Author Topic: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap  (Read 3572 times)

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Offline plurn

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #50 on: April 07, 2019, 07:21:09 pm »
If it is anything like my Harman Kardon PM640VXi (HK6500 certainly looks similar externally and is about the same age), make sure you try cleaning out all the pots on the front panel with some electrical clean and lube (or just electrical clean) - volume, balance, tone etc. Behind the front panel there is a little hole in each pot for you to insert the spray tube. You spray a bit in and work the pots back and forth to loosen the grime, and spray a bit more to flush the grime out. Put a cloth around the pot to catch the liquid to stop it getting everywhere. Let it dry out before using it.

Over time my PM640VXi got quite noisy (hiss) and the noise got louder with volume setting. Volume and balance were very touchy with dead spots - had to fiddle with the controls to get the a setting that worked well until I eventually gave up. I thought it would need repairs and ended up buying another amp instead. Went back to it a year later when I got more adventurous with fixing electronics and cleaned the pots and it was like new. No noticeable noise any more, no dead spots. It was quite a surprise as I assumed it needed new caps etc.

I have heard this is a common problem with Harman Kardon amps of that era. They just need the pots cleaned from time to time.

Might not fix your problem, but it is very easy to try and can't hurt.
 
The following users thanked this post: ExtraThiccBoi

Offline ExtraThiccBoi

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #51 on: April 07, 2019, 08:19:03 pm »
If it is anything like my Harman Kardon PM640VXi (HK6500 certainly looks similar externally and is about the same age), make sure you try cleaning out all the pots on the front panel with some electrical clean and lube (or just electrical clean) - volume, balance, tone etc. Behind the front panel there is a little hole in each pot for you to insert the spray tube. You spray a bit in and work the pots back and forth to loosen the grime, and spray a bit more to flush the grime out. Put a cloth around the pot to catch the liquid to stop it getting everywhere. Let it dry out before using it.

Over time my PM640VXi got quite noisy (hiss) and the noise got louder with volume setting. Volume and balance were very touchy with dead spots - had to fiddle with the controls to get the a setting that worked well until I eventually gave up. I thought it would need repairs and ended up buying another amp instead. Went back to it a year later when I got more adventurous with fixing electronics and cleaned the pots and it was like new. No noticeable noise any more, no dead spots. It was quite a surprise as I assumed it needed new caps etc.

I have heard this is a common problem with Harman Kardon amps of that era. They just need the pots cleaned from time to time.

Might not fix your problem, but it is very easy to try and can't hurt.
Very interesting, glad to hear yours was a simple fix! I am afraid i can't clean the pots more than i already did. I sprayed more than a couple ml of car contact cleaner (very aggresive stuff, eats away resins and enamel from coils) into those pots and spent good half an hour turning them back and forth.
I will try it once more and see if it helps. Thanks for the comment!
 

Offline ExtraThiccBoi

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #52 on: April 07, 2019, 08:25:43 pm »
Adding a JFET or CMOS voltage follower to buffer the potentiometer is probably the easiest way.

Or the bipolar differential pair could be replaced with a JFET differential pair.
Could replacing the input pair with 2N5088 or 2N5089 help? Matching them as close as possible of course.
 

Offline plurn

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #53 on: April 08, 2019, 04:06:07 am »
ExtraThiccBoi wrote:
"... I sprayed more than a couple ml of car contact cleaner (very aggresive stuff, eats away resins and enamel from coils) into those pots and spent good half an hour turning them back and forth. ..."

I would never recommend using an aggressive cleaner like that for pots. That stuff sounds dangerous. I have typically heard you want cleaner specifically for cleaning electrical potentiometers that is safe for plastic as they often have plastic in them.

Anyway sounds like I guessed wrong since you had already cleaned them. Was hoping it would have been an easy fix for you.

Good luck and I hope you figure it out soon.
 

Offline Richard Crowley

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #54 on: April 08, 2019, 04:35:50 am »
Where did the myth come from that capacitors (or the replacement of capacitors) had any significant impact on noise floor?  Granted that we regularly see failure from dried-up electrolytic caps in vintage gear, and increased ESR and leakage in vintage paper capacitors (c.f. Mr. Carlson).  But when has that ever demonstrated to have any real impact on noise floor?  IME, the list of suspects for circuit noise (Johnson noise) has active components (firebottles, transistors, ICs.) at #1 on the list (with a bullet). followed (at a distant second place) by resistors, especially in high impedance circuits.  Do capacitors even make the Top-5 Most Most wanted on the noise suspect list?  Bad/cheap circuit design and/or component-sourcing seems way ahead of capacitors as another suspect cause.
 

Offline plurn

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #55 on: April 08, 2019, 06:19:25 am »
Where did the myth come from that capacitors (or the replacement of capacitors) had any significant impact on noise floor?  Granted that we regularly see failure from dried-up electrolytic caps in vintage gear, and increased ESR and leakage in vintage paper capacitors (c.f. Mr. Carlson).  But when has that ever demonstrated to have any real impact on noise floor?  IME, the list of suspects for circuit noise (Johnson noise) has active components (firebottles, transistors, ICs.) at #1 on the list (with a bullet). followed (at a distant second place) by resistors, especially in high impedance circuits.  Do capacitors even make the Top-5 Most Most wanted on the noise suspect list?  Bad/cheap circuit design and/or component-sourcing seems way ahead of capacitors as another suspect cause.

If it is a myth - you have taught me something so thank you for that. If we accept that it is a myth I would think that it has come from the following:

- electrolytic capacitors have become the number one thing to fail in relatively modern electronics (that is what I am lead to believe - could be another popular myth) so they tend to be the first thing blamed for any fault whether they are the cause or not. It is my go to reason for faults (until further troubleshooting) so I am guilty of that.

- bad capacitors can be a cause of mains hum in audio products (that is my understanding anyway - could be wrong) which is a type of noise - not a big stretch for people to also think it is a cause of other types of noise.

- many people without much electronics experience are aware of bad capacitors as a cause of issues, yet these same people may not be aware of other causes, so capacitors typically get the blame and this information spreads.

I would be in the category of having not much electronics experience so you can judge my comments accordingly.
 

Offline magic

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #56 on: April 08, 2019, 06:46:18 am »
Where did the myth come from that capacitors (or the replacement of capacitors) had any significant impact on noise floor?
It is a well known fact which some "audio objectivists" still refuse to acknowledge that modded equipment always sounds better than unmodded :P (unless you totally screw it up)
This explains the origin of virtually all audio voodoo and also why there is so much of it.
 

Offline David Hess

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Re: Vintage amp (HarmanKardon HK6500) mystery hiss - after full recap
« Reply #57 on: April 09, 2019, 04:54:45 am »
Adding a JFET or CMOS voltage follower to buffer the potentiometer is probably the easiest way.

Or the bipolar differential pair could be replaced with a JFET differential pair.

Could replacing the input pair with 2N5088 or 2N5089 help? Matching them as close as possible of course.

If the original transistors are damaged, it would help a lot assuming they meet the voltage, current, and power requirements.  The BC549C or BC550C are also high gain and what would typically be used.
 


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