Harman Kardon 6550 amp fault finding

earl of sodbury · 11th July 2007, 05:01 PM

Solely on the basis that I know which end of the soldering iron to hold, I agreed to look at a mate's faulty HK6550 amp.

He had experienced it losing volume over a short period, fading to nothing in about a minute. On restart - nothing.

When it got to me, apart from surprisingly shoddy build, eyeballing it revealed nothing - no blown caps or burned boards, startup produced no tortured screams or puffs of smoke (more's the pity...)

SFAICT voltages were fine up to the output stage, but there was nothing at the loudspeaker terminals.

To make myself feel useful I cleaned all earth connections, sockets, switches and pots.

Now I have *some* voltage at output, it plays music and nothing shows obvious signs of strain or excessive temperatures, but if I go over about 20% on the volume pot there is no rise in volume but a HUGE rise in distortion...

It's going to be a PITA to strip down, so can anyone suggest where I should look first for this problem?

thanks, eofs

Netlist · 11th July 2007, 05:42 PM

Do you have a schematic? Does it have an output relay?

/Hugo

earl of sodbury · 11th July 2007, 05:47 PM

I don't have a schematic - not managed to find one on the net, but will be contacting HK.

I believe it does have output relays.

Netlist · 11th July 2007, 05:51 PM

Alright, i bookmarked this thread. Perhaps others here have a (similar) schematic.

/Hugo

earl of sodbury · 11th July 2007, 06:04 PM

Thanks

earl of sodbury · 12th July 2007, 02:17 PM

Not had much time to devote to this, but besides the other issues described, I notice the amp is completely out of phase (polarities reversed at output).

Any thoughts? Anyone?

11th July 2007, 05:01 PM	#1
earl of sodbury diyAudio Member Join Date: Jun 2006 Location: West Oxon	Harman Kardon 6550 amp fault finding Solely on the basis that I know which end of the soldering iron to hold, I agreed to look at a mate's faulty HK6550 amp. He had experienced it losing volume over a short period, fading to nothing in about a minute. On restart - nothing. When it got to me, apart from surprisingly shoddy build, eyeballing it revealed nothing - no blown caps or burned boards, startup produced no tortured screams or puffs of smoke (more's the pity...) SFAICT voltages were fine up to the output stage, but there was nothing at the loudspeaker terminals. To make myself feel useful I cleaned all earth connections, sockets, switches and pots. Now I have some voltage at output, it plays music and nothing shows obvious signs of strain or excessive temperatures, but if I go over about 20% on the volume pot there is no rise in volume but a HUGE rise in distortion... It's going to be a PITA to strip down, so can anyone suggest where I should look first for this problem? thanks, eofs

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11th July 2007, 05:42 PM	#2
Netlist diyAudio Moderator Join Date: Jan 2003	Do you have a schematic? Does it have an output relay? /Hugo

11th July 2007, 05:47 PM	#3
earl of sodbury diyAudio Member Join Date: Jun 2006 Location: West Oxon	I don't have a schematic - not managed to find one on the net, but will be contacting HK. I believe it does have output relays.

11th July 2007, 05:51 PM	#4
Netlist diyAudio Moderator Join Date: Jan 2003	Alright, i bookmarked this thread. Perhaps others here have a (similar) schematic. /Hugo

11th July 2007, 06:04 PM	#5
earl of sodbury diyAudio Member Join Date: Jun 2006 Location: West Oxon	Thanks

12th July 2007, 02:17 PM	#6
earl of sodbury diyAudio Member Join Date: Jun 2006 Location: West Oxon	Not had much time to devote to this, but besides the other issues described, I notice the amp is completely out of phase (polarities reversed at output). Any thoughts? Anyone?

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