Powered monitor tweeter death

PolarityMan

So I was tracking some bass yesterday which involved me taking my studio monitors (the mixing kind not the put your foot on while solo'ing kind) and at one point one of the monitors made a really weird noise and the tweeter went mental (the dust cover extruding really really far out).

I thought it might have been a signal issue so I turned the monitor off, unplugged the speaker cable and then turned it back on, same thing happened again but this time only for a split second and then the tweeter went completely dead. The main driver works absolutely fine.

So my question is, given that these are powered monitors is it likely that the tweeter might have been blown by a fault in the amplifier of crossover etc meaning that if I replace the tweeter the same thing will happen again?

Also any thoughts on how plausible it is to replace the tweeter and is it cost effective, these are KRK rokit 5's for reference.

I know amps prob isnt the right section for this but maybe @ICBM or @ecc83 has some knowledge in this area?

ICBM

Yes, it's an amplifier fault. The tweeter dome coming really far out even with no signal input means it's a DC fault in the power section that drives the tweeter - the speaker will be biamped (many modern ones are), which is why it only affected the tweeter. A passive crossover would have blocked it, anyway.

In which case, do not replace the tweeter - it will just blow again the next time the amp does the same thing, which it almost certainly will even if the fault isn't now permanent.

PolarityMan

so with that in mind is it  a deader given that ebay seems to have second hand replacements around £60 for the whole speaker?

ICBM

PolarityMan said:

so with that in mind is it  a deader given that ebay seems to have second hand replacements around £60 for the whole speaker?

I'm not familiar with this particular model, but my experience with other powered monitors is that the labour cost just to get at it and then put it back together could be as much as that, or more - even if it's a simple single-component failure (which it may not be). Add the cost of the IC and the tweeter and it's almost certainly uneconomical.

Someone else who has actually seen inside one of these could give a more accurate assessment though.

PolarityMan

Cheers for the advice, I'll hold out to see if anyone else comments but sounds like it might be best to just grab another one on ebay. I bought them second hand a few years back anyway at a pretty good price so I cant really complain too much.

Is there anything I should watch for on the second one in case it has a similar fault, especially if it may have been caused by either moving the speaker (last few months its been in and out of car about 8 times although alwyas fitted so speakers were protected and unable to move freely) or through particularly hard use by the previous owner?

ICBM

I don't know. Sometimes physical shock can crack solder joints between the power IC or transistors and the PCB, if the heatsink moves - but not very likely, they're usually well-secured, which is why they're often very hard to take apart. Overdriving the speaker could stress the tweeter amp in particular, but they're usually pretty well-protected.

Apart from that it just seems to be pot luck really. Some designs are a bit more robust than others both physically and electrically.

DJH83004

I think IC is bang on the money, I have not looked at the 5, but one of its bigger brothers from local studio and on that one there were definitely separate TDA..... driver chips for the bass and tweeter. So you will probably have to replace the output chip and the tweeter, plus labour equals 'beyond economic repair' I'm afraid.   

ecc83

Really surprised they did not put a 4.7mfd 'ish foil cap in the tweeter circuit (a non Pol' electro would do for me but you know how the 'tweaks' are!) .

I know top line stuff HAS to be all DC path but then they have servo amps to keep a zero output offset and very sophisticated protection circuits. You never read of Focal or PMC monitors going wrong.

VERY bad luck chap.

Dave.

PolarityMan

I have some more vocals and stuff to track and edit which is doable in mono so I think  if the remaining one holds out until thats completed ill pick up another matching one, if it dies too then ill prob go for something different as some googling seems to show quite a few issues with blown tweeters on these.

Danny1969

I haven't repaired the Rokit 5's but I have repaired the larger Rocket 8's. Not massively impressed with the design, the amps are difficult to work through and the transformer has an internal fuse which warrants buying a new one when the secondary goes OC 
The output I.C is meant to be protected against going D.C

crunchman

Aren't the Rokit 5s the ones with the hideous yellow cones.  I'd just take it as a sign that you should get something else that's less of an eyesore.

PolarityMan

I quite like the yellow cones

ICBM

Danny1969 said:

The output I.C is meant to be protected against going D.C

Along with being thermally, short circuit, over-voltage and safe operating area protected - power ICs never ever fail, ever!

There must be some other explanation for all the countless blown ones we've all changed... aliens or something probably.

ecc83

ICBM said:

Danny1969 said:

The output I.C is meant to be protected against going D.C

Along with being thermally, short circuit, over-voltage and safe operating area protected - power ICs never ever fail, ever!

There must be some other explanation for all the countless blown ones we've all changed... aliens or something probably.

Ha! Same goes for 78 series regulators. VERY reliable but not totally bombproof.

To be honest tho' IC, it is probably designers (I use the term loosely!) who use a device AT its limit to reduce cost and/or do not take into account top mains voltage possibilities?

Dave.

Sporky

I would just like to say that "Powered Monitor Tweeter Death" is a fairly decent band name.

crunchman

Sporky said:

I would just like to say that "Powered Monitor Tweeter Death" is a fairly decent band name.

You could abbreviate that to PMT death.