Hafler DH-120 thumping & cracking when power on - diyAudio
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Old 27th February 2006, 10:51 AM   #1
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Unhappy Hafler DH-120 thumping & cracking when power on

Hi all,

I have recently purchase two second hand Hafler DH-120 using as mono block. One of them is starting to give me trouble. Sometimes it makes a big thumping and cracking noises when I first turn it on. I am using a classA Parasound pre-amp, but don't think that is the problem as I disconnect it before I turn the Amp on and the problem is still there, even with the speakers swapped around. I suspect it could be the power switch. I tried to open the cover today but don't have the right socket for it (it's imperial rather than metric!). Other than that, it is the best sound system I have ever heard with a pair of Ohm speakers.
Could anyone give me some suggestion please.

Thanks alot
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Old 27th February 2006, 05:17 PM   #2
anatech is offline anatech  Canada
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Hi smoken_hot,
Try replacing the electrolytics on the driver PCB's. Most are defective I have found. Check the series resistors to the supply rails for the filters. It also helps to match Q1 and Q2 for gain.

These are good amps. The outputs are seldom defective.

-Chris

Edit: spelling
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Old 27th February 2006, 05:50 PM   #3
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Use a 1/4" socket (nut runner). Remove the top 4 screws then also loosen the screws to both heat sinks. This will allow you to lift the lid.

Put your amp in Stereo mode. Attach speakers to each channel.

Then, tell us if the faulty sound comes from both channels equally? If so the problem is probably in the power supply. If not, you will know which channel has a problem.

The sounds you describe point to a leaky capacitor, an old electrolytic capacitor. The electrolytics in your amp are now around 20 years old, past their normal lifetime.

I, too, like the sound of the DH-120 and am using 3 of them in my Home Theaters. Great sound for a small amp. The Hafler 9130, Pro 1200 and SE-120 all use the same circuit design. Good luck on repair
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Old 28th February 2006, 09:13 AM   #4
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Thanks very much gents for your advice. I will try to get hold of 1/4'' socket sometimes this weekend and pull it apart. I am getting a bit rusty with electronics in these days. I was an electonic technician many years ago (agriculture electronics) so no experience at all in power amp. I might need to get my books out again!
I will take your advise and let you know.

Thanks again
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Old 5th March 2006, 06:26 AM   #5
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Hi gents,

Finally I got time to buy 1/4'' socket! (Thanks, Dick West)
I put it on stereo and connected a speaker to the left channel, then the right channel and both are still cracking (no thumping this time).

I had a quick look at the driver PCB, but I am not familiar with the circuit. I don't know which/where electrolytics and the series resistors to look for.

What is the next step?

Thanks again.

PS. When I opened the cover, I was a bit "shocked" that there is no earth connected to the frame. Is this normal to stop earth loop?
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Old 5th March 2006, 05:17 PM   #6
anatech is offline anatech  Canada
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Hi smoken_hot,
See all the electrolytics on the river boards? Change them.

If you can match transistors for DC beta, now is the time to do that. Also have a look for bad solders.

-Chris
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Old 9th March 2006, 11:28 AM   #7
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Hi Chris,

I will try to get hold of those electrolytics and let you know the result.

Thanks again for your advice
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Old 9th March 2006, 11:01 PM   #8
anatech is offline anatech  Canada
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Did I actually type river boards ???

I meant driver boards! Your figured it out though. By all means, let us know how the new caps fix things up.

-Chris
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Old 10th March 2006, 02:51 PM   #9
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Hi Chris,

I couldn't find any "river" in there, I thought it was a spelling mistake anyway

We have got a long weekend in Australia (labour day on Monday), I should have time to fix it.

Thanks
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Old 10th March 2006, 04:47 PM   #10
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Smoken,

If your problem sounds come equally from both channels then I first would look at the power supply -- as it is supposed to supply both channels equally.

Do you have a spare pair of electrolytics with screw tops that you could use to replace -- temporarily -- the ones in your amp? Perhaps the noise problem comes from the large filter caps and replacing them will solve your problem.

I presently have 4 DH-120 amps here, they run my home theater. None of them have any electrolytics replaced -- as yet (fingers crossed).

If the bad sounds were coming from just one channel I would take a can of cool spray and a hair dryer to it and cool then heat various components to find which one seems to have an effect on the bad sounds. This process will help isolate a faulty transistor.

Also, did you shoot some lube spray into the level control pots on the rear? Ditto for the stereo/mono switch. I once "fixed" a DH-120 that had symptoms similar to yours by just lubricating and working back and forth a few times the stereo/mono switch.

Keep us informed on what you find there as we all learn from experience, sometimes even other's experiences.
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