Fluke 199B Scopemeter Repair

[grasshopper]

G'day all,
I am looking to repair my Scopemeter. There is some obvious heat damage to the battery connector outer shell and what appears to be a "fried" component adjacent to it in the photograph.

Can anyone please identify the damaged component as I have yet to source a wiring diagram and service manual for the unit?

Appreciate your help.

Cheers

Grasshopper

 

[KrisBlueNZ]

It's hard to say; there's not much of it left!

It could be a diode for reverse battery protection. If it has disintegrated, it would have failed to protect the rest of the circuit, so you may find lots of damaged components, if you connected the battery backwards.

What did you do to cause this damage? What were you doing to the unit when it happened?

 

[grasshopper]

G'day Kris,
Battery was being changed and yes accidentally touched battery connection in reverse and instant flash over. This was a while ago now and I decided to give a repair my best shot and maybe learn something in the process. Yes user stupidity could have caused more damage but this component is the most obvious victim and I am hoping maybe sacrificed itself to save the unit.
Good news is I now have Service manual 206 pages and attached is the power circuit diagram.
I am looking to examine the unit now I have the diagram, carefully remove the component and see if I can identify it and source a replacement.
Any advice would be greatly appreciated.
Cheers
Grasshopper

 

[grasshopper]

Just adding Scopemeter Power Supply Block Diagram.

 

[KrisBlueNZ]

OK, on the battery connector, pin 1 (presumably the left end on the photo in post #1) is positive, and pins 2 and 4 are negative.

The damaged component seems to be connected directly between pins 1 and 2, but I don't see any component on that schematic diagram that's connected directly across the battery connection points.

Assuming it's a diode for reverse voltage protection, you can test the scopemeter with it completely removed, to see whether the rest of it has survived or not.

If it still works, take another photo from directly above, with a ruler (steel if possible) sitting beside the damaged component, and we will suggest a suitable replacement diode.

 

[grasshopper]

G'day Kris,
Please see image attached. Battery connector is orientated correctly and I have added the pin numbers I believe are correct and agree with your advice.

The red cable battery positive +7.2V (I have confirmed by checking battery itself between red pin 1 and ground pin 2)
).

Not sure if my wiring diagram is accurate for my machine especially as I do not have the fan shown on the right of the diagram 9-15.
I'm just about to remove the component.

Many thanks for your support.

Grasshopper

 

[grasshopper]

Component removed, area cleaned no obvious visual damage.

 

[grasshopper]

Component Length. 5 mm nominal?

 

[grasshopper]

Component Width. 2.5 mm nominal ?

 

[grasshopper]

Component depth at each end, thickest places. 0.5 mm Nominal?

Ok Kris,

There it is the story so far. If you can identify the c omponent and suggest a suitable replacement I can try I would be most grateful?

Once again thanks for your patience and support.

 

[grasshopper]

Hi Kris,

If I look at the circuit diagram I look at R4101 which is as the attached page from the manual. That's it the grasshopper has reached the limit of his wisdom Master.

 

[(*steve*)]

It looks to me to be a current sense resistor. You can notice what appears to be a thin trace connected to one end of the device. This may be a Kelvin connection.

Have you measured the resistance of this device?

 

[grasshopper]

Hi Steve,

Resistance shows as 0.4 ohms.

On both sides of device looking very closely here there are what look like scribed parallel lines. Please see sketch attached as camera cant pick it up very well.

 

[(*steve*)]

Probably a 0.39 ohm resistor

 

[KrisBlueNZ]

Steve, he seems to have identified it as R4101, 0.15Ω ±1% TC70 (presumably ±70 ppm/°C). See the asterisk on the parts list in post #11. Yes it is a current shunt. It's in the top left area of the schematic in post #3.

There is no diode connected directly across the battery; the fault current must have flowed through L4100 and L4101, and through V4105, the catch diode in the charger. I don't expect it would have damaged them. What do you think, Steve?

It is not shown as a four-terminal resistor on the schematic.

Here's one: http://www.digikey.com/product-detail/en/WSL2010R1500FEA18/WSLF-.15CT-ND/713474

 

[(*steve*)]

Gee that's gone pretty high.

To burn up a current shunt of that size suggests to me that something has gone badly wrong elsewhere (I see a suggestion of power connected with reverse polarity).

Yeah, I would check the mosfet, the diode, and the inductor (in that order )

With luck, the relatively low impedance would mean that other components were not subject to a significant reverse voltage.

 

[KrisBlueNZ]

Yes, he said he connected the battery backwards.

 

[grasshopper]

That's great guys thank you. Time to order the component and replace it. I will update as soon as.

Cheers

grasshopper

 

[grasshopper]

Ok Guys, resistor now replaced.

On pressing start key I get a normal beep, screen back light starts up but screen is a hash of lines with some clear text in the background (suggests software starts to boot?).

After a very short time there is an audible "click" and the unit shuts off.

I do have a video of what happens but I am not sure if video can be uploaded to the forum?

Any advice or ideas appreciated.

 

[KrisBlueNZ]

I don't have any suggestions, sorry :-(