Chieftec CFT-750-14c

[pdavid]

This psu was returned to me after I cleaned it. The owner said that his pc shuts down after a few seconds. Now I have to fix it since "I broke it probably" It turns on but soon after the 5v rail starts dropping to 4 something volts without load. All the other voltages are rock solid and within spec. With load 5V even gets lower like 3.6V and the psu turns off. I doubt only a recap would help on this one. The modular cable soldering is a mess, but there are no short or anything on the pcb. Has anyone experienced this issue with this psu. It's a cwt psh model.

[pdavid]

Re: Chieftec CFT-750-14c

I'm going to dissasamble this unit and rework the soldering. The controll board has a lm393, ps229 and a weltrend supervisor chips. I hope it's not one of these giving some trouble. It could be some of the tiny capacitors on this board...

[pdavid]

Re: Chieftec CFT-750-14c

I checked the diodes for each voltage rails, changed the filter capacitors. The problem remains. I was playing around with the control board. "Unpluged" the 5 volt protection functions and the unit won't start up. Small capacitors measure ok. I guess that part works fine, but I'm not entirely sure how all the protections work. The problem is with the 5V regulation.
Need more pics?

[ben7]

Re: Chieftec CFT-750-14c

Does the PSU have minimum load resistors inside? If so, check those and see if they are all reading fine.

[pdavid]

Re: Chieftec CFT-750-14c

There is only one 15Ohm 5W resistor on the 12V rail. It's the one on the supervisor pcb. The rest on the secondary are smd resistors. I wired the standy to the 5V voltage input on the ps299 and the symptom remains. All voltages are fine except 5V. The problem may be somewhere else.

[pdavid]

Re: Chieftec CFT-750-14c

I figured that the C2 ceramic capacitor was shorted so I changed it to a 0.5 uF. Now 5 volt is 0.1 higher... but still something like 4.2V without load. Still regulation is screwed up so I'm tracking back things.
Btw should I install the missing vr1 trimmer, if so what value?

[pdavid]

Re: Chieftec CFT-750-14c

I still haven't solved this 5V problem. I've been lectured by some pros that it's something with the feedback and that missing vr1 pot isn't important. There may be a faulty component on the primary...

[mscserbia]

Re: Chieftec CFT-750-14c

Use another core,new one for 5V rail and rewind it.I had the same problem with 3.3V rail.Try that.

[rtstorm]

Re: Chieftec CFT-750-14c

hi, i have the same problem. +5V go to 3.6V widouth load. Have you managed to get it working?

[ben7]

Re: Chieftec CFT-750-14c

Tried checking and/or replacing the 5V rectifier diode? It might be leaky too.

[rtstorm]

Re: Chieftec CFT-750-14c

Ok does anybod have a schematic for this so i can find diode?

[pdavid]

Re: Chieftec CFT-750-14c

hi, i have the same problem. +5V go to 3.6V widouth load. Have you managed to get it working?

I had much trouble fixing this problem on that CWT PSH. As far as I remeber it was something trivial. A bad smd resistor somewhere in the secondary.
There is no schematic for this psu platform yet. The 5V diodes are in the middle on the third heatsink.
I wish I could help more. Good luck!