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Dead 770X
Posted: Tue Nov 29, 2005 9:18 pm
by Lomax
I have a completely dead 770X, no lights, no boot, nothing. Any clues what is the most liekly cause? Power supply is fine, works in other machines.
Lomax
Posted: Tue Nov 29, 2005 10:02 pm
by LtTPfan
Sounds like it's the DC-DC board to me.
Posted: Wed Nov 30, 2005 10:04 am
by Lomax
Is this a common fault on the 770 series?
Lomax
Posted: Wed Nov 30, 2005 12:46 pm
by LtTPfan
Lomax wrote:Is this a common fault on the 770 series?
Lomax
Yup, and if you have an ohmmeter and are adept at soldering small surface mount devices, you might be able to replace just a fuse.
Posted: Wed Nov 30, 2005 1:36 pm
by Lomax
What a nightmare the 770 is to open up compared to the 600 series! I have now removed 30 odd screws, 8 sub-assemblies (including the screen) and finally got to the DC-DC card. Thank god for the HMM, re-assembly would be tricky otherwise...
Looking at the DC-DC card it has no less than 12 fuses (FU1-FU12), only three of which are dimensioned for serious power (FU1 6.3A, FU3 2A and FU8 also 2A). If it's one of these I'm sure I could perform a replacement as they're pretty large SMDs. The other ones I would need help with.
With my limited knowledge I would suspect FU1 is the most liekely candidate as it has the highest rating and as the system is completely without power which would suggest a failure high up in the power supply chain. Is this correct?
Also, if this is a common failure, would you recommend replacing the faulty fuse with one of a slightly higher rating (or slow burn instead of fast)?
Thanks,
Lomax
Posted: Wed Nov 30, 2005 7:36 pm
by LtTPfan
My four DC-DC card failures have all been self induced. The first two times I attempted to plug in the wrong power supply, one of the opposite polarity that didin't even fit the jack but blew a fuse on contact. For the last eight years I've double and tripple checked what adapter I'm plugging in, even when I know there are no others within reach. My third episode involved a spill, and the fourth when I was testing a bunch of video boards, one of which was bad. The latter case did not blow a fuse.
If your board went spontaneously it might not be a fuse but rather another component. I wouldn't increase the fuse size as you'd likely blow components or burn traces rather than the fuse.
Posted: Wed Nov 30, 2005 7:47 pm
by Lomax
I bought this machine as dead (£35) as it had the 1280x1024 13.7" screen (and 8Mb graphics card). The only thing I know was that the previous owner had tested it with a power supply known to be working only to find it dead. I suppose it is not impossible that that power supply was incorrect and actually caused the failure.
After checking the voltage/amperage and polarity requirements of the 770X I decied to test it with my 600X PS which has same plug,voltage and polarity and exceeds the power requirements. Still dead of course.
Can I measure the fuses without de-soldring them from the DC-DC card?
Looks like a nice machine though! It would be great to get it running again, I really want to see what that SXGA screen looks like!
Lomax