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I have a Hafler P225 two channel power amp. This amp has a common power supply but two separate amp boards. The inputs, amps, and outputs are separate. when I play either amp the output will fade. I can watch the amplitude decrease on the scope. If I turn the volume control down the signal output will get louder. The problem gets goofy when I get the same results on either side. I have completely checked the PS. The PS voltages remain constant as this problem happens. I have used a dual trace scope to watch both input signal and output. The in stays constant but the out changes. Any body ever seen this???
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Both channels affected? Look for something they share. is there a mute circuit?Education is what you're left with after you have forgotten what you have learned. -
Hafler p225 power supply filter caps
The power supply is the common section to both amps. It consists of a transformer, a bridge rectifier and these two big momuus: SANGAMD Type DCM 10,000uF 75VDC with this stamped on it; DCM103U075BC2A. Then there is a number 658-8443-119. They are about the size of a coke can (little smaller). They are white with screw terminals.
I've looked around online and nobody seems to know about these. Like what the DCM stands for. I find several are available on ebay.
There is one for each DC rail. When I place a voltmeter from each to ground, the plus side starts at 64V and goes down in about 20 seconds to 47VDC. The negative side starts at 32VDC and goes up to the 47VDC. The meter from the plusterminal of one cap to the negative of the other reveals 122VDC that drops to 94VDC.
As I stated in my first post, the changes in the voltages change the volume of the output on each respective amp. Dual trace scope shows channel 1 stays constant in amplitude but channel 2 increases in amplitude. After the 20 seconds the volume equals out. On occasion the volume increases on channel 2, then there is a dull "pop" and the volume drops instantly to about zero but then recovers to normal.
Whats the DCM? Can I change these out with any other electrolytic cap? Any other input?Comment
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Sangamo makes various kinds of caps, the one you have is their type DCM. It isn;t an industry term or anything, just their model number, so to speak. SO it is a 10,000uf 75v screw terminal cap.
Nothing cosmic about them, I'd look in Digikey, Mouser, Allied, etc for one.
In your first post you mention PS voltages remain constant, then in your next post you mention PS voltages drifting up and down. I am confused. If the voltage across each cap is stable and there is no AC voltage there to speak of, then the caps are probably OK.
It really kinda sounds like you have a missing ground to part of your circuit, to me.Education is what you're left with after you have forgotten what you have learned.Comment
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floating ground
The first symptom I noticed was a change in audio envelope amplitude. Today I was measuring voltage on the filter caps trying to determine if they were leaking dc. After I wrote the last post I realized that the ground switch was on "floating" instead of "chassis" ground. It was late in the day. In the AM I will renew my efforts with a better attitude.Comment
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Well if it is that simple, great. But in my head i was thinking some element withing the circuit had lost its path to a ground reference. I was not worried about the earth ground. Hell, I hope I am wrong and your life gets simple.
But I get an impression not unlike a triode tube with its grid resistor removed. The grid can then charge up over a brief while.Education is what you're left with after you have forgotten what you have learned.Comment
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Does this amp have some DDT-like protection circuit? You know the setup with a clipping detector that drives a compressor in the input."Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"Comment
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