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H/K AVR 25II - protect mode only with center channel

 
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ModemJunki



Joined: 31 Jul 2010
Posts: 2

PostPosted: Sat Jul 31, 2010 1:12 am    Post subject: H/K AVR 25II - protect mode only with center channel Reply with quote

Hi,

First thanks for the tech manual/schematics! Wonderful start. I can sort of read schematics and I can solder but I'm no EE.

I have already determined that I might need to replace certain capacitors in the volume control circuit on the advice of the technical manual but was hoping for a more knowledgeable opinion.

I have an old Harman/Kardon AVR25MKII receiver. When Pro-Logic mode (surround or 3-channel) is active, using the volume control causes amplifier protection to trigger. This only happens when the volume pot moves, there is a scratchy sound and then the system shuts down. I think the clue is the scratchy noise - i suspect it overdrives the preamp section somehow.

If Pro-Logic mode is turned off (stereo only) protect mode does not trigger.

The receiver also makes a "tick" (pop) when switching sources or using the "direct" (tone control bypass) button (it did not do this in the past).

Last, it was noted that the center channel output had to be increased to match the channel volumes - my son made the adjustment some months ago and didn't mention it. He had to max out the level to get it close.

As I type this, the volume pot is drying from contact cleaner, I will apply some contact lube and see if that helps the situation at all.

I will be also be searching for caps suitable to replace the ones indicated by the technical manual. I have a collection of old motherboards and PC power supplies that I have used in the past to harvest caps, would these also be suitable for audio use?

Any advice given is appreciated!
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sat Jul 31, 2010 2:45 am    Post subject: Reply with quote

In general, your listed symptoms are usually caused by DC voltage induced in the signal path. Classic symptoms are scratchy volume and protection is usually triggered on most any receiver.
Most of the time it is caused by one or more switching IC(s) in the signal path, such as electronic volume control, CMOS switching or even muting transistors can leak DC. You will usually notice the fault even without any source material going thru the unit. I used to run in to this type of failure all the time when I was in consumer electronic field. The key is to locate where the DC voltage gets injected in the signal path?(There should not be any)
ie, if/when the unit is "acting up", you will be able to measure some sort of a dc level along the signal path which will be automatically transferred all the way to the final stage where the protection will detect it. Also you will notice that it will vary according to the volume level if the fault is located before the volume control.
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ModemJunki



Joined: 31 Jul 2010
Posts: 2

PostPosted: Sat Jul 31, 2010 5:40 pm    Post subject: Reply with quote

(Oops - forgot to login. Can the "guest" post be deleted?)

Thanks, vtech!

What you describe is accurate - this happens without any source, and the protect is triggered when the volume level is advanced beyond about 20%.

So, if I am understanding this, I must use the schematic (which helpfully shows the center channel signal path) and follow this path with a meter looking for DC voltage (where the signal should be purely AC).

And, since it happens with change in volume, the problem should be somewhere before the volume control (or possibly in the volume control IC).

It will be some time before I can start this task - I must finish some rather more analog projects involving a scraper, sandpaper, paint, a paintbrush, and some windows before I can return to the basement workshop Confused

In the meantime, I found a replacement receiver (another AVR25MKII) for 10% the price of what it cost new, and it works perfectly in our video setup - so I don't have my family bugging me to go and buy something new Very Happy

But I will reply with my results, perhaps next month I will have time!
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