Re: Reviving Spock

[bieling@terra.es (Jorge Chamorro Bieling)]

BLuRry <brendan.robert@gmail.com> wrote:

> 
> (e.g. if the 5v is really 4.8v will it fry something?)
>

A modern P.S. should have better line/load regulation than an old one
(?) and should be able to keep its outputs closer to +5v. (that is, if
it's current rating is enough)

Therefore I think it should not fry anything.

> Unless someone knows offhand how I could go about diagnosing this one

When a diode is shorted the fuse blows when you turn it on, either the
bridge rectifier or the switching transistor or both are gone.

The easiest way to fix this is to replace both the bridge (the 4 diodes)
and the switching transistor from the beginning. No measurements
involved. Easy. Fast. And cheap.

If you want to check the diodes with a multimeter look for one that has
the "diode check" function (*): It goes like this:

**First disconnect everything from the mains plug**
**Then wait ten minutes**
Or check and make sure that the big capacitor(s) coming just after the
bridge rectifier are discharged.

The diode check funtion will tell you the voltage drop across the diode.
You have to check the diode in both directions (check, reverse the
probes, check again). A diode that is ok will give you a reading of 0.x
Volts (less than 1) in one direction and out of range (= several hundred
volts) in the other. If both readings are close to 0v the diode is
shorted (broken, bad). Both readings out of range will also mean a
broken diode, replace it too, but this is not the usual mode of failure
of a diode, and this has nothing to do with the blown fuse.

Once you have found the bad diodes, replace them. 
For a few cents more, I would also replace the switching transistor now.

Connect it, cross your fingers, and switch it on.

If the fuse keeps blowing and you did not replace the switching
transistor... I told you.

If the fuse keeps blowing and you replaced both the bridge and the
switching transistor, you have discovered a new smps failure mode.

If the fuse does not blow, but it still does not work, most likely
there's a toasted resistor somewhere. If there's not, unless you've got
the patience and the knowledge and/or the schematics, unless you've got
a good reason, I would say, better forget it.

Good luck !

(*)Before, when the multimeters were called testers, when the testers
were not digital, when they did not have the "diode check" function, the
usual way to check a diode was to measure its resistance. But given that
the resistance of a diode is a function of the voltage across it, *not
all* the multimeters can measure it properly.
-- 
Jorge Chamorro Bieling