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Old 13th April 2013, 02:56 PM   #1
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Default Replacing caps in my hk770

Hi.I have just bought a Harman kardon hk770 daul mono stereo amp.I paid $20.00 and i seems to work ok.Hard to complain for the price but i noticed some poping sounds coming through my speakers after playing it for awhile.When i opened it up it looked the the caps have leaked as they are stained on the bottom.I also notice one side is hotter than the other.Can anyone tell buy the pictures if they should be replaced.I am not sure if they are original but if they are...they would be at least 30 years old.It still has plenty of power.8x 4700uf(m)50v 4x2200uf(m)63v elan.Are these caps hard to find and are they expensive.Thanks Sorry for all the questions.Total Newbie
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Old 13th April 2013, 03:23 PM   #2
PMI is offline PMI  United States
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Quote:
Originally Posted by whateverittakes View Post
...it looked the the caps have leaked as they are stained on the bottom....
Make sure that you are not just looking at some glue that was holding the base of the caps for mechanical stability. Some glue used that long ago started out clear, then turns brown-ish after a long time in use. Regardless that, replacing them is a good idea b/c they are so old.

The caps are standard value, and probably standard size (can't tell size from the pics). The caps will cost $2-4 each, depending on manufacturer and where you buy them. Try to find the same lead spacing and diameter, if possible.

Digikey, newark, mouser, have search engines that work very well to locate the correct value and size of most capacitors.
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Old 13th April 2013, 04:11 PM   #3
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That's a lovely looking amp. $20 very well spent. Try getting the two heatsinks for that money!

If one side is hotter than the other, it may be worth checking that the rail voltages are the same on each side. This could be the caps but I would have thought they'd have had to degraded an awful lot. Also, you'd probably find one of them getting hot differently from the rest.

But changing them is definitely worthwhile. I would try to find the same ones as you have there and resist the temptation to 'improve' things. If you can't, then try and find the original specs (and incidentally it will be worthwhile getting the original schematic - I suggest this because I'm slightly surprised the two sets are different voltages, though it could have a regulated supply) and get a not too expensive replacement with something near the same ESR. (You'll probably find Elna do something similar still, though the price will have gone down.)
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Old 13th April 2013, 04:24 PM   #4
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Thanks for the reply's.I think i might take this in to a tech...as i have really no electrical skills.I just didn't want to put more money than what its worth into it.
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Old 13th April 2013, 04:29 PM   #5
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Originally Posted by ChristianThomas View Post
If one side is hotter than the other, it may be worth checking that the rail voltages are the same on each side. This could be the caps but I would have thought they'd have had to degraded an awful lot. Also, you'd probably find one of them getting hot differently from the rest.
Bias, or offset out of spec? ... regardless, very nice purchase for $20, but the OP has his work cut out, this would not have been my pick to work on for someone totally new. Nevertheless, everything is in the manual, assembly, schematics, BOM:

http://manuals.harman.com/hk/Service...hk770%20sm.pdf

I would do a search, there are other threads that mention this amp here.
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Old 24th April 2013, 11:14 PM   #6
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Well i got this back from the tech yesterday..With a clean bill of health.He adjusted the idle current and balanced current.No parts needed he said.When i got home and hooked it up it sounded good for about 2 songs in.Then the poping started though not as much then the left channel started heating up.I then shut it down and realized i had been using my cheap patch cords...so after i swapped them and hooked it back up with a decent set of interconnects the amp was dead quite.The amp was warm (not hot) and was evenly dispersed left and right side.I think i will use this for awhile like it is then maybe have the caps done latter.I would have never thought the patch cords would have been the culprit.Thanks

Last edited by whateverittakes; 24th April 2013 at 11:37 PM.
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Old 25th April 2013, 03:12 AM   #7
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Oxide layer on old plated surfaces?
Give the sockets a clean and polish too, to head off future annoyances with poor contacts. I imagine they are nickel plated surfaces which are porous and somewhat prone to harbouring contamination. Otherwise, replace with another plating type.
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Old 25th April 2013, 11:19 AM   #8
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Thanks.Any suggestions on the product to use for cleaning and or polishing?
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Old 26th April 2013, 09:51 AM   #9
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Use any paste type metal polish suited to hard metals like chrome. Auto chrome polish (not so popular now that car trim chrome is plastic) would be fine but brass, copper polishes will still work - you need to really clean the metal more than polish but ideally, avoid wetting the metal with water based (liquid) compounds to reduce the chance of more oxidation. Polishing is just the easiest way to thoroughly clean the metal. No good reason to do this, otherwise

There really are commercial cleaning products for pro. use in studios, where they are warranted, due to hundreds of jacks and expensive downtime potential. I don't know a lot about their composition but obviously there is always some fix in a bottle for anything you care to name.
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