Kicker night of the vaporizing fuse

[Scanlines]

I got a kicker in few days ago, it's one of the smaller "ZR"s (slide-in module)
I can't find the model # on the unit. Must be on the plastic side-piece that's missing.

It keeps popping the little surface mount signal ground fuse on the under-side of the circuit board.

Last time I replaced it, the amp worked great on the bench until it was reinstalled in the car. Then, it popped again.
I just replaced the fuse again. It worked fine on the first test but when I hooked it up on the bench again, It popped once more.

I can believe that the customer may have a signal ground-DC ground offset problem, but I don't on my test bench.
(This amp's case becomes signal grounded when you bolt the board in so I could easily believe that the case may have touched
the car chassis. But not during my test)

There were no markings on the fuse, so I've been using a 1amp pico. Is that the correct size? do I have a problem elsewhere ?

Here's some pic's of the board.
http://members.aol.com/ta7205/rca.jpg
http://members.aol.com/ta7205/board.jpg

Appreciate any advice.

 

[Perry Babin]

It sounds like you have an intermittent short in the transformer.

On some of the amps like the ZR1000, the screw near the transformer was on the copper for the secondary ground. If that's the case with this amp, use a nylon screw to mount the board to the sink. Make sure the standoff is under the board so the board can't short to the mounting boss. This won't cause the fuse to blow unless there is a defective insulator but it will cause engine noise if the amp is mounted to the body of the vehicle.

 

[ppia600]

Is it possible you don't have a secure ground where its mounted in the car? Try putting a ground loop isolator (for a test) in between the rca inputs of the amp and see if it still comes on or still pops the "fuse".

That's all I could think of, besides voltage being on the input grounds like you both are referring to. I was assuming it hadn't been worked on before, oops.



(haha, yet again I type too slowly :lol: )

 

[Perry Babin]

The fuse was originally a littelfuse 430 series. The 468 series is the new lead free device. The original was marked TH (1 amp).


ppia600:

 

[Scanlines]

Perry and PPIA;

I took your advice and insulated the mounting screw by the toroid. But first, I replaced the fuse again, and tightened
that screw on the board right into the case without the insulator. Made sure the case was signal grounded with an ohm meter.

I fired the amp up again and tried to wiggle the toroid around. I also whacked on it with a small piece of wood several times.
The intermittently shorted transformers I've seen usually react to that but this one didn't.

If I don't see anymore problems, I'm going to send it out with the insulator and standoff in there. I will make sure that I'm present when the amp is reinstalled just to see.

Thanks, I'll let you know how I make out.

 

[Perry Babin]

With the secondary grounded to the sink, you wouldn't have seen any reaction without having the sink grounded.

If you have a signal source with an isolated ground, the problem may not have been obvious.

With the amp connected to the power supply (B+ and ground) and your scope grounded to the ground terminal of the amp, monitor the secondary center-tap voltage with the scope (there should be no voltage). Try twisting the transformer again to see if any DC shows up on the secondary center-tap. If it doesn't, the transformer may be OK.