Amp repair help

[RichIOM]

Hi guys,

I've been looking at fixing my brother in-laws amplifier and I was wondering if you could help?

It's a Harmon Kardon AVR4000 home cinema amplifier, and basically it doesn't sound as loud as it used to. It's over 10 years old so I'm guessing it's capacitors breaking down. I've looked at them by eye and can't see any noticeably bulging or anything.

I've also tested the amplifier output on a scope and it measured 46.9W at 0DB using a 100Hz test tone, when I put a dummy load on it, the load was 9ohms however as I didn't have any 8ohm power resistors to load on it (speakers used are 8ohm).

The spec suggests it should be putting out 50W per channel, so I would say power output is fine.

The waveform was also clean on the scope, until +5db when it begins to distort but I would expect this.

However volume isn't as loud audiably as it used to be, and we were able to push it to +db listening on surround, where as that would be deafening in the past with the exact same speaker setup. Normally -32db is getting loud but this isn't the case now.

The quality is also noticeably more flat.

I was just hoping someone may be able to steer me in a direction to look at, as there as so many component areas to look at and I'd rather replace caps if there's a feel that they're worth replacing (likely to be bad).

Thanks.

[mashley_nad]

Do you get exactly the same result (i.e reduced volume) on all inputs?

My first thought would be a failed electrolytic cap in the preamp section that is affecting an op amp gain in some way.

[cbdb]

I've also tested the amplifier output on a scope and it measured 46.9W at 0DB using a 100Hz test tone,

0db means nothing without a reference. Find the amps sensitivity (ie 100 mv ) and use this as an input level. If you still get 50 watts the amp is fine and you may have a problem in the gear feeding the amp.

[cbdb]

Reread your post, do you mean 0db on your volume control? You still need to feed the amp the right level signal re its sensitivity.

[sofaspud]

All the midrange drivers are blown?
Seriously though, please make sure there isn't a variable being left out. You have an odd "failure" as presently described.

[RichIOM]

Thanks for the feedback.

0DB is as displayed on the screen when turning up the volume, I think it starts somewhere like -72DB

I'll take a look at the sensitivity, I presume you mean measure the voltage in from the source?

To confirm the source is varied as it's not my amplifier so it's definately not the input equipment, and the speakers aren't a factor either as I have exactly the same speakers as my brother in-law (who's amp it is), and it sounds the same on mine.

I know it's an odd failure to have which is why I'm a bit stumped at the moment.

Are electrolytic capacitors the most comon failure would you say when it comes to components breaking down with age?

I presume microchips don't suffer as much from age?

Thanks.

[jaycee]

Electrolytics are indeed subject to age and are the most common failure - especially combined with temperature.

Very old IC's can be subject to failure mainly due to flaws in early manufacturing, but there won't be anything of that age in this particular amp.

[RichIOM]

Thanks, that's what I thought, when I've read most people complaining of faults in equipment, it's either an obvious blown component, dry joint/poor connection or the electrolytic capacitors.

I've tested all inputs now so far as audiably as I only had the scope and multimeter on loan, my multimeter is more geared towards dc and so only covers 200V upwards on ac.

There's no audiable difference on all inputs, including the input on the front, which allowed me to discount the main input banks which are on a circuit board soldered to the main motherboard circuit, which looks like it's a card that could be removed, but the pins are actually soldered.

The amp only really starts sounding half decent at around -25db on the volume, which as I said starts at -72db do it needs to be wound up to have any volume coming through.

I appreciate the output is only as good as the input, which I learnt most from my days as a car audio engineer, however this amplifier isn't driven by a preamp, so there is no actual gain, it's just standard source equipment. I've tried various sources and they all have the same volume.

So based on the above, that leaves me with either the motherboard circuitry or the power supply circuit.

Any further ideas or input would be welcome.

Thanks