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Old 10-23-2011, 12:22 AM   #1
momaka
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Default Need help with Shuttle SB83G5A - fans spin but that's all

Iíve been trying to fix this Shuttle SB83G5A computer for a while now. When it was given to me, the motherboard had 1 visibly bad cap on V_DIMM and all 4 bad caps on what I believe is the Northbridge/Southbridge VRM (3 on low side, 1 on high side). The bad caps were all OST. I replaced them with equivalent/superior Nichicon caps. The motherboard is a FB83 I think.

The problem is that the fans spin and nothing else happens.

What Iíve tried so far (with no further success):
- replaced a few more caps other than the obviously failed ones, but that didnít help.
- tried using 2 other known-good power supplies (one is a fully recapped 300W Macron Power, the other one is this) besides the original one, which has no visibly-bad caps.
- I reset the CMOS Ė in fact, after the first few tests, I completely removed the CMOS battery.
- tried the motherboard with and without RAM. The memory sticks Iíve used for the test are known good, and I am using them in other computers right now without a problem.

What seems to be working so far:
- V_DIMM voltage Ė it shows 2.5V stable, with and without RAM. Some of the other smaller linear voltage regulators also seem to be working fine.
- The Southbridge, since I can shut down and start the computer from the button successfully, as well as restart it.

Iíve measured some of the derived voltages on the motherboard. Thereís a number of them that arenít coming up (they jump up a bit then settle to a low value), including the CPUís V_CORE. Iíve checked all MOSFETs on these rails, and none are shorted.

I think the problem is with Northbridge/Southbridge VRM, since it shows 0.6V only and Iím almost certain 0.6V canít be high-enough for the NB and SB. My logic is that if the NB/SB voltage is not right, then perhaps the other voltages on the motherboard wonít be either. The only reason I suspect the NB/SB VRM is because the lower MOSFET *seems* to have a slight bubble on it as best as I can tell.

Iíve circled the NB/SB VRM with red. The lower MOSFET is connected between 3.3V rail of the PSU and NB/SB VRM. The upper MOSFET is connected between ground and NB/SB VRM. Caps with red dots on them are on NB/SB VRM low. Yellow Ė NB/SB VRM high. Green Ė V_DIMM 2.5v

In any case, my question is, does anyone know what that NB/SB voltage should be? Perhaps someone who has this motherboard can measure it and tell me if itís not too much of a hassle?

I would really like to get this PC fixed, since it has a PCI-E slot (a first for me). I like the small form factor of the case as well.

Also, if anyone has any other suggestions, please let me know. At this point Iím open to any and all ideas. In fact, Iíve very tempted to just insert 1.5V on that spot and see what happens.
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