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Old 20th December 2009, 02:54 AM   #1
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Default Need help to diagnose a Hafler DH200 problem...

I upgraded a DH200 (about one week ago) with new caps on the circuit boards and also the two big power supply caps (upgraded to 18000mf). The amp worked fine for a few days and did sound improved.

However, today the amp developed a strange problem. About 2 seconds after I turn it off there is a strange whistling noise in both channels that decreases in pitch and volume over about 5 seconds, then everything is quiet. It sounds like a capacitor discharging but I have no idea how to isolate this.

I am wondering if it could be the small caps on the input connecters but it seems unlikely that both would fail at the same time. These were not replaced.

Has anyone encountered this problem before?

Thanks in advance for any suggestions you might have.
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Old 20th December 2009, 04:59 AM   #2
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Hi,
Did you check the polarity of the capacitors? Make sure they are installed with the polarity right ?
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Old 20th December 2009, 09:12 AM   #3
djk is offline djk
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Put a pair of 47F bypass caps on the 18,000F and see what happens.
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Old 20th December 2009, 04:36 PM   #4
eyoung is offline eyoung  United States
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Hola Y'all...

Recheck your solder joints on both boards, but if it's in both channels it's more likely in the P/S. I also bypassed my big caps with smaller caps and a 6800 Ohm 2w resistor parallel to my bypass cap.

Regards, Elwood
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Old 20th December 2009, 05:28 PM   #5
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Hi,
Also make sure the large capacitors are 75 volts or higher.
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Old 20th December 2009, 05:32 PM   #6
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When the amp on, there is some noise?
Which was the previous capacitance?
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Old 20th December 2009, 11:21 PM   #7
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Thanks for advice guys. I will work on this tomorrow night when I have a minute. To answers the questions asked in support of a diagnosis.

- There is mechanical noise which I believe is coming from the transformer. I have a replacement part arrive tomorrow to swap in.

- The new power caps are rated 75volts (18,000mfd). The original caps were 10,000mfd 75V.

- The polarity looks fine to me. The are electrolytic so I assume they would have blow up right away.

My current thinking is that it is the small cap that is solders on top of the rectofier. This could effect both channels if it is not working properly.

Thanks again for all the ideas. I will report back on Tuesday.

-
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