hafler dh-200 damage?

[george a]

Hi All

Last night I was tweaking my speaker xovers (rat’s nests as my wife calls them) when I touched one of the component leads with the soldering iron I heard a pop sound come from the loudspeaker. Unfortunately I had forgotten to disconnect the amp and worse still it was still powered up. (getting tired)
In short the speakers are fine but the amp now distorts as volume is raised .
All fuses have been replaced but the sound is still distorted , much like the sound of a blown fuse but with some popping sound again.
Other channel of amp is fine. Voltage at idle on speaker terminals also seems fine .
Can anyone offer a suggestion as to what may have been damaged (my first uninformed guess is that one or more of the output mosfets may be damaged)
Any suggestions would be welcome- have not tested rail voltages or current draw as yet.

Thanks

[george a]

Hi All

I have had a further look -can't measure bias current at moment but output offset is approx 20mV to affected channel (14mV to good one).
Interestingly to the touch 3 of the 4 output mosfets feel warm/hot to the touch , the other feels relatively cool (aside from some heat transfer through the heat sink.(?)
Again any comments suggestions would be welcome - ...anyone..??

[SY]

Dumb question: have you checked the output fuses? The way the 200 is set up, when the fuse pops, it doesn't go silent, it just sounds thin and distorted.

[george a]

Sy

Yes I checked and replaced all fuses.
I have a couple of spare mosfets - hopefully one is not blown...
Thanks

[Zero Cool]

I had this same problem, turned out to be bad solder connections on the Diodes used in the constant current source for the double diff input stage. there are 2 diodes in series and the solder connections didnt look bad, but, if i pushed on the board they would pop, then the channel would start to work for a bit. then it would pop again and distort and the heat sink would cool off.



Zero

[george a]

Thanks Zero
I'll have a look - but I don't think my actions would necessarily cause such a defect.
Thanks

[george a]

Hi All

Closer examination and measuremant shows that bias current and offset appear ok.
However the problem is actually in both channels and also includes a low level buzz, power turn off caused a tweet type sound (instability ?).
I suspect that some earthing issue may now be at fault.
I have heard a Hafler once drop an earthing resistor and it was not pretty...
Again any suggestions would be welcome
Thanks