Harman/Kardon HK6500. Loudness not working when button is pressed

[KrisBlueNZ]

OK, so is Q908 emitter at around +12V now?

 

[Rickedafied]

Yes.
I notice the heatsink for Q519 and Q520 is getting hot though

 

[KrisBlueNZ]

Can you measure the voltage ACROSS R555, and the voltage across R556?

 

[Rickedafied]

R556 showing -48V on both ends and R555 is showing +48V on both ends

 

[KrisBlueNZ]

Measure ACROSS the resistor. Disconnect the black meter lead from 0V and put one probe on each end of the resistor. Then repeat for the other resistor.

 

[Rickedafied]

Oh sorry. R555 is showing 0.43V and R556 is showing -4.70V and seems to be climbing slowly

 

[KrisBlueNZ]

OK, that voltage across R556 is too high. It means the current is 470 mA. R556 will be dissipating 2.2 watts and is probably getting very hot!

Can you unplug CN401 and CN402 and check the voltage across R556 again. It should be almost zero with CN401 and CN402 disconnected.

 

[Rickedafied]

Wait, the black probe wasnt touching the pin that good. Its -0.43 not -4.70

Do I still disconnect cn401 and 402?

 

[KrisBlueNZ]

No, that's OK. If you have 0.43V across R555 and the same across R556, that means there is 43 mA flowing through each regulator. That's fine. That will produce about 2 watts total dissipation in Q519 and Q520 which will make the heatsink warm, but not too hot to touch, right?

So does the amp work properly now?

 

[Rickedafied]

Yes its working now.
The heatsink, I can't leave my fingers on it too long. Almost as hot as those other 8 components that I told you that were getting hot. Normal I guess?

 

[Rickedafied]

Its too hot to touch now. Would plugging CN403 back on do anything?

 

[KrisBlueNZ]

No, that's not right.

It's only Q519 and Q520 on that heatsink, right?
And there's 0.43V across R555 and the same across R556? (polarity doesn't matter)
And there's about +22V on Q519 emitter and about -22V on Q520 emitter?

No, don't reconnect CN403. That would just cause more problems.

 

[Rickedafied]

Yes to all.

 

[KrisBlueNZ]

Don't leave it running if anything is too hot to touch.

Are the other eight transistors still getting very hot?

Can you upload close-up photos of both sides of the board in the area of the heatsink and the resistors please.

 

[Rickedafied]

 

[KrisBlueNZ]

Those are good photos and you've done a good job replacing those components. Everything looks good.

Try this.

1. Disconnect CN401 and CN402.
2. Power up the amp and measure the voltages across R555 and R556.
3. Wait to see whether the heatsink gets hot again.
3. Power off, reconnect CN402.
4. Power up and measure voltages across R555 and R556 again.
5. Power off, reconnect CN401 as well.
6. Power up and measure voltages across R555 and R556 again.

I'm going to sleep now so just post your results and I'll get back to you.

 

[Rickedafied]

Thanks. Ok CN401 and CN402 are the two Connectors between the new capacitors right? They don't say CN401 and CN402, I just see one writing next to one saying JP46B

 

[KrisBlueNZ]

Right.

The wire that goes into CN401 goes off to PCB 9 the tone control board.

The wire that goes into CN402 goes off to PCB 10 the "BASCON" board.

 

[Rickedafied]

Both CN401 and CN402 disconnected
R555 and R556 showing 00.0V heatsink not hot

CN402 connected alone.
R555 and R556 showing 00.1V heatsink not hot

CN401 connected alone.
R555 and R556 showing 00.4v heatsink not hot.

CN401 and CN402 Connected, heatsink starts to warm up again.

 

[Rickedafied]

What are Q442 and Q444 purpose?