Harman Kardon HK6950r repair

rss388 · 29th January 2012, 08:15 PM

Hello all,

I just got my hands on an HK6950r. The unit would work on power up for a couple minutes and then the protection light comes on and it stops working. I had taken the unit apart and found that the unit would work just fine if the power amp assembly does not contact the chassis. I was able to set the bias and everything but soon as I assemble it back into the chassis, where according to factory construction only one side of the heat sink is galvancally tied to the chassis, the protection fires again. I looked through the protection circuit and believe it to be firing due to excessive current through one channel. In fact, if I disconnected the protection signal the main power fuse would blow, leading me to conclude that the protection circuit is not false triggering.

Is it possible the unit is suffering from some kind of stability problem where the oscillation gets triggered by the heat sink/power amp assembly being tied to the chassis? Does anyone else has any similar experiences? Unfortunately I do not have a scope other than a good quality Fluke DMM.

I have the service manual in pdf and I would be happy to email to you to look through, although the schematics are scanned and are broken up in pages, making it kind of hard to follow.

Many thanks in advance.

rss388 · 2nd February 2012, 08:39 AM

Hello all,

BUMP. I have figured out that the output stage is a triple compound configuration. One channel works fine but another channel works OK if the heat sink is not screwed onto the chassis but soon as I touch the heatsink, the current shoots up and trips the protection circuit. I am theorizing a stability problem but I have no scope to confirm. I tried putting base resistors 1.5 ohm and 10 ohm on each of the output devices (2SA1302/2SC3281 triple paralleled) but things did not improve. I did not want to increase the Miller cap on the pre-driver so as not to upset the compensation.

Does anyone have any suggestions? Could an output device have gone bad? The unit had worked for a few years and an electronics shop had repaired this symptom once but it is not clear where the repair was.

Thanks!

prasanna gunasinhe · 12th February 2012, 02:43 AM

Is there any dc voltage between heat sink of the 'bad channel' and the chassis?

29th January 2012, 08:15 PM	#1
rss388 diyAudio Member Join Date: Dec 2007	Harman Kardon HK6950r repair Hello all, I just got my hands on an HK6950r. The unit would work on power up for a couple minutes and then the protection light comes on and it stops working. I had taken the unit apart and found that the unit would work just fine if the power amp assembly does not contact the chassis. I was able to set the bias and everything but soon as I assemble it back into the chassis, where according to factory construction only one side of the heat sink is galvancally tied to the chassis, the protection fires again. I looked through the protection circuit and believe it to be firing due to excessive current through one channel. In fact, if I disconnected the protection signal the main power fuse would blow, leading me to conclude that the protection circuit is not false triggering. Is it possible the unit is suffering from some kind of stability problem where the oscillation gets triggered by the heat sink/power amp assembly being tied to the chassis? Does anyone else has any similar experiences? Unfortunately I do not have a scope other than a good quality Fluke DMM. I have the service manual in pdf and I would be happy to email to you to look through, although the schematics are scanned and are broken up in pages, making it kind of hard to follow. Many thanks in advance.

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2nd February 2012, 08:39 AM	#2
rss388 diyAudio Member Join Date: Dec 2007	Hello all, BUMP. I have figured out that the output stage is a triple compound configuration. One channel works fine but another channel works OK if the heat sink is not screwed onto the chassis but soon as I touch the heatsink, the current shoots up and trips the protection circuit. I am theorizing a stability problem but I have no scope to confirm. I tried putting base resistors 1.5 ohm and 10 ohm on each of the output devices (2SA1302/2SC3281 triple paralleled) but things did not improve. I did not want to increase the Miller cap on the pre-driver so as not to upset the compensation. Does anyone have any suggestions? Could an output device have gone bad? The unit had worked for a few years and an electronics shop had repaired this symptom once but it is not clear where the repair was. Thanks!

12th February 2012, 02:43 AM	#3
prasanna gunasinhe diyAudio Member Join Date: Nov 2010 Location: Sri Lanka	Is there any dc voltage between heat sink of the 'bad channel' and the chassis?

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