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Discussion Starter #1
Looking for any suggestions on an issue I have with my Harman Kardon AVR8000. First, this problem has existed for about 15 years and I use the receiver pretty regularly, so I can live with it if it can't be fixed. With that said, it would be nice to fix it.

When I try to turn on the receiver from a 'cold' start, you hear a relay click (3 seconds), then the amp clicks off. Note that it never fully turns one. Then I push the button again you hear a relay click (3 seconds), then the amp clicks off. The third time, you hear the first relay click (5 seconds), amp comes on like it should.

If the amp is 'warm' you can shut it off and most of the time it will come right back on like it should. After about 10 minutes, it's the 'cold' start procedure with fail, fail, success.

I assume there is some sort of fault that is tripping a protect circuit, but I have no idea where to begin. Any thoughts?

Thank you in advance.
 

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Unplug the unit, and take the top cover off.
Next to the big toroidal power transformer is a vertical circuit board with many fuses. Take out each fuse and bend back in slightly their respective mounting tabs, as they tend to loosen up with age.. Make e a note of each fuse value and order in the circuit board, reinstall the fuses and try it now. Note that I was the original project design engineer for the AVR8000.

Just my $0.02... ;)
 

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Discussion Starter #3
First, thank you for the reply. I removed four fuses (three on one side and one on the other). As you can see in the picture below, some of the metal contact had rubbed off. I rotated them to make a good connection and reinstalled. Unfortunately, there is no change in behavior of the amplifier. Any other thoughts?

3040941
 

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Discussion Starter #5
So it looks like I will be diving in deeper. After removing the back plate I noticed the following capacitors bulging. Do these all need to be replaced? I will pull all of the applicable sub-boards this weekend and get a better idea of what's under the hood.
3042441
3042442
 

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Discussion Starter #7
I tore into the amp and it felt like open-heart surgery. So many board and connectors! Once I got to the power supply board, I popped the covers off the tops of the caps and saw that there was no expansion on the tops of the caps. I got it all back together and the good news if that I didn't blow it up. The bad news is that the behavior is the same.

3042932
 

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Discussion Starter #8
OK, so a bit more digging. I went through the service bulletins and tried to get everything in line. First I measured and adjusted the idle current. A couple channels were about 25% high, but not too bad. I got all channels within 2% of spec (10.5mv).

Next I decided to test the DC Offset. Here are the readings:

Front R: 122mV
Front L: -84mV
Center: -3.4V (yes, volts)
Surround R: -4mV
Surround L: 2mV

I think I have located a problem! The front channels are bad, but the center is off the charts. Any idea where to start to get this amp back in line?
 

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Discussion Starter #9
Update: I did a ton of testing and I am pretty sure the issue is on the center channel amp board. To move forward, I had to de-solder all of the transistors on the board. Absolute PITA. See below:
3050808


Note that I tested all transistors and the pencil marks are the hFE and mV readings.
 

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Discussion Starter #10
I started pulling the smaller transistors and found that QC901 was reading fine when testing as a diode, but the hFE was 30 (compared to similar transistors at ~390). I think this is the culprit and would make sense as it is part of the initial transistor pair on the schematic:

3050809


Because the KTC3200 is no longer available, I bought 100 KSC1845 and I will hunt for the best pair.
 

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Discussion Starter #11 (Edited)
One more quick update. I pulled the 901/902 pairs on the other two channels on this amp board (because why not). Here are the measurements:

Front Left:
901 hFE: 285 mV: 615
902 hFE: 546 mV: 627

Surround Left:
901 hFE: 601 mV: 605
902 hFE: 548 mV: 627

It looks like the amp was either poorly matched, or the KTC3200 have drifted quite a bit. These differences in hFE might also explain the DC offset in each channel as the differences in each pair correspond to the differences in the DC offset. With that said, the center channel is certainly farther off than I would have expected, but maybe the small differences end up compounded as the signal goes through the various stages of amplification.

At this point, I will probably replace all of the KTC3200 and KTA1268 on the board with the fresh transistors. I don't like the "shotgun" approach of making a ton of changes, but removal of the board takes about an hour plus an hour of cleanup, so I would like to avoid that if possible.

It's also worth mentioning for anyone else reading this and thinking about the same thing, the pinout on the existing transistors is the reverse of the pinout of these replacements, so the "round side" will face the other direction. Edit: This was incorrect. I misunderstood the diagram.
 

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Keep us posted. All I know is my HK 7550HD started having the exact same power issue a few years ago. On the third time, it would usually power up and stay powered on like you said, then after it was warm, I could toggle on/off without issue. It was only from a cold start that it didn't work properly. I also started noticing some pink noise in my center channel about the same time as the power issue started. I never did have time to diagnose the issues. I boxed up the receiver and it's been in a closet since then.
 

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Discussion Starter #13
Keep us posted. All I know is my HK 7550HD started having the exact same power issue a few years ago. On the third time, it would usually power up and stay powered on like you said, then after it was warm, I could toggle on/off without issue. It was only from a cold start that it didn't work properly. I also started noticing some pink noise in my center channel about the same time as the power issue started. I never did have time to diagnose the issues. I boxed up the receiver and it's been in a closet since then.
Sounds exactly like my issue. Are you able to check DC Offset with a multimeter? I wonder if you have DC in that channel just like I do.
 

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Discussion Starter #17
I got the new transistors in and put the amp back together. It still faults into protection, but when I get the amp on, the main 30W power resistor (R451) smokes. (Crap!)

Back to the drawing board.
 
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