ds1102e - burned out psu resistor?

[sjones]

This is going to seem super stupid, so I apologize in advance.

Just bought my first scope, DS1102E. Holy crap the fan is loud, so I went to replace it. After pulling it apart, I foolishly didn't note the polarity of the fan header. Plugged it in, put a multimeter on it, found the polarity and as I was verifying the polarity before turning it off, a resistor on the power supply board let off its magic smoke. No idea why. No leads were connected to anything except the multimeter probes to the low voltage fan header.

In any case, the resistor is trashed. (Hopefully that's the only thing.) The confusing part is that the silkscreen says this should be an oscillator (L5), not a resistor. Every photo I've found of the board shows a blue oscillator. The only schematic[1] I could find for this board appears? to suggest this component should be an oscillator. Assuming I need to replace the burned resistor with another resistor, but because of the damage to the resistor, I can't see all the bands to determine its value.

Hopefully to show that I'm not crazy, attaching a photo of the board with the burned up L5 resistor.

The board's date code is stamped 20141024. Is there any chance anyone has a comparable board and could pass along the resistor value of "L5"? Or provide some insight as to why that was not populated by an oscillator? Thanks. I really appreciate it. Frustrating to have killed this thing

thanks!

[1] http://rigol.codenaschen.de/images/3/35/DS1052E_PSU_schematic.jpg
[2] https://www.dropbox.com/preview/Public/IMG_2307.JPG

[Alex Eisenhut]

L is an inductor, not an oscillator or resistor. It's probably a simple way to keep the fan's brushless controller's noise out of the scope's supply. By plugging the fan backwards, you burned out L5 because of excessive current. These types of inductors can't carry lots of current.

Why do you think it's an oscillator?

[tautech]

Plugged it in, put a multimeter on it, found the polarity and as I was verifying the polarity before turning it off, a resistor on the power supply board let off its magic smoke. No idea why. No leads were connected to anything except the multimeter probes to the low voltage fan header.

DDM leads in the amps not volts sockets ?

[DC1MC]

Hi, if the German DHL make me nice surprise and delivers this:
https://www.ebay.de/itm/SCYTHE-Mini-Kaze-50mm-Gehäuselüfter-z-B-HTPCs-Northbridge-externe-Festplatten/361944246434
I will do the same operation on a MSO version, hopefully without the magic smoke  , and peek at the actual value.

As it looks now, it's a 5.1 ohm resistor at 0.5W, that was replacing the inductor  not oscillator, (I've had a similar brain fart confusing drain and source of a mosfet, really bad flu).
It could be that the fan was connected to some voltage higher than 12V and was being noisier and wearing off faster, therefore they replaced the inductor with a resistor to reduce the fan voltage a bit.
As other member have said it could be that you've left your DMM leads on current or just a plain shorcircuit.

 Good luck,
 DC1MC

[Performa01]

As it looks now, it's a 5.1 ohm resistor at 0.5W, that was replacing the inductor ...

What makes you think it is a resistor - have you measured it?
What is the actual color code on it?

[DC1MC]

As it looks now, it's a 5.1 ohm resistor at 0.5W, that was replacing the inductor ...

What makes you think it is a resistor - have you measured it?
What is the actual color code on it?

Look at the posted pictures please.

[genghisnico13]

I would expect the value of L5, assuming it is a resistor, to be the same as R30(on the right of the picture). When mass manufacturing you try to minimize the amount of different value passives you use.

[DC1MC]

I would expect the value of L5, assuming it is a resistor, to be the same as R30(on the right of the picture). When mass manufacturing you try to minimize the amount of different value passives you use.

I have assumed the same, plus all but one of the color code lines are visible in the picture.

[SeanB]

Looks like it was a 5R1 0.3W metal film resistor. you can replace it with pretty any value from 4R7 to 6R8 and the fan will work just as well. Using 10R will probably slow the fan down.

[sjones]

Thanks for the quick replies and all the feedback.

You're correct. The part that the silkscreen indicates for this location is an inductor, not an oscillator. That's my mistake.

Yes, I'm sure it's a resistor. No, it can't be directly/reliably measured because it's toast.

In retrospect, I should have just checked continuity between the two fan header pins and any ground (ie the screw next to the header). There was no reason for me to plug it in and power it up to check the polarity. I'm an idiot, and I know better. I thought my multimeter had a 1M resistor built in to keep from creating a dead short like probably happened in this case. Either way, powering it up was totally unnecessary. Lesson learned. (ie don't do what I did...)

The solder on the back of the board (all connections) is a gray-ish color, not the typical bright-ish silver so I suspect it has some stuff in it (ie lead) that I definitely don't want to breath. Have to grab some kind of ventilation apparatus and a new resistor, and then go about making the repair. I'll follow up with results once I have them.

Thanks again, I appreciate all the replies.

[tautech]

The solder on the back of the board (all connections) is a gray-ish color, not the typical bright-ish silver so I suspect it has some stuff in it (ie lead) that I definitely don't want to breath. Have to grab some kind of ventilation apparatus and a new resistor, and then go about making the repair. I'll follow up with results once I have them.

Yer what ? 
What BS on this good earth have you been watching/reading/listening to ? 

Many members on this forum, experienced and not, use ONLY lead based solders and don't/won't use that Pb free muck !
As for ventilation apparatus, yeah right. If you're sitting at a soldering station all day maybe, but really, to replace one component.....pull the other one !

[Performa01]

It's actually the other way round. Leaded solder joints (as it is preferred for repair work by most) are shiny silver, whereas the lead-free stuff mostly looks pretty much like a cold solder joint, i.e. dull grey.