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Old 8th February 2009, 11:56 PM   #1
noctal is offline noctal  Portugal
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Default DH500 repair advice

My DH500 has one channel making loud popping sounds and I clueless regarding what to do. I chose Mechanical Eng after all.

Can anyone recommend a trustworthy shop/person who could do a general check up on my amp?

I do realize this is a DIY forum but I just dont have the heart to learn the electronics way. Electricity scares me and the sheer size of the 2 big caps inside turn me off.

Thanks in advance,
Noc
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Old 9th February 2009, 02:01 PM   #2
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Being in the USA, I'm a bit too far away.
You want someone local.

The schematics are online.

I'd ask you if you have a scope?
Or maybe an old analog voltmeter?

Please check the DC levels on both channels with the amp on, no music playing?
They should be <~0.1vdc maximum

The channel that pops... did it just start one day, or did you notice a woofer dishing in and out once in a while?

How often does it pop?

Is the pop a "tick" or more of a "tock" or lower frequency sound?
Or is it the woofer dishing in and out?

Does it sound like an electrical charge "arcing" across a gap, and then stopping for a while, happening once in a while?

Does it do it without anything plugged in? (if not, swap the input wires, maybe it is in ur preamp)

If you take the cover off, you'll see a relay that is in series with the output (speaker line). It is a good idea to use a very bright light and a magnifying glass to inspect the contacts... see if they are bright, or if they are blackened or pitted.
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Old 10th February 2009, 07:47 PM   #3
noctal is offline noctal  Portugal
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Thanks for the reply.

I dont have a scope but do have a digital multimeter.

The popping is loud (tock, not tick), random (but several times a minute) and occurs with only the amp connected, no preamp.

This condition has been present for a few months but would go away if the amp was left a few hours to "warm up", (perhaps humidity inside?).

Recently it refuses to stop popping and started making a constant loud rushing sound.

I will check the relay.

After checking posts on these posts I realize i have one of the early boards, the 10 and that one of the caps (i think its a cap) with a funky wing like heat sink, there are 3 or 4 per board, has leaked and is somewhat loose. Could that be it?

Thanks again,

Noc
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Old 10th February 2009, 08:07 PM   #4
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The metal, winged cans are the driver transistors. Q8, Q11, Q12, and Q13.

If you haven't already found it, the service manual is available at hafler.com
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Old 10th February 2009, 08:16 PM   #5
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Ummm... the black winged things are heatsinks probably.

The transistors have heatsinks.

Sounds like you have a bad (noisy) transistor.

You can find it by buying a can of "freeze spray" from your local electronics store or mail order. Wait for the thing to make the noise. One by one spray the transistors. Spray, wait 30 seconds. If the noise abates, reduces substantially, changes in character once the transistor gets cold, that is the bad transistor.

Watch out for overspray, it can fool you if you cool an adjacent transistor. But even so, when you hit the bad one, it will be rather obvious. Try them all for certainty.

So, turn it on, spray a transistor, wait, spray another,wait ... repeat until you find the bad one.

Once you find the bad one, you need to identify it on the schematic via the parts placement diagram that Hafler supplies in the manual - just download the manual.

Then get on here and someone will tell you what it is, and where to find one.

All that is left after that is lifting the PCB off the heatsink, desoldering and soldering in the replacement part.

Any local repair shop can handle that once you've done all the hard work.

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Old 11th February 2009, 01:08 AM   #6
djk is offline djk
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The problem is more likely to be one of the many plastic TO92 devices, less likely the metal ones with the winged heatsinks.

These amps are all over 20 years old now, I routinely replace all the small signal parts, they cost very little.
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Old 11th February 2009, 02:36 AM   #7
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I'd also suggest pulling the output mosfets out of their sockets, check for burned pins, and then re-seat them.
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