Hello,

I set out to repair a subwoofer that would not turn on because I read online that a common problem was a zener diode overheating. I read a few tutorials online about switching out the diode and I took on the project with no background in electronics aside from a Physics II course I took in college. I am now a proud owner of a Gardner-Bender GDT-311 multimeter and a 25 watt soldering iron. I located the diode and tested it with my multimeter and determined it was bad so I replaced it in the circuit. I tested the diode before soldering with the ohmmeter setting and it read as expected. After soldering I tested again with my multimeter and it read a changing value (number increasing positively) and then switched the leads and it read a changing negative number (number increasing positively). Confused I desoldered it and checked the component again and it was still ok. Can any one explain to me why this may have happened?
I have also read that a resistor can open up if the diode is fried but I checked it and it was reading close to the intended resistance although outside of the tolerance (10 ohm with 5%, read at 11.5 ohm).
Please correct me if I have used incorrect language, I am just beginning but I am very interested in continuing with this as a hobby so I would like to learn as much as possible.

http://www.fixya.com/support/t13733461-mcs_160_subwoofer_no_power

Cheers!

 

I tested the diode before soldering with the ohmmeter setting and it read as expected. After soldering I tested again with my multimeter and it read a changing value (number increasing positively) and then switched the leads and it read a changing negative number (number increasing positively).

Measuring components in-circuit can be confounded by the connected components. It's as simple as that. In your case there may be a capacitor charging through that diode, or something like that. If it's OK off the board, and you don't roast it with your iron, I wouldn't worry about goofy results you see while it's on the board.

Is there any doubt about which way to place the part? If you have any doubt at all, just post a photo here of your board and diode.

 

Thanks wayneh! Unfortunately, I will have to look for another potential source of error now as that did not fix the problem. After reassembling the board in the subwoofer I plugged it in and the fuse blew. I believe I have the diode in as I was careful to place it the same way as the previous diode as it seems to match the orientation of the picture printed on the board.
Do you know what are the most likely components to contribute to a blown fuse?

 

look at the components that are fixed to a heatsink. Test them for a short.

 

Do you know what are the most likely components to contribute to a blown fuse?

For years about the only thing I could repair on a board was a bad bridge rectifier or an obviously blown component. I knew how to identify rectifiers in a power supply, they were easy to find in a local store, and they are often the cause of blown fuses. Blown parts that could still be identified were easy enough also.

More recently I've learned that electrolytic capacitors are subject to aging, and have much shorter lives than just about anything else on the board. They can go bad (by ESR) with no outward indication and can even still have rated capacitance. A bad cap can definitely cause a fuse to blow.