auhn (ALDI-brand) AC-42F22 no 5 volt supply

[dkneyle]

I can't find any obvious component failure. I'm attaching the circuit for the power supply. It uses a generic model of SMPS Model Number MLT198TX. The switching FET Q9 has 400VDC going into it, it just doesn't appear to be oscillating. Nothing on the cold side of the transformer T1. Somebody with better circuit analysis knowledge than me might be able to explain what makes the FET switch? There's an 8 legged SMT IC that I see for sale on E-Bay as a part for this power supply which makes me wonder if it is a common cause of failure? Also that FET does look like it's been changed out at least once which is very suspicious. DC testing and the FET doesn't appear obviously at fault. Wondering if it might be a failure under stress of 400VDC?
All suggestions gratefully received.

[dick_barton]

Re: auhn (ALDI-brand) AC-42F22 no 5 volt supply

Here is an example of a supply using that chip which may help you.

[dkneyle]

Re: auhn (ALDI-brand) AC-42F22 no 5 volt supply

thanks Dick. It certainly appears it needs that IC to kick off the oscillation process. I might change out that driver FET, but if that doesn't bring it to life then I'll assume, in the absence of any other failed semis, it must be that SMT chip, which is beyond my capabilities to replace I fear.

[jetadm123]

Re: auhn (ALDI-brand) AC-42F22 no 5 volt supply

I suggest the following:

1) check Q6 and Q9 for shorts. Also the resistance values, R64, R65 and R40 around Q6.

2) there's a startup capacitor for IC1, EC4. You can try replacing it, if step 1 checks ok.

3) if still no go, the VCC pin of IC1 requires a minimum voltage to start (probably something like 11-15 volts?). You'll have to download the data sheet for IC1 to verify the minimum start voltage. If you don't see that voltage, then consider replacing IC1.

[dkneyle]

Re: auhn (ALDI-brand) AC-42F22 no 5 volt supply

Can a supply like this be run unloaded and still start, at least start the 5V Standby? It's just that gaining access to test points can really only be done from the underside of the board. (I'm aware the board is 'hot' and would be taking extreme care.)

[jetadm123]

Re: auhn (ALDI-brand) AC-42F22 no 5 volt supply

A lot of the test points underneath the board can be measured from the top of the board. For example, the voltage of the Vcc pin of IC1 can be measured at capacitor EC4. Any points you cannot access from the top, you can solder small lengths wire to that point and run them to clear the board.