Having recovered from the 'flu and caught up with other stuff, I had another go at the Delta Electronika SM1540D bench PSU last night. The later schematics that Delta provided is a great improvement. It addition to matching the unit, the layout has been improved so you can follow the circut "flow" much more easily. There is no circuit description so it's work it out as you go along. To recap (no not that) It came in dead. So far a missing fuse, shorted (tracked) 100pF 1kV ceramic (known fault according to Delta. I put a 3kV one in) and open circuit contact on local / remote switch were fixed. Now it was lighting up but no output. The 100pF cap was killing the auxillary DC supplies. This is a small independent switch mode supply. So now we have Over Voltage Protection LED and CV mode LED ON. No signal to the main switch drive circuit. The OVP LED actually covers multiple faults so a bit of a pain.
The first significant component in the control loop is a HEF4046 PLL IC. On the original schmatic the function of this wa not at all clear. On the new schmatic it is apparent that it is generating a 50% duty cycle "clock" for the main power switcher that is phase locked to the PWM signal of the auxillary supply switcher. The 4046 VCO had no output. It was replaced and gave a nice output but still no drive. Next in line was a MOSFET lnverter / level shifter. Common source with pull-up to 12V on Drain. Drain stuck at 12V. The trusty Peak Atlas confrmed a dead small signal MOSFET. It's a BS170. I didn't have one in stock. A close match that I had was a VN10KM. Checking the datasheets showed a diference in pinout. So when fitting it I checkedthe track layout to be sure. Something seemed wrong so I double checked. The latest BS170 / MMBF170 daasheet on the ONSEMI website (
https://www.onsemi.com/pdf/datasheet/mmbf170-d.pdf ) dated Dec 20121 has the wrong pinout for the BS170
I've let them know.
So the VN10KM fitted (identical pinout) and we have life
. The output is on, sets correctly and tranfers from CV to CC as expected. However the OVP LED is still on, the CC LED does not come on and the CV LED only dims slightly on transition from CV to CC operation.
So investigation of the LED drive logic is for later today. I'm happy that the power circuit is functioning for now. Been a while since I've had a unit with so many individual confirmed failed components.