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Old 15th August 2007, 10:34 AM   #1
poko is offline poko  United States
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Unhappy Suddenly Loud Hum in Hafler DH-101

I have a Hafler DH-101 preamp that was working fine but suddenly developed a loud hum in both channels while it was in use.

The hum is constant in amplitude and not dependent upon any of the controls or switches -- it remains even with the volume control all the way down. I've checked all external cables and inputs, and have now determined that it is internal to the DH-101.

Are there any Hafler DH-101 experts out there who can point me towards a likely cause? I built this preamp myself from a kit over 30 years ago and applied the DH-106 mods back then by just following instructions, but I don't understand enough about the circuitry to troubleshoot this problem without some guidance.

TIA for any help someone can lend me.
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Old 15th August 2007, 11:30 AM   #2
Zen Mod is offline Zen Mod  Serbia
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Default Re: Suddenly Loud Hum in Hafler DH-101

Quote:
Originally posted by poko
I have a Hafler DH-101 preamp that was working fine but suddenly developed a loud hum in both channels while it was in use.

The hum is constant in amplitude and not dependent upon any of the controls or switches -- it remains even with the volume control all the way down. I've checked all external cables and inputs, and have now determined that it is internal to the DH-101.

Are there any Hafler DH-101 experts out there who can point me towards a likely cause? I built this preamp myself from a kit over 30 years ago and applied the DH-106 mods back then by just following instructions, but I don't understand enough about the circuitry to troubleshoot this problem without some guidance.

TIA for any help someone can lend me.

replace all electrolytics in power supply , check , and start from here changing all other (even if few bip biggies can be problematic to find new ones);
probably that's only culprit
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Old 15th August 2007, 11:34 AM   #3
poko is offline poko  United States
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I was looking at the circuit diagram and being that the hum is on both channels equally, I'm wondering if it could be in the power supply. Could a bad positive or negative voltage regulator cause this?
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Old 15th August 2007, 12:22 PM   #4
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by poko
I was looking at the circuit diagram and being that the hum is on both channels equally, I'm wondering if it could be in the power supply. Could a bad positive or negative voltage regulator cause this?
first thing first

change caps , then try further,if needed .

I saw 78xx and 79xx survived shorted cap
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Old 15th August 2007, 12:42 PM   #5
Nordic is offline Nordic  South Africa
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Confirm all ground tabs and contacts are good useing a multimeter... have had hum issue this month with corrosion forming...
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Old 16th August 2007, 09:04 AM   #6
poko is offline poko  United States
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Thanks Zen Mod and Nordic for your quick replies. I checked the ground contacts and seem to be fine. I have ordered replacement electrolytic caps and voltage regulators for the power supply, and should have them in a few days. I'll let you know if that does the trick. Thanks again!
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Old 16th August 2007, 09:21 AM   #7
Nordic is offline Nordic  South Africa
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It is probably those lytics...

Can problematic electrolytics cause an intermittant random scrathing/crackleing sound...

My amp makes this scratchy sound on one channel, which has completely been rebuild (trying to find a cure) appart from the PSU... Thought I had it all fixed... but after standing on for a few hours with no signal, the crackle came back... It is not terribly annoying, can't hear it above... Also it seems to take very long to reach this point... amp was on yesterday for probably 15 hours before it started again...

The quiet channel heatsink was just above ambient, while the noisy channel's sink felt cooler...
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Old 16th August 2007, 01:21 PM   #8
jblmar is offline jblmar  United States
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Don't overlook diodes in the power supply. Another source of hum.

Ron
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Old 22nd August 2007, 08:48 PM   #9
poko is offline poko  United States
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Well, I replaced all the electrolytic caps in the power supply, but that didn't fix the hum. So, I replaced both the pos and neg voltage regulators -- nope, still loud hum on both channels. The hum is constant and doesn't change regardless of any controls/switches.

I guess I'll try replacing the diodes as jblmar suggested. There's really nothing else left to replace on this power supply board, so I hope that's it!
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Old 25th August 2007, 05:25 PM   #10
poko is offline poko  United States
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Bad news... I replaced the 4 rectifier diodes but the hum remains. Since I've replaced all the caps, regulators and diodes on the power supply, the problem is elsewhere.

Anyone have any ideas where I should be looking now? Again, the hum is equal on both channels, remains constant regardless of the volume setting or input selector settings.

Thanks in advance for any help you can give me!
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