Numark Dimension 4 Amp - Common Fault?

[djwiggy50]

Hi all,

Just picking your brains. Recently started having problems with a Numark Dimension 4 amplifer - mains fault.

It was tripping the fuse in the lead every time it was switched on.

On opening up and boards checked..... showed significant damage to one side of the output amp side PCB (massive burn out - not very pretty). But the other side appears to be fine. Tried powering up amp with the faulty output board disconnected from the power PCB - powers up fine & no apparent problems on the remaining ''good side''

Just wondering if anyone's had a similar problem? What was the outcome? Was the fault located to the burnt out board? or did it originate earlier down the line (power caps)?

Would be good to get up and running again.

Many thanks

[djeddie]

The fault is, (at least I'm 99% sure without seeing inside it), a shorted channel. In other words the output devices have blown. Whether it's a common fault I couldn't say but seeing the 2nd hand prices on eBay it might not be cost effective to repair.

[djwiggy50]

I'll be honest, that's what I'm expecting the fault to be, too just a short on that output board.

I suppose curiosity makes you wonder why it shorted out.....

[DJ Jules]

I'll be honest, that's what I'm expecting the fault to be, too just a short on that output board.

I suppose curiosity makes you wonder why it shorted out.....

That'd be a common fault on cheaper amps - the output devices fail under load and either short circuit completely, or connect the base of the transistor to the source or sink and fry the components upstream of the final output stage.

Either that or someone has bridged a 4ohm load across both channels.

Julian

[djwiggy50]

Well, replacement output board on its way from numark. I'll let you know how I get on!

[Daryll]

That'd be a common fault on cheaper amps - the output devices fail under load and either short circuit completely, or connect the base of the transistor to the source or sink and fry the components upstream of the final output stage.


Julian

Agree with that . My old berry 2500 had a channel die . the output transistor had shorted out , that takes a host of other components with it , luckily the transistor protected the fuse for a while......, the amp was scrapped , but after 6 years of trouble free service , I still think its a good amp.

Daryll

[djwiggy50]

I've just got a spidey sense tingling that it might have orginated from the power supply board to the output board.

Could a fault on the main capacitors/voltage regulator be linked to this?

[DJ Jules]

Could a fault on the main capacitors/voltage regulator be linked to this?

Yes, but it's difficult to know if it's the symptom or the cause. I would have thought that the amp would run both channels from the same power rails (otherwise Bridging wouldn't work) so if the power supply was faulty it would have fried both sides of the amp. I'm not 100% familiar with the amp, but looking at the weight I'd think it's using a large round transformer to power it and not a switch mode power supply, so I also wouldn't expect there to be a voltage regulator between the transformer and the amp modules, I'd expect the power just to be rectified and smoothed by a couple of power capacitors. If the capacitors failed they'd either go open circuit and you'd get a VERY noisy output or closed circuit - but that would have resulted in blue smoke and blown fuses in the power lines, not wildly unregulated power.

Trace the wiring to see if both sides are running from the same power rails. If they are, then you know the power supply is ok as the other side of the amp is still working. If the power rails are separate, disconnect the amp modules, fire it up, and measure the output voltages on both sides using a multimeter - if they're similar on both sides then it's probably fine*.

Julian

* Disclaimer - if you're not sure about what you're doing, don't do it. Amplifiers contain high voltages and high currents which can be lethal. The author accepts no responsibility for any injury caused by following my advice

[ckpr2]

I suppose curiosity makes you wonder why it shorted out.....

A short on the output.
A large glitch on the mains.
Running the amp too hard for too long.
Poor design using cheap components and possibly under rated components.