SMPS Enermax EDL4223V repair. Mini 5v 12v

[dogchainx]

I'd like to get this old external SCSI HDD drive up and running.  I know you can buy something like this for $50 on ebay, so this is more of the hobby of understanding SMPS repair than trying to save money.  I have only basic knowledge of SMPS and never have repaired one.  I know...safety first. I'm being extremely careful with all voltage measurements.

This is an Enermax EDL4223V repair.

On the right is AC in, which is fine at 120v. 

-Fuse is not blown
-Filter main cap shows ~160-165v.  Holds charge after power off for 30 seconds, going down to 5v after 10 seconds or so.
-I don't detect any shorts on the high voltage side.
-Seems all traces have voltages going through it, including the switching controller board (just below the big filter cap).

On the left is the +5v, +12v out to power molex SCSI hard drive.
-12v, 5v is DEAD. nothing is shown (DC setting on mutimeter double checked)
-Nothing past the transformer in the middle shows any voltage. All traces show no voltage.

My questions are, what are the most likely culprits for this?

Is the transformer dead? I've seen transformers much more discolored than this that still work, but maybe this has an internal temp probe that killed it?
Could the switching controller board have any issues with controlling the transformer?
Are caps on the 5v,12v side cause an issue?  Say one is dead, that would kill any activity on that end?

Any insights to the next step in diagnosing this would be great.



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[magic]

If it's absolute zero volts everywhere on the secondary then very likely the primary switching controller isn't switching at all.

Start with identifying that chip and finding a datasheet. There is usually an example schematic there and this board looks so simple that it is likely copied from the datasheet almost verbatim.

A likely cause is failure of power supply to the primary chip. Dead small low voltage electrolytic capacitor, or open circuit hundreds-kΩ resistor, or a small diode.

[dogchainx]

Thank you!  I was just drawing this up when you posted.

I'll go over the switching PCB again to see what I can gleam from it.

There is a diode and resistor on the MAIN PCB circuit from OUTPUT on the small switching PCB that I'll test. 

[magic]

That's gate drive circuit. It's unlikely to have problems, unless the power FET is also dead.

Report the three voltages on the small board connector with respect to GND.
Remember that GND is not ground but negative mains voltage from the diode bridge.
Don't touch anything there and try not to short things with probe tips.

[fzabkar]

There should be a startup resistor feeding the rectified mains supply from the bulk capacitor to pin #7 (Vcc) of the KA3842 IC. It appears to have been desoldered from the daughter board. :-?

https://www.ti.com/lit/pdf/slua143

[magic]

There is no HV on the small board, so that resistor has to be on the main board.

But IMO the first thing is to simply check VCC voltage. If it's good then problems must be elsewhere.

[wraper]

In cases when MOSFET is not blown, the most common failure is small electrolytic capacitor on the primary side. And check Vcc as already suggested, sometimes Rstart resistor fails. Check Dbias too.

[fmashockie]

As some others have said I'd be looking at electrolytic caps on the secondary side.  But also, check your diodes involved in the inverter switching and output rectifier stages of the SMPS.  They are usually found close to the primary and secondary sides of the transformer.  You might need to desolder them from the board to fully test them - use the diode test function on your multimeter.  I've fixed several SMPSs with the exact symptoms you describe where one of these diodes was the issue.  Good luck!