Harman Kardon 6550 amp fault finding

[earl of sodbury]

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]

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

/Hugo

 

[earl of sodbury]

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]

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

/Hugo

 

[earl of sodbury]

Thanks

 

[earl of sodbury]

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?

An externally hosted image should be here but it was not working when we last tested it.