strange thud/pop when powering up, should i be worried?!

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Aug 10, 2019
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hi guys,

you may have seen in the show your system section that i am currently running a pretty old (circa 1983) harman/kardon hk770 power amp as part of my system.

i've only had it fitted for a week or so, but something is really bugging me! i've disconeccted the speakers now and hooked them back up to my azur 650a until i can be satisfied that this won't harm them.

basically, when you switch it on there is a short delay (circuits powering up i suppose) followed by a clicking relay, at exactly that moment the right hand speaker will pop quite loudly. if you take the cover off the speaker you can actually see the drivers 'pump' outwards before settling sort of 3/4 of the way back from where they started (if that makes any sense?!) if you switch the amp off the drivers will fall back to their natuaral position.

it is not affected by turning down volume etc, and if you switch on with speaker set b engaged (nothing connected) then switch over to 'a' it still happens, it even occurs if the pre amp is off.

if i unplug the right hand speaker, i still hear the clicking relay, but no sounds/ driver movement from the left hand speaker whatsoever. Thi leads me to believe that what is going on with the right speaker is pretty abnormal and potentially detrimental to it!

I should add that there does not to seem to be any distortion or background noise when playing, and music sounds great through it!

i have read some information about cheking the dc output balance on the terminals, so i will check that tonight (could this be caused by an imbalance?) i suppose there could be a dodgy relay for the right channel or something.

thanks in advance for any assistance, fingers crossed its nothing to bad, i do love the sound that this has given to my setup!

cheers

Dan :help:
 
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Anonymous

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Ok, quick update, checked the voltages accross each channel, getting 60mv on the left and a massive 7.4v on the right, I guess thats the issue then! I've downloaded a service manual which seems to suggest that there are variable resistors of some sort to adjust the dc voltage balance, but surely they can't drift that far off their settings! question is where do I look next?
Cheers
Dan
 

matthewpiano

Well-known member
Certainly don't use the HK until you've got it sorted. That sort of voltage is going to do some damage. Unless you really know what you are doing I'd get it looked at by a qualified service person. You could easily fry your speaker if you get things wrong.
 
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Anonymous

Guest
There's no chance my speakers are going anywhere near that thing until I'm sure it's sorted!
I've tweaked the pot for the right channel and got the voltage down to 5.8v clearly something between the transformer and the pot is letting too much voltage through. I've checked the resistance across the pot at both ends of it's adjustment and it matches up with the values in the service manual. I suppose the next step is to check everything between the transformer and the pot!
Can I really be bothered? Hmmmm. Does anyone have a recommendation for a hifi service/repair place in the southampton area?

Cheers
Dan :)
 

Blackdawn

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May 7, 2010
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Not sure if this helps. My dad has a NAD 3020 from around the same time. Its always made a thud/pop when you turn it on and off. No relay clicking noise though. Only the one thud/pop sound when switching on and off. Music still plays perfectly fine through this amp. No idea of the voltages its putting out. I wouldn't worry if there is a thud/pop when you turn it on and off as long as it doesn't do this while playing music all the time. Older amps have these quirks. It wouldn't hurt to have it serviced though especially as the HK sounds good.
 
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Anonymous

Guest
Your amp has a fault called a dc-offset on the right hand channel, and you should get a service centre to repair it for you. A high dc offset like to one you are reporting causes a large current to flow through the bass/ midrange unit of the attached speaker, which can quickly cause the voice coil of the drive units to overheat and burn out.

Most likely one of the driver and/ or output transistors are faulty. It may not be economically viable to repair, given it dates from 1993, and is not generally considered a 'classic amp'.
 
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Anonymous

Guest
I had read that some older amps have a tendency to cause the speakers to pop or thud when switched on, unfortunately this is totally abnormal. The amp is putting almost 8volts through the speaker at idle, according to my service manual it should be 0v+/- 10mv!
The clicking relay i've tracked down to speaker selector, there's one relay for each speaker set 'a' and 'b'
The only other thing i can think to check is called 'idling current adjustment' there is another pot on each channel to adjust this, and it looks like this one sits before the output pot in the circuit, so possibly either duff or way out of adjustment letting too much voltage through?
I'll check that as soon as i can get my hands on a second voltmeter.. After that i'm either going to have to check everything between the transformer and the output pot or just hand it over to someone else to repair for me! I guess it depends how lazy/brave i feel!
I've still got an old set of speakers around that i dont mind sacrificing in the name of sorting this amp, it really is a nice bit of kit, and seems a shame to let it go to waste in the name of potentially a few cheap components!

Cheers
Dan
 
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Anonymous

Guest
Thanks for your input 'hifi gear'
Maybe i'll see what sort of money we're looking at for a repair then.. It will almost certainly be less than a new power amp though!
Looking at the service manual there are a total of 8 transistors on each channel labelled either driver or pre driver, perhaps one of these is the culprit then... Just need to test them somehow.. If thats the case the parts should be less than £50 and i have a friend who is pretty handy with a soldering iron.. Since the amp cost me nothing, i dont have anything to lose in trying to sort it out... Obviously it will be thouroughly tested with my old speakers before letting it near my kefs again though!

Thanks
Dan
 
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Anonymous

Guest
Got an update for those that are interested.. I.have tested all of the transistors on the right chanel board (took a while!) I have tested them all several times and also tested again comparing the readings with the left chanel (2 identical boards) I've narrowed it down to one of the pre driver transistors, which seems to be dead.. Next job is to take it out and test again to be certain. Fingers crossed this will be the culprit though! Any more input is much appreciated! Cheers Dan
 
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Anonymous

Guest
Final update...it's fixed! Replaced one 39p transistor on the right channel and BINGO!
Unfortunately I managed to damage the left channel on reassembly (long story!) And took out several resistors and a couple of transistors, luckily I managed to did all that for about 30 quid.

So in all it's cost me less than 40 quid and a few hours of my time to get the amp working again, suffice to say I'm very happy!

Cheers
Dan :)
 
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Anonymous

Guest
Forgot to add... I now have 5mv of dc voltage on both channels (previously 63mv and 7.4v!)
Please excuse any typos, I'm on my phone!

Dan
 

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