Fluke 8846A - Fuse instantly blows

[mdszy]

I have a Fluke 8846A that I'm repairing and the problem it's having is when I power it on, the fuse instantly blows. I've verified that there isn't any problem with the transformer since I used the transformer from another unit and it still blew the fuse. I'm kind of stuck on how to troubleshoot this, since I can't actually power the unit on to test voltages and such, and the power supply is part of the main circuit board, not its own separate unit.

Any pointers for where to start looking or suggestions for troubleshooting steps?

[GreyWoolfe]

A dim bulb tester should help with that.  Google Fu will give plenty of examples.

[mdszy]

A dim bulb tester should help with that.  Google Fu will give plenty of examples.

Oh nice, thanks. I was thinking using a variac could be useful, but I don't have one and don't particularly want to spend the money on one. But this looks like an awesome alternative that I'll try out tonight hopefully. Thank you!

[ArthurDent]

I don't have a 8846 but the EEVBlog has a video of a teardown  that might help. If you have another working unit you got the transformer from there are some other things to check. I'm assuming you had replaced the fuse with the correct value with the voltage selector set correctly and all the push-on connectors on the fuse/input assembly from the transformer primary are in their correct location.

There appear to be separate connectors to the circuit board from the transformer primary and secondary. If you unplug the larger connector from the secondary and plug the power cord in and turn the power switch on and it doesn't blow the fuse, the problem is on the circuit board. The most likely suspects are diodes and filter caps. Look for discoloration on the board and signs of leakage/bulging on the caps.

Test and replace any diodes that are shorted or caps that are leaky. Because it is blowing the fuse instantly I'd suspect a bad diode that is shorting out a secondary winding. A good hand multimeter should be able to identify a shorted diode in circuit so try this first before removing random parts. Caps often (but not always) fail open and would cause other problems and not blow a fuse. Generally problems like this are power supply related and replacement parts are readily available and cheap.

Here's a photo of the power supply area I copied from the teardown.

http://www.siliconvalleygarage.com/eevblog/8846teardown/supplies.JPG
   

[mdszy]

I don't have a 8846 but the EEVBlog has a video of a teardown  that might help. If you have another working unit you got the transformer from there are some other things to check. I'm assuming you had replaced the fuse with the correct value with the voltage selector set correctly and all the push-on connectors on the fuse/input assembly from the transformer primary are in their correct location.

There appear to be separate connectors to the circuit board from the transformer primary and secondary. If you unplug the larger connector from the secondary and plug the power cord in and turn the power switch on and it doesn't blow the fuse, the problem is on the circuit board. The most likely suspects are diodes and filter caps. Look for discoloration on the board and signs of leakage/bulging on the caps.

Test and replace any diodes that are shorted or caps that are leaky. Because it is blowing the fuse instantly I'd suspect a bad diode that is shorting out a secondary winding. A good hand multimeter should be able to identify a shorted diode in circuit so try this first before removing random parts. Caps often (but not always) fail open and would cause other problems and not blow a fuse. Generally problems like this are power supply related and replacement parts are readily available and cheap.

Here's a photo of the power supply area I copied from the teardown.

http://www.siliconvalleygarage.com/eevblog/8846teardown/supplies.JPG
   

Great! Thank you so much for the tips. I'll check all of this stuff out!