I Finished the Delta Eletronika SM1540D bench power supply repair today. Previously I'd got it functonal but the OVP LED was still on and the constant voltage (CV) and constant current mode indicator LEDs were not operating correctly. The circuit is a bit odd. The LED anodes are connected to the plus 12V rail (diamond 2 in cct) the cathodes to an individual NPN transistor collector. The transistor emitters are conneted together and then to the minus 12V rail (diamond 3, Diamond 1 is 0V). Each transistor base has a series resistor and the other end of that is diode OR'ed to two or more opr-amp outputs. The op-amps are conneced as comparators and outputs swing between -10 and +10V. To be honest I've not fully worked out the logic of the comapators. A key learing point was that measurements have to be made relative to the commoned emitters. A few voltage measurements indicated Q613 that drives the OVP LED was faulty. Pulled it and the Peak Atlas confirmed a Collecter - Emitter short. It's a PH2222A plastic version of a 2N2222A. I put in a E-line MPS2222A as I have plenty. Thay got the OVP LED working correctly but CV and CC were still not makinn any sense. More measurements pointed to Q614 that drives the CV LED. Pulled it and the Atlas said it was a diode array. Further checks showed it was leaky C-E nd C-B. I replaced it and Q612 because it's bit of chore getting to the PCB. With 2 of 3 already gone I didn't think it would last long even though it tested OK.
It finally works
![Smiley :)](https://www.eevblog.com/forum/Smileys/default/xsmiley.gif.pagespeed.ic.R8GFI-pF6f.png)
I did get a bit carried awy and turned it up to 40A on my load and melted the test leads
![Embarrassed :-[](https://www.eevblog.com/forum/Smileys/default/xembarrassed.gif.pagespeed.ic.zcm3EmNu9T.png)
The final repair tally was:
F300 (main HV DC supply fuse) missing.
F501 (Aux DC supply mains side soldered fuse) blown.
C508 (AuxDC supply switch mosfet snubber) Shorted with signs of tracking / arcing.
SW850 (voltage remote program select) Open circui in Local position.
D620 (TL431 reference diode inn OVP circuit) Changed because diode tested differently to known good diode. Not confirmed faulty.
IC900 (HEF4046 PLL IC). VCO not oscillating.
Q902 (BS170 MOSFET level shifter driven from IC900 VCO output) open circuit.
Q613 (PH2222A OVP LED driver) C-E short circuit.
Q614 (PH2222A CV LED driver) C-E andC-B leaky.
Q612 (PH2222A CC LED driver) Precautionary replacement due to Q613/4 failures.
I've not had that many confirmed component failures in unit for a long time. It's possible that the arcing of the aux supply switch MOSFET snubber caused transients on the LV rails resulting in the damage.
It's a nice PSU with no fans and clear green LED displays.