Hi all, I have a home audio amplifier that I am trying to fix and I was wondering what category I should post my thread. There is a bridge rectifier on the power supply board that has quite a large spark when I first power it on. The spark appears to jump from the positive lead to the negative lead. I've replaced the bridge rectifier and the 2 small caps next to it, but it still sparks. The amp still seems to work but I do want to fix the large spark.

 

It still works while sparking?!

We need more info such as model, photo of bridge area, your precise description of what you see, etc.

 

I definitely have more info, just wasn't sure where to post...

Model: AudioSource Amp300

It only emits a single large spark when I initially power it on, then it continues to operate normally. I can even power off and on again without spark. It will only spark again after it has remained off for a certain period of time (haven't tested to see what that period of time is).
I know that the spark is making the jump between the + and - terminals on the bridge rectifier because after I removed the old one, I could see burn marks on those 2 pins. I took a short video of the spark:

 

There isn't (shouldn't be, anyway) enough voltage to make that spark, so it is much more likely to be a duff contact that is sparking. That may be one of the bridge crimp terminals (but I doubt it). When the amp is switched on there will be a large current surge through the bridge charging up the capacitors. From the circuit diagram, this current switched by a relay and the sparking may be at the relay contacts.

Can you identify the relay?

(Schematics shamelessly stolen from another site)

 

Yes, the relay would be that small white component labeled RL01 on the board. But the spark is most definitely occurring underneath the bridge, confirmed by the burn marks on the legs:

 

Immediately to the right of the bridge there are two electrolytics. The board underneath them has some brown marks. is the board underneath the bridge a clean yellow colour or are there brown marks under that too?
Has any liquid managed to get in there?

 

Wow! That's quite a spark. Very odd. Here's that single spark frame from your video.

 

that discoloration under those caps is the reason I replaced them first. The original caps were sitting directly on the board and I thought that brown was electrolytic material leaking out the bottom of the caps and that the caps were shorting to that "staple" looking piece of metal between them. But when I removed the originals, they were fine, no leaking. I could then see that the brown is that glue-like substance they use to hold some components in place on the board, although it is definitely discolored from the sparking. Taking it frame by frame, you can see the spark originates under the left side of the bridge (positive terminal far left, ac terminal near left) but I can't tell exactly which pin it starts at and can't tell at all where it finishes:

 

Incredible!

Maybe a poor solder joint between the bridge and the PCB?

 

I've replaced the bridge since that video with no change in behavior. Both the original solder joint and the new are solid. I can try to take another video.

 

Is the PCB under the bridge a nice clean yellow colour?

 

its pretty clean looking. and now I know it has to be powered down more than 7 minutes to generate a new spark on power up. I took a new video and didn't reveal much - perhaps more evidence that it originates at the +

 

That heat sink is screwed on the bridge. Is the bolt too long, creating a smaller spark gap to some other potential?

 

No sir. the screw goes from the underside up into the heat sink.

 

I'd like to see the back side of the PCB in that area and the best pic you can get of the area under the bridge. Any chance you can observe the spark moment from below? Just wondering if there are any clues there.

 

 

 

That it only does it after it has been switched off for a little while suggests it is the current surge charging those big capacitors which causes the problem

I wondered whether it might be the relay contacts bouncing or poor contact interrupting that current and generating a large back emf in the inductance of the transformer secondary but I really don't think it can be that. Any large back emf would hit the relay contacts first and the tracks under the board as the gaps are smaller. It is a long way between the bridge leads.

I dunno.

 

Would replacing the large caps and/or the 2 voltage regulators (IC01 & IC02) that are discolored be worth doing?

 

Have you watched for the spark on the back side? I'm still wondering about a gap between a bridge leg and the board. If that's the problem, it may also be obvious from the back.

But I dunno either.