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Old 21st February 2008, 03:33 PM   #1
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Default Hafler Powered Studio Monitor

Greetings, This is my first post.

I have a Hafler TRM8-1 powered studio monitor that has been acting up lately. It is a solid state bi-amped reference monitor speaker system. Here is a link to the manual which includes schematics.

Hafler Manual

The amplifier works fine on an intermittent basis. When I turn the unit on, I soon begin hearing hiss and random crackling from only the woofer. Sometimes it remains in this state for many minutes and other times the hiss and crackling slowly builds in volume until a pop occurs, at which point the unit returns to normal operation. This occurs even when the unit is isolated with no audio source plugged into the input.

I have noted that the woofer appears to be pushed outward and remains pushed forward until an audible pop happens (DC bias leaking?) at which point the hiss and cracking are gone and the woofer returns to its normal position. The speakers and amplifier still function in either state. This condition happens 2-4 times per hour and normally lasts for several minutes. I have begun leaving the unit off for fear that I may damage other components. I have a schematic which can also be found in the pdf manual that Hafler offers online.

To me this sounds like DC current is occasionally leaking through a filter capacitor or another component. I have opened the enclosure and examined the pc board. There is not any obvious damage or evidence of heat There was some dust that had acculmulated on the pc boards which I cleaned off. The next step I am considering is to open the unit up again, run the amp, and see if freeze spray on components will kill the noise when it occurs.

Am I on the right track here, or is there an obvious answer to this problem? Thanks for any assistance.
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Old 21st February 2008, 04:46 PM   #2
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I'd definitely check for bad solderings first.
Be careful, because what you describe is DC at the output.

Meanwhile, I moved this thread to Solid State and corrected the url.

/Hugo
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Old 3rd March 2008, 04:30 PM   #3
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Default Still No Luck

I appreciate the comment. I am still having no luck repairing this unit. Hafler will repair at standard rate which is very expensive. I will send it too them if I have too. But I am still thinking that this can be repaired at home. I recently talked to a local audio repair shop in Houston who recommended just sending it to Hafler as they do not do PC board repair on active monitor speaker. They pointed out that the attempt of repair often leads to blown speakers or other components making the repair not cost effective. Although this seems somewhat reasonable, it has occured to me that the speakers could be disconnected via the switching which is offered on the back of the unit for troubleshooting. Also they rightly pointed out that more than one component has likely failed.

They may be correct, but considering that the unit still operates correctly at least for the first minute of operation, and that it passes signal even when the problem occurs, it seems reasonable that a single component such as a filter capacitor is leaking DC current. Is it reasonable to carefully trace back with the scope to discover where the DC current is leaking? Or am I laboring under wishful thinking.
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Old 4th March 2008, 04:25 AM   #4
taj is offline taj
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Have you done as Hugo suggested and retouched all the solder pads on the woofer's PCB? It does sound like a bad solder connection on the circuit board.

And as he also alluded, you may want to connect up some junker speakers for testing rather than the monitor drivers. That DC holding the cone out of position -will- burn out your voice coils pretty quickly if it's left there for any amount of time.

There aren't many 'user serviceable parts' in there. If any of the op-amp chips are socketed, remove and re-seat them, and/or use some kind of good contact cleaner on them. An eraser will clean the IC pins okay, but won't clean their socket connections.

I hate all those little dip switches for that reason. Good chance of flaky connections in those, considering the dust you found. With the power disconnected, you could try flipping them back and forth a bunch of times to scrape their contacts. Of course, make sure they end up in the same position they started in. If that fixes your noise/DC problem, consider replacing all the dip switches.

The bias and offset adjust trimmers could be flaky too, but if you replace them, you must make sure the circuit's expected resistance is adjusted into the new ones before turning on the power. Or you are adept enough with amplifiers to set the bias and offset yourself. Incorrect adjustments there could release significant smoke at worst, or really screw up the amp's sound at best.

The amp isn't really old enough for the electrolytic caps to start dying, unless they were faulty to begin with. All the other caps are generally reliable (aside from their solder connections).

Don't touch the MOSFET transistors, they are static sensitive.

If that doesn't do it, send it to Hafler. Unless someone else here wants to walk you through some really in-depth circuit troubleshooting. (But somehow I doubt that will happen.)

Good luck.

..Todd
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Old 5th March 2008, 08:38 PM   #5
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Default Thanks

That was very helpful advice. Thanks. I did take a further look at the pc board. I had reconnected all connectors before I had even posted on this site. I did not consider the DIP switches being that the symptom of the speaker getting DC current seemed to point to another issue. There was what appeared to be some evidence of heat on one leg of a MOSFET. I did a static discharge and soldered that one leg in case there was a problem. But there was no change. To get to the underside of the PC board would require me to remove all the MOSFETs from the heat sinks. I assume there is a heat transfer compound and there is also a plastic looking sheet sandwiched in there that I assume could be damaged. I didn't want to risk damaging anything. So I buttoned the whole thing back up and I am shipping it to a Hafler repair facility today.
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Old 5th March 2008, 08:38 PM   #6
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Default grounding the culprit?

One thing I should mention that may have precipitated the problem. I recently set up my studio in a rented house built in the 1920's. Although the wall plugs had modern three prong plugs, I soon began to wonder if the ground was actually connected as I was having a variety of problems with my computer and studio gear. Sure enough, I checked and the ground prong was not even connected. I did a little wiring job myself and made the ground connection. But before I had fixed this the SCSI ultrawide drive in my mac failed. And soon after I had fixed it the ground, the problem with this active speaker manifested. So my question is, would this likely have created the problems I experienced, and if so, could it also point towards a particular component failing in my Hafler?
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