5500XT Memory VRM Short???

[nipsta]

Trying to repair a Powercolor 5500XT. I checked all the VRMs, found a short on the Vcore. I thought the culprit was a bad HiFet, as shown in the pic, I removed both Hi and Lo, only to find that the Hi was bad and the Lo was good.

Unfortunately, the short still present. So I lifted the inductors (open circuit) and measured. As you can see the top 2 inductors (Memory??) still reads short, 0.3 ohms. The other inductors reads around 30 ohms. And the bottom inductor (PEX??), reads 2.5 ohms.

Also I measured the caps to the right side of the inductors, they are short. The caps on the left side of the mosfets are good.

I'm not sure what to do at this point. I initially assumed it may be another shorted mosfet, but after lifting both Memory inductors, they both read short. So that would mean 3 mosfets total would have shorted, which I don't believe happened. also the caps toward the left of these mosfets reads good.

Please help. THanks in advance.

[didi79]

Re: 5500XT Memory VRM Short???

Inductors should have almost no resistance. It's just a wire.
Or do you mean to ground ?

Capacitors: You'd have to remove them to be sure they are short. Most likely they are not. If you measure them on the board, you are measuring the same short between VMem/VCore to ground (almost all caps are on ground at one side).


Before desoldering more stuff, measure the MOSFET's:
- source to gate resistance
- source to drain resistance
- gate to drain resistance

Whenever you measure to gate, put the black probe of the multimeter (COM or minus or whatever) on the gate pin not the red (+), otherwise you may switch the MOSFET on and have a bad reading.

You may notice a difference even when they are on the board.

I learnt that using this guide: http://electronicsbeliever.com/how-t...-is-defective/

Keep in mind that whenever a MOSFET dies, there's a high chance the doubler or driver is taken out too. So short may be there too.

If nothing obvious is found, a voltage injection with a thermal camera may show something. But know what you are doing. Like: don't put 12V directly on RAM or GPU