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Old 28th February 2012, 10:31 PM   #1
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Location: Wheaton, Il
Default Hafler DH-200 problems...

Before starting with my latest problem, I'd like to thank a lot of folks on this forum. I am one of those folks who re-capped, and replaced my transistors to get near zero DC offset, etc. without having to post a single question - everything I needed to know was already here!!


Now on to my newest question...

First, I need to admit - I have little/no formal electronics training and all I have learned I have learned from decades of tinkering. I am definitely still a novice, so frequently rely upon the web and forums to help me.

After replacing all the electrolytics on the driver boards + transistors Q1-6, everything was great!! Amp was quiet, and DC offset was near zero.

However recently, I've noticed a very quiet squeal + hiss + hum coming from the speakers. Keeping it in perspective, you can barely hear it unless you get pretty close to the speakers, but definitely audible for critical listening.

Using my DMM (not the one in the picture), I measure 0-1mv of DC and 1.8-2.5mv of AC. In addition here is a picture of how it looks on my scope. The lower trace (flat @ 20 mv/div) is the signal at the input to the driver board, and the upper trace is at the speakers (a mess @ 2mv/div). Horizontal is at 2ms/div.

The noise always there, but does drift up and down in amplitude a very small amount.

The right and left channel exhibit the same signal (on scope) but the right channel is a little higher amplitude (+~50%).

R39 and R45 which can go bad and cause hum are good (if you're a DH-200 person, you know about them and most will tell you to check them first)

Finally, another observation. If I bring my hand close to C1, which is very close to the line level inputs into the driver board, the signal on the scope goes up dramatically. I can put my hand anywhere else around the circuitry with little effect.

I have reflowed a lot of solder, but if this sounds like a cold solder I will do it again.

Ok, those are the clues I've gathered. What's next?


Schematic is in this http://www.hafler.com/techsupport/pd...00_amp_man.pdf

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Old 28th February 2012, 11:08 PM   #2
jaycee is offline jaycee  United Kingdom
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Does it go away if you short the input to ground?
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Old 29th February 2012, 01:02 PM   #3
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Try shorting the scopes grd to the amp chassis grd. Shorting the inputs is very important if your looking noise or hum...
The stuff you don't know always humbles us.........
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Old 29th February 2012, 01:31 PM   #4
eyoung is offline eyoung  United States
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Location: Sunny SC,USA 15 min south of Charlotte NC
cold solder joints are common on haflers , from your fuzzy picture looks like bias is too low crossover distortion at your zero point. that being said the bias generator trannies frequently fail, if I recall a common 2n2222 or mps2222a, parts list specs a NP2222.

regards, E
"Lead me not into temptation...I can find it myself."
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Old 29th February 2012, 03:07 PM   #5
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Finally, another observation. If I bring my hand close to C1, which is very close to the line level inputs into the driver board, the signal on the scope goes up dramatically. I can put my hand anywhere else around the circuitry with little effect.
Your body works as an aerial receiving all the dirty electromagnetics fields laying around.

You induce them into input C1 when aproaching your finger.
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