Scanner repair mystery

[Peabody]

A friend brought over an HP ScanJet 3570C flatbed scanner to see if I could fix it.  It's dead, Jim.

Turns out the fuse is blown.  Attached is a diagram of the power circuit.  There's an 18R high-watt resistor in series after the fuse.  That resistor is good, and the 7805 regulator is good.  With the regulator removed, the output through-hole isn't shorted to ground.  And the AC adapter is good.

So my first question is - with the resistor in there, how does this fuse get blown?  12V divided by 18 ohms is 667mA even with a dead short to ground.  How does a 1A fuse blow?

Second, I don't have a replacement axial 10mm 1A fuse, or know where you would get one locally with RS being gone.  I do have a large slow-blow (313) 1A fuse, but I would have to solder wires to the ends.  Can you do that to a fuse without melting whatever is supposed to melt when it's supposed to blow?  Given that the AC adapter must have some limiting built in, and with the 18R resistor there, would it make sense to just wire across the blown fuse at least to see if the thing is going to work at all?  If it's still dead, then there's nothing more to be done anyway (it's all tiny SMD stuff, and I don't have a schematic).  But if it works, then I could order a replacement fuse.

[capt bullshot]

If you keep an eye on it (smoke ...), there's little issue replacing the fuse by a wire in this particular case. I'd definitively do it that way.
Sometimes fuses just "wear out" over time and usage and go open for no obvious reason, though this should be rare.

[james_s]

I've had low current fuses fail mechanically several times. It's a thin and fragile wire element, vibration and shock can break it.

[mariush]

V = I x R

Seems like the 18 ohm resistor is there just to drop some voltage before the 7805 regulator, which only needs around 7.5v to output a smooth 5v. (the bigger the difference, the more heat the regulator has to dissipate)

For example, let's go with 0.25A of current .... V = 0.25x18 = 4.5v ... so the regulator sees 12v - 4.5v = 7.5v  and will only dissipate  (7.5v-5v)x0.25a = 0.625 watts

[mikerj]

A friend brought over an HP ScanJet 3570C flatbed scanner to see if I could fix it.  It's dead, Jim.

Turns out the fuse is blown.  Attached is a diagram of the power circuit.  There's an 18R high-watt resistor in series after the fuse.  That resistor is good, and the 7805 regulator is good.  With the regulator removed, the output through-hole isn't shorted to ground.  And the AC adapter is good.

So my first question is - with the resistor in there, how does this fuse get blown?  12V divided by 18 ohms is 667mA even with a dead short to ground.  How does a 1A fuse blow?

Possibly by someone plugging an incorrect AC adapter into the scanner.  Also are you 100% sure that none of the scanner circuitry is powered by 12v i.e. connected prior to the resistor?  I'd be a little surprised if e.g. the back light and motor drivers were all supplied by the 7805.

[Psi]

Did you read or measure the 18R to determine that value.

Sometimes its hard to read them, could it be 1.8R (1R8) ?

[amyk]

If you keep an eye on it (smoke ...), there's little issue replacing the fuse by a wire in this particular case. I'd definitively do it that way.
Sometimes fuses just "wear out" over time and usage and go open for no obvious reason, though this should be rare.

Also use a current-limited supply to power it up the first time.

[Peabody]

Also are you 100% sure that none of the scanner circuitry is powered by 12v i.e. connected prior to the resistor?  I'd be a little surprised if e.g. the back light and motor drivers were all supplied by the 7805.

You're right.  The 12V supply goes elsewhere as well, including to the motor.  So that could explain why/how the fuse blew.

Well I got the fuse replaced using an inline fuse holder.  When I power it up, USB is working, and my XP tells me a new device is connected, and it wants to install a driver if I have one.  At this point I'm going to let the owner take over and see if he can get it to work since he has the software.  If it still doesn't work, I don't have anything left to try.  So either it works or it doesn't.  But at least the fuse didn't blow again.

One bad sign is that while USB appeared to work, the scanner tray did not move to the index position.  My guess is it would do that if working properly.  But maybe it won't do that until the software is installed.  Anyway, thanks for the comments and suggestions.

[mariush]

If you install the drivers, you can then use freeware image viewer IrfanView to try to scan some stuff.

Just go in File menu and Select SCAN/ TWAIN Source ... select the scanner, then use the Aquire / Batch scanning menu option.

Scanners tend to warm up the lamp inside (maybe not the case with leds if yours has leds) then most scanners do a preview, so the scanner should then move inside to scan whatever's inside.
On my old Canon scanner, it was around 20s to warm up the light and then I could scan tens of pages without delays.

[TheMG]

One bad sign is that while USB appeared to work, the scanner tray did not move to the index position.  My guess is it would do that if working properly.

Some of them don't do anything at all until the drivers tell it to.

[Peabody]

One bad sign is that while USB appeared to work, the scanner tray did not move to the index position.  My guess is it would do that if working properly.

Some of them don't do anything at all until the drivers tell it to.

That's the case with this 3570C.  When the driver was installed, it came alive, and now works perfectly. Still no indication of what blew the fuse.  I guess that will just remain a mystery.

[Gyro]

A quick web search revealed another 3570C unexplained blown fuse. Maybe it's just rated a bit close.

[TheMG]

That's quite possible. If a fuse is repeatedly loaded very close to its rating, thermal cycling can over time result in the fuse element breaking, as it heats up slightly, therefore expanding, every time the load is applied (scanning a page), then cools back down.