Tips for finding shorted component

[byoung]

One of my Dynaudio Bm6a monitors had a catastrophic issue and just fuzzed out (it's not the relay, I've already dealt with that a couple of times). It became really distorted and then stopped passing audio. When I first examined it, I was getting low resistance on one of the mosfets so I replaced them all along with their resistors and transistors as per the service manual recommendation. I put it back together and it is still blowing it's two fuses for the +45/-45v lines. The +15/-15v(preamp) lines still seem to be functioning fine, the AC transformer seems ok, its just the power amp blows it's fuses instantly when I turn it on. I noticed at the airpax switch heatsink/ground connection that I was getting continuity when I shouldn't be based on my functioning speaker, so I realized I had a short, I swapped the airpax switch as well thinking it may be the problem... but it wasn't. There is still a short in a component somewhere and I haven't  been able to locate it.  I suspect maybe a electrolytic cap or diode failure, possibly one that only shows up when power is applied? Any thoughts on how to find the bad component with a multimeter without being able to power it up?  There aren't too many parts in the +/-45 circuit so I'm hoping somehow I can find it.

 

[CJ]

you could hook up up to a variac and bring it up real slow,  then start sniffing around the board, feeling for heat with your fingers,  keep the voltage low enough so you don't blow the fuse or get shocked while feeling components,

it will probably  be something that is across both rails if both fuses are blowing,

 

[JohnRoberts]

Perhaps look for something telling it to pull up and down at the same time.. perhaps in the driver or pre-driver circuity.

Variac and light bulb in series with line cord to current limit can be useful when troubleshooting.

JR

 

[pucho812]

Whenever  faulting finding...

I always find it best to first do a  detailed visual inspection.  This is a good way to find resistors that have burned up, capacitors that may have blown(mushroomed), and so forth.

 

[12afael]

nothing work 100%.

- There is an ultra low impedance meter that should work but neehhh
- you can put a lab power supply and add some current. put some alcohol and see where it evaporate faster.
- add more current and burn the short if it is possible.
- start changing parts blindly. (try tantalums first)
- search for typical failures on internet.

some luck is useful too

 

[walter]

I have used freeze spray to find hot components, you need the freeze spray that frosts. I have also had good results with locating shorted smd caps by measuring  with the ohm meter. Most of the components on the shorted rail will read .3 ohms and the bad one will read .2 ohms or something like that. You could also remove components from the circuit and power on  to check if you have removed the offending part, or if there is a way to isolate half the circuit by removing a series component to help divide and conquer.

 

[pucho812]

the compressed air that freezes when sprayed upside down is great for finding cold solder joints.

 

[byoung]

Thanks for all the input. I have gone over the board front to back multiple times visually, nothing stands out. everything on the board is through hole, no smd, and all the solder joints seem really solid. I'm going to see if I can get a hold of a lab power supply... or does anybody know of one I can build? And when we are talking freeze spray, are we just talking about spraying compressed air upside down or is there an actual 'freeze spray'?

 

[JohnRoberts]

Thanks for all the input. I have gone over the board front to back multiple times visually, nothing stands out. everything on the board is through hole, no smd, and all the solder joints seem really solid. I'm going to see if I can get a hold of a lab power supply... or does anybody know of one I can build? And when we are talking freeze spray, are we just talking about spraying compressed air upside down or is there an actual 'freeze spray'?

Freeze spray is a real thing and handy for troubleshooting intermittent thermal issues. The spray cold will make it easier to see hot spots.

I'd check all transistor junctions with a VOM... some modern VOM even have diode scales.

JR