Astar Power Supply Troubleshooting Help

[drpj23]

I have an Astar power supply that keeps blowing the fuse. Upon opening up the set, I also saw that the main filter cap was blown.

I replaced that to see what would happen and of course the fuse blew again. The thermistor too also had a hole in it.

The bridge rec and FET tested ok and diodes seem to be ok from checking.

I have noticed there is a short between the two pins of the main filter cap, which presumably is causing this.

I just need some guidance; could it be that transformer LEQ30?

[drpj23]

Re: Astar Power Supply Troubleshooting Help

Well transformer is ruled out.

I don't believe that 24v power module IC the case either.

I have included a picture to show where the negative pin of the Cap is connected to on the underside (since I don't have a schematic), maybe that will help someone.

I guess I'll start removing those diodes see if something changes, or missed one.

[momaka]

Re: Astar Power Supply Troubleshooting Help

Looks like this is a power supply with APFC. Check the APFC rectifying diode (attached on the heat sink on the primary, right under U2) and MOSFET (right below the main cap spot in this picture).

When testing a faulty power supply that blows fuses, it is usually helpful to have an incandescent light bulb wired in series with the live wire on the PSU. If the fault is still present when you try to test the PSU, the incandescent light bulb will become fully lit and the fuse won't blow (components on the board can still fry, but it may take them a few seconds, so if you see the light bulb stay fully lit for more than 3 seconds or so, the problem is likely still there).

[drpj23]

Re: Astar Power Supply Troubleshooting Help

The MOSFET I believe is OK. The diode I checked en-circuit and nothing gave me a reason to remove it (with a meter) but I will tomorrow.

And I have been meaning to make that series lightbulb load device but time never seems to be a friend these days. Maybe I can make it a saturday project tomorrow

[budm]

Re: Astar Power Supply Troubleshooting Help

The short may be from that U2 Power driver module, it drives the transformer.

[drpj23]

Re: Astar Power Supply Troubleshooting Help

Well removed the U2 driver module and the short is gone.

Testing on the board with that removed pins 8 and 23 along with 4 and 23 create a direct short across the filter cap.

On the device 8 and 23 have resistance in the kohms so that's ruled out.

4 and 23 are shorted on the device. Looking at the datasheet: 4 is MOSFET1 source and 23 is MOSFET2 drain. The MOSFET1's drain is connected to MOSFET2's source. Pin 19 is that connection. Using those pins, we find our culprit

[budm]

Re: Astar Power Supply Troubleshooting Help

I hope you can get that module. It looks like a half-bridge drive setup, so it needs two MOSFETs.

[drpj23]

Re: Astar Power Supply Troubleshooting Help

eBay has one in N America, so I'll order that and post results.

[drpj23]

Re: Astar Power Supply Troubleshooting Help

Got the parts in.

Well it stopped blowing fuses but upon plugging in, the main filter cap has 360v and just continuously drops. There was smoke too but I cannot source where it came from.

[drpj23]

Re: Astar Power Supply Troubleshooting Help

I plugged in a second time and smoke reappeared after 20 secs and seemed to come from that biggest transformer.

Well I plugged in a again to see what voltages where coming up. And it didn't smoke this time. All voltages where present but unstable. The 24v rail was 24.8-5.1. The filter cap was jumping between 260-380, just dropping till 260 then reset to 380. That transformer has a small hum to it but also ticks, and goes back between the two sounds randomly.

[drpj23]

Re: Astar Power Supply Troubleshooting Help

Ok I'm just confused on this.
The ticking and all happen during standby but it switches itself on and it goes to high whine.
And I accidentally shorted out the filter cap (was watching it guess my hand got lazy) to another trace and it now just stays on, wont switch off. But, the voltages are all very steady (ha!)

[drpj23]

Re: Astar Power Supply Troubleshooting Help

Well I put the power supply in the TV and hey what do you know, it works and cuts on and off too!

But something is telling me this is going to be short lived haha.....well I guess time will tell lol

Thanks to all that chimed in to aid me in this repair