Harman Kardon PM655VXI sound problem

[coto37]

Hello,

I am new to this forum, looks very helpfull and lots of interesting stuff is posted here so I wanted to take a go and see if you could help me out.

I bought this amp around 12 years ago on ebay and it has never given me a problem untill now. I have a sound problem at low volume my right speaker distorts. But only does as soon as it starts heating up a little, does not have to be on for a while, maybe a min or two. The more I turn the volume up the less it distorts, untill its fairly loud and I can not hear it.

I have tryed changing speakers round, still does it on the right. I have tryed changing cables, input and output ones.
So I decided to open the amp up and clean it up. I reed that dust is bad for sound, so I blowed all the dust out from top, bottom, and front panel.
After trying this, i found no difference, still distortion.
So I decided to adjust the sound pots, I moved them around untill I it stopped making a scraching sound and set them where they were.
I noticed while I was adjusting the sound pots, my righ woofer did move alot more than the left one, the left one did not practicly move at all.
Also noticed that the left channel heat sink side heats up faster than the right chanel side.

So for now no luck, the amp is from 1985 (if im correct). Read about people recaping, even if there is no visible leaks, becouse of age just in case, I can see some marks under a couple of capacitors but I am not sure if they are leaking. Maybe a transistor? I have no clue, just guessing...

Thanks,
Isaac.

PS: Sorry about my spelling, I am Spanish.

Here are some pics of the inside:

 

[Raynald]

Very nice amp, I was selling them in the 80's! That series is notorious for the little square selector buttons causing problems. They are hard to clean properly apparently but I would start with them as it is known issue. I can't guarantee that is your problem but it is worth a try and while you are at it clean all of the rotary controls if you have not done so already.

 

[onplane]

Many HK units have pre-outs and main-ins.

If your does, remove the plugs and replace with a standard audio cable except reverse the left and right.

Reason for doing this is to see whether the problem moves. If it does, your problem is in the right power amp. If it doesn't move, the problem is in the right pre amp.

Jerry

 

[coto37]

Very nice amp, I was selling them in the 80's! That series is notorious for the little square selector buttons causing problems. They are hard to clean properly apparently but I would start with them as it is known issue. I can't guarantee that is your problem but it is worth a try and while you are at it clean all of the rotary controls if you have not done so already.

Hello Raynald,

Thanks for your response. I knew about those buttons. Once a week or so I press them all a few times, I think it cleans them out. I also move all the knobs. Do you think that is enough or would you try also cleaning with contact cleaner?

Thanks,
Isaac

 

[coto37]

Many HK units have pre-outs and main-ins.

If your does, remove the plugs and replace with a standard audio cable except reverse the left and right.

Reason for doing this is to see whether the problem moves. If it does, your problem is in the right power amp. If it doesn't move, the problem is in the right pre amp.

Jerry

Hello Jerry,

Thanks for your response. I will try that. And let you know.


Thanks,
Isaac.

 

[Raynald]

Hello Raynald,

Thanks for your response. I knew about those buttons. Once a week or so I press them all a few times, I think it cleans them out. I also move all the knobs. Do you think that is enough or would you try also cleaning with contact cleaner?

Thanks,
Isaac

On a unit of that age I would clean everything with a good contact cleaner.

 

[coto37]

That type of problem sounds like corroded contacts on the speaker protection relay often they are of a plug in type clean the contacts also the switch contacts in side its case the contacts tend to get pitted and need to be filed sooth use a fine grit sand paper not emery paper Clean all the dust when you are. Done

Hello Harryconover,

I am not sure if my amp actually has any speaker protection relays. I can not identify them, and I can not find the relays in the shematic. Is it possible it does not have any? Where would they usually be placed?

Thanks.
Isaac.

 

[coto37]

Many HK units have pre-outs and main-ins.

If your does, remove the plugs and replace with a standard audio cable except reverse the left and right.

Reason for doing this is to see whether the problem moves. If it does, your problem is in the right power amp. If it doesn't move, the problem is in the right pre amp.

Jerry

Hello Jerry,

Just tryed that out and the distortion still on the right channel. So its a power amp issue?

Thanks,
Isaac,

 

[onplane]

Hello Jerry,

Just tryed that out and the distortion still on the right channel. So its a power amp issue?

Thanks,
Isaac,

Yes, if the Left pre-amp is the input to the right power amp and the problem remains on the Right, then your problem is in the Right power amp.

OK, so the very first thing to clean is your speaker selection switches. These are usually the last thing after the amp and before your speakers.

The other thing to clean well would be your headphone jack. In some amps, the headphone jack will cut the signal to the speaker's.

Once this is done and if the problem remains, look for a relay. If there is none, you have a bad component in your right power amp and will need equipment to trace the source of distortion.

Regards,
Jerry

 

[coto37]

Yes, if the Left pre-amp is the input to the right power amp and the problem remains on the Right, then your problem is in the Right power amp.

OK, so the very first thing to clean is your speaker selection switches. These are usually the last thing after the amp and before your speakers.

The other thing to clean well would be your headphone jack. In some amps, the headphone jack will cut the signal to the speaker's.

Once this is done and if the problem remains, look for a relay. If there is none, you have a bad component in your right power amp and will need equipment to trace the source of distortion.

Regards,
Jerry

Thanks Jerry,

Will clean it all up tomorrow and let you know.

Isaac.

 

[coto37]

Its possible that you don't have a relay but quite unlikely it is generly on the power supply board or by the output transistors on the scmatic it is between the outputs and any speaker switching

I cant seem to find any relays, here are some pics.

 

[onplane]

Isaac, just want to make certain that when you removed the plugs and replaced with an audio cable taking the left pre-out to the right main-in, that you pushed the main-in switch on the front panel. Right? If you didn't push that switch, you learned nothing.

Forget cleaning the headphone jack. It doesn't interrupt the signal in your amp. Instead, add to your list the pre-out main-in switch to clean.

Regards,
Jerry

 

[coto37]

Isaac, just want to make certain that when you removed the plugs and replaced with an audio cable taking the left pre-out to the right main-in, that you pushed the main-in switch on the front panel. Right? If you didn't push that switch, you learned nothing.

Forget cleaning the headphone jack. It doesn't interrupt the signal in your amp. Instead, add to your list the pre-out main-in switch to clean.

Regards,
Jerry

Hello Jerry,

I did push that switch, it was still the same. I plan on cleaning everything tomorrow with contact cleaner. I will let you know what happens.

Thanks
Isaac.

 

[coto37]

On a unit of that age I would clean everything with a good contact cleaner.

Its possible that you don't have a relay but quite unlikely it is generly on the power supply board or by the output transistors on the scmatic it is between the outputs and any speaker switching

Isaac, just want to make certain that when you removed the plugs and replaced with an audio cable taking the left pre-out to the right main-in, that you pushed the main-in switch on the front panel. Right? If you didn't push that switch, you learned nothing.

Forget cleaning the headphone jack. It doesn't interrupt the signal in your amp. Instead, add to your list the pre-out main-in switch to clean.

Regards,
Jerry

Hello Guys,

I cleaned out the amplifiers pots, speaker select, and buttons with contact clean yesterday.
All went really well, no distortion on the righ speaker, I could adjust the pot on the left speaker properly and the woofer would not be pulled in anymore, the amplifier was running much cooler, the heat sink heated up equally.. Had the amp on all day practicly, and even the sound was nicer.

But today, just now actually, I was watching a series, and I heard like a strange "poping" noise from the left speaker, and suddenly the woofer was being pulled in again and the amp was heating up like before. But still no more distortion on the right speaker.
So I just cleaned everything again with contact cleaner and I am waiting for it to dry and try again.

Any suggestions? why does this happen?

Thanks,
Isaac.

 

[coto37]

By the way, there is also a small humming noise coming out of both speakers.

Thanks,
Isaac.

 

[onplane]

Well you have the worst problem of all ...an intermittent problem. They are the worst of all, because when you attempt to trace the source, everything works perfectly.

Often times these are caused by a cold solder joint. It works fine and then doesn't. Sometimes vibration will cause the cold solder joint to "flip". That is, give it a smack when it's not working and it'll start working. Also that same smack will flip it from working to non-working.

Finding these is a pain. Look closely at all the solder joints in the right power amp. Look for joints that are discolored. Look for parts that can wobble.

Nothing easy here...
Jerry

 

[coto37]

Well you have the worst problem of all ...an intermittent problem. They are the worst of all, because when you attempt to trace the source, everything works perfectly.

Often times these are caused by a cold solder joint. It works fine and then doesn't. Sometimes vibration will cause the cold solder joint to "flip". That is, give it a smack when it's not working and it'll start working. Also that same smack will flip it from working to non-working.

Finding these is a pain. Look closely at all the solder joints in the right power amp. Look for joints that are discolored. Look for parts that can wobble.

Nothing easy here...
Jerry

Hello Jerry,

Will do that and see if I am lucky. Also, I reed that the power supply cappacitors for the power amp can cause humming. Is this possible too? They do have some marks around the base, but I am not sure.
Also looks like the left channel is getting too much power so the woofer is allways being pulled in.
I have had this problem for a while, but usually I used to adjust the pots and it was all fine. Now, the pots wont let me adjust it enough becouse looks like the can not handle the power getting to the speaker.

Also I hear very rearly some static on the left speaker for just a second and it stops.
If I knock or bang the amp, there is no difference.
But I will still check the solders on the power amp.

Thanks,
Isaac.

 

[onplane]

Hello Jerry,

Will do that and see if I am lucky. Also, I reed that the power supply cappacitors for the power amp can cause humming. Is this possible too? They do have some marks around the base, but I am not sure.
Also looks like the left channel is getting too much power so the woofer is allways being pulled in.
I have had this problem for a while, but usually I used to adjust the pots and it was all fine. Now, the pots wont let me adjust it enough becouse looks like the can not handle the power getting to the speaker.

Also I hear very rearly some static on the left speaker for just a second and it stops.
If I knock or bang the amp, there is no difference.
But I will still check the solders on the power amp.

Thanks,
Isaac.

Isaac, certainly the powers supply caps will cause ripple if they aren't working properly. Remember, though, a power supply problem will impact both power amps, and rarely do they cause intermittent problems. Usually, once they fail, they rarely start working again. Check the solder joints on them, nevertheless.

The woofer is being pulled in because of a negative DC bias. Lift the amp a couple of inches while it's on and let it drop.

Vibration will often give you a clue that a cold solder joint is the problem.

Finding them, however, is another issue. Heat often causes the solder joints to become bad, so look closely at any parts with attached heat sinks.

Regards,
Jerry

 

[coto37]

Isaac, certainly the powers supply caps will cause ripple if they aren't working properly. Remember, though, a power supply problem will impact both power amps, and rarely do they cause intermittent problems. Usually, once they fail, they rarely start working again. Check the solder joints on them, nevertheless.

The woofer is being pulled in because of a negative DC bias. Lift the amp a couple of inches while it's on and let it drop.

Vibration will often give you a clue that a cold solder joint is the problem.

Finding them, however, is another issue. Heat often causes the solder joints to become bad, so look closely at any parts with attached heat sinks.

Regards,
Jerry

Hello Jerry,

Will do and will let you know how it hoes. I have done the lifting and letting drop thing to the amp and nothing happens, no change in anything or sound.

Also the internitent problem, only became intermitent as soon as I cleaned everything up with contact cleaner seemed to be working nicely and suddenly back to not working properly.

I will check components and solders near heatsinks. In the big main heatshink I just have transistors under it, and there are 2 seperate heatsinks holding up on 6 legs on the power amp section I will check also.

Thanks,

Isaac.

 

[njcanuck]

I have owned four HK PM series amps and they have all exhibited similar symptoms to what you are hearing. In every case it was dirty switch contacts that was fixed with DeOxit cleaning. The main problem switches are the Main/Preamp switch and the Speaker Selection switch. It can be quite difficult to get cleaner into the Preamp switch. I take the faceplate off to get at it and then soak it and work it ~50 times. The rotary speaker switch is actually located at the rear of the amp at the end of a long shaft. Again, work it after spraying it.

Note: If you are using phono inputs, the MC/MM switch is at the end of cable at the back of the amp. It also causes problems.

There are no speaker relays in these amps.

Good Luck