Hi Guys (and Gals of course). I'm trying to fix a Harman Kardon HK6600 amp. (see circuit diagram: http://manuals.harman.com/hk/Service Manual/HK6600 sm.pdf ) It has a 4 V DCoffset on one of the outputs. I've replaced Q404,406,408,410 and 446 allready, and measured all releveant resistors, but no cause yet. Any ideas?
thanx, Philip
thanx, Philip
Hi Philip,
The first thing to do is see if the diff pair is attempting to correct the offset. Measure the E-B junctions to see if any are reverse biased and make sure the CCS is running. Measure voltage drops across R418, R412 and R414 to show current flow.
Q446 is a regulator, so you can measure the voltage drops. The DC offset control has an effect so the stage may be working fine.
Where ever you see a resistor on an emitter, measure the voltage to confirm current flow. Also measure E-B drops. You can do this at reduced voltage, be careful not to short anything.
Beware the no burn resistors. They can look fine but be open. Sometimes it's just as easy to discharge the supplies and measure the darn things.
-Chris
The first thing to do is see if the diff pair is attempting to correct the offset. Measure the E-B junctions to see if any are reverse biased and make sure the CCS is running. Measure voltage drops across R418, R412 and R414 to show current flow.
Q446 is a regulator, so you can measure the voltage drops. The DC offset control has an effect so the stage may be working fine.
Where ever you see a resistor on an emitter, measure the voltage to confirm current flow. Also measure E-B drops. You can do this at reduced voltage, be careful not to short anything.
Beware the no burn resistors. They can look fine but be open. Sometimes it's just as easy to discharge the supplies and measure the darn things.
-Chris
Hk6600
HI
I'm sorry about my bed English ,but i'll try .
Becouse u have the same faults on both two channels . U have to looking for any problem on common circuit - May be "Mute-protector" circuit.
To be sure that the power amp' is well .u have to insulating the circuit and check it with double power supply.
Dudu
HI
I'm sorry about my bed English ,but i'll try .
Becouse u have the same faults on both two channels . U have to looking for any problem on common circuit - May be "Mute-protector" circuit.
To be sure that the power amp' is well .u have to insulating the circuit and check it with double power supply.
Dudu
HI Chris & Dudu,
thanx for your replies. First, the problem is in the right channel only. Second: during powerup, there is no DC due to the muting circuitvia Q418.
I will try your suggestions Chris, first thing monday. By the way, what is CCS?
nice weekend everybody,
Philip
thanx for your replies. First, the problem is in the right channel only. Second: during powerup, there is no DC due to the muting circuitvia Q418.
I will try your suggestions Chris, first thing monday. By the way, what is CCS?
nice weekend everybody,
Philip
Hi Philip,
Sorry, a CCS is a constant current source. It allows wide variations of collector voltage (in this case) while sourcing or sinking the same amount of current. They are most commonly used for tail current on a differential pair. Q408 / Q410 make up the "CCS" in this example.
Q418 is a voltage regulator, around 14.4 VDC. There may be a small delay formed by the 22K resistor and 47uF cap. Maybe around one second. Probably less as one time constant is beyond 14.4 V, 0.63 X the supply voltage.
-Chris
Sorry, a CCS is a constant current source. It allows wide variations of collector voltage (in this case) while sourcing or sinking the same amount of current. They are most commonly used for tail current on a differential pair. Q408 / Q410 make up the "CCS" in this example.
Q418 is a voltage regulator, around 14.4 VDC. There may be a small delay formed by the 22K resistor and 47uF cap. Maybe around one second. Probably less as one time constant is beyond 14.4 V, 0.63 X the supply voltage.
-Chris
I'm having same DC offset problem. Starts at 4-6 volts then gradually goes down to normal after about 15 minutes. Did anyone find solution to this problem. It's in only 1 channel.
Sounds like a duff electrolytic capacitor somewhere in the signal path that eventually recovers, in which case you should follow the advice given above. (And maybe try an RCA cable with a the other end shorted on the right MAIN IN in order to see whether that changes anything. Don't forget about the loudness circuit as a potential trouble spot.) Might also be oscillation though - does that side get unusually warm? There's 3 power-related electrolytics in the power amp, too.
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I've changed most of lytics with exception of one across zener. It has identical voltage across compared to one on other side so I left it in. I will try shorted rca across main & look at loudness circuit. I did notice that when you select main in with nothing attached as opposed to using it integrated that DC ofset jumps even higher mabye 10 volts. Thanks for advice. This is a tough one. Donald
Went ahead & changed rest of lytics with no succcess. I also tried shorted rca input and toggled loudness switch on/off with no voltage change. Already change the 2 main transistors & drivers. The schematic is not readable on many voltage points-looks like a weak scan which makes it more difficult. What a challenge.
In case someone else runs across this I found the problems. It ended up being several components. The parts that made most difference were. C407 a 2 pico mica cap and R429-560 ohm resistor. Replacing C407 made DC ofset drop from 4 to 2 volts. Replacing R429 made drop from 2 to .5volts. I was able to get to 10mv by replacing several other resistors in ofset circuit. 47K and 39K next to offset pot(sorry cant remember numbers)and R449. None of the resistors were out of tolerance very far even when out of the circuit. The only way I found was using a hot air station and moving over the components and noting the greatest Offset voltage change. Hope this helps someone in future and thanks for all the input. Donald
Not sure if this thread is still alive, i have a 6500, after recap and new power transistors, the dc offset in Rch likes to drift from my initial setting to like 40mV-50mV. Lch is mostly stable. Idling current is 40mV in both, i had to modify the idling circuit for Lch to get it right but it's fine now. Any ideas what to change? There are resistors near the pot, R421 (1.8K) and R419 (33K). Maybe those?
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