.com/MRT/go/msnnkwxp1020093182mrt/direct/01/