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Filter Caps Going Bad?
Filter Caps Going Bad?
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Old 9th January 2007, 04:50 PM   #1
DreadPirate is offline DreadPirate  United States
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Default Filter Caps Going Bad?

I have a Harman Kardon PM665 Integrated (dual powered) that I've cleaned up nicely with deoxit a few times. Got it used ($7). I cannot get rid of an intermittent static in the left channel. It does sound just like dirty volume controls (I'm very familiar with this issue), but occurs constantly regardless of button and knob use.

Are the filter capacitors the likely culprit? Is this a symptom of filter caps going bad?

Is there a way to quickly check if the filter cap is bad? Can I quick solder a capacitor in parallel of somewhat close uF (>voltage of course)? I will check with a voltmeter prior to messing with these, I understand they like to bite.

I do have an esr meter, but currently do not have a capacitance meter. Will this meter reveal the problem without disassembly?
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Old 10th January 2007, 05:33 AM   #2
unclejed613 is offline unclejed613  United States
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noise, like intermittant hissing or crackling is more likely caused by bad solder or a transistor junction breaking down (or sometimes a bad zener diode)....... i have seen some electrolytics go bad from time to time where they have an internal intermittant connection. the ones i have seen this happen to most often are Sprague caps with red epoxy on one end (if it's a radial cap, it's the end with the wires coming out of it). every single one i've seen fail like this, had either the red epoxy blackened around the positive lead (from the connection burning inside the epoxy) or the positive lead burnt to the extent that it actually fell out while removing the cap from the board. you could also have a dirty speaker relay (if the amp uses a speaker relay). try tapping on various components with a screwdriver handle and see if you can cause the static (or make it go away) (it's also possible to find bad solder this way). a bad transistor or zener will not usually respond to physical shock, but gets noisy (or quieter) with temperature only.

back to the Sprague caps, i once repaired an HP voltage standard that had 10 of those caps in parallel in the power supply, 8 of them had gone bad. i found the first one when i happened to bump it while trying to attach a scope probe. after replacing it i noticed that the problem had improved but not gone away, so i began removing the other 9, and found 7 of them bad. i replaced all 10 of them with nichicons, instead of taking the chance of replacing them with the same type and having the same failure happen later. since then i have seen many different types of equipment, audio, banking, radio transmitters, etc... with similar caps with exactly the same failure with that type of cap, so now if i see one, good or bad, i replace it.
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Old 10th January 2007, 12:34 PM   #3
DreadPirate is offline DreadPirate  United States
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Great reply! I need to get this unit going, seems the best one in the heap of vintage I own to power my recent purchase of Thiel c3.5s. This thing is "twin powered" with two massive transformers. Its calling card is an HICC of 60A, hoping that will do the trick until I get something better.

The crackling is coming through the headphone jack, I have not put speakers on it yet. It does NOT seem to be affected by the operation of any knob or button. Several posts here and there indicate these are problematic and I have found the rotary knobs on other older HKs to be noisy and get noisy again after 6 mos or so after cleaning. What I would really like is a lead on the pots for my HK730, they all have a loose feel to them since cleaning and are beginning to get crackly again.

Back to the PM665, I think you are on the right track with the cold solder or defective component call, I will tap around when I open her up this weekend. I have since looked into the failing leaky filter caps, seems these result in a hum, not crackling, when they are failing due to age.

Thanks again.
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Old 10th January 2007, 03:55 PM   #4
djk is offline djk
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Noise on a 730 volume control can be DC leakage from the coupling caps.
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