It just keeps blowing...

[madan1]

Re: It just keeps blowing...

wow wow wow.. what happened here.. guys, no beef, please!

I just wanted to share with you my headache with that obsessed amplifier... and since that's a pretty common amp design I was hoping that someone has already seen that gremlin and knew how to fight it


In the next days I will provide more accurate data about what blew and whats ok... now I'm noting down things to check.


by the way, could a transistor be faulty, but still giving good resistance reading on the multimeter?

[petehall347]

Re: It just keeps blowing...

yes a transistor can behave like that .

[budm]

Re: It just keeps blowing...

Your meter does not put out the operating Voltage that the Transistors are subjected to so it will not be 100% accurate.

[keeney123]

Re: It just keeps blowing...

This is my last post. Promise!

[budm]

Re: It just keeps blowing...

What is the yellow line for?

[rievax_60]

Re: It just keeps blowing...

This is my last post. Promise!

If the current from R606 and R608 did follow the blue path on its way to the voltage amplifier transistor(TR828), a large voltage difference would develop between the bases of the driver transistors(TR532,TR534), turning on the output transistors hard which would then pass destructive rail to rail current.
TR528 with its associated components, functions as a voltage clamp. It passes most of the current from R606 and R608, holding the voltage between its Collector and Emitter at about 2V.
This voltage difference is shared between the 4 Base Emitter junctions of the driver and out transistors causing them to slightly turn on, passing just a few milliamps from rail to rail. This is called "Idle Current"

[madan1]

Re: It just keeps blowing...

Finally found some time to put the denon apart.
I measured the main amp pcb and here are the results.
I'm really surprised that there are that few blown parts:
TR536
TR538
R620

The emitter resistors gave good readings ( and almost the same as those of the working channels ), but still they are going to the trashbin.

The VR504 pot shows 0-4.2K which I think is OK ( still wondering why it should be set to max before powering up the amp for first time... I don't believe that denon or any other company makes amps that blow when the pot is turned to min ).

I do not like the readings of TR528, but seem to be the same as on the other channels... still, most likely I'm replacing it.

Now I'm starting with the pre-stage part of the amp on the other pcb.

[madan1]

Re: It just keeps blowing...

The other board looks good - no shorted resistors or transistors. I have already bought 2sc2705 + 2x sa970, so probably I'm changing them.
I also checked the traces continuity between the components and everything on the main pcb seems to be fine.

By the way is it safe to power up the unit with replaced r620, but without the main transistors?