It just keeps blowing...

[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?

[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.

[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"

[budm]

Re: It just keeps blowing...

What is the yellow line for?

[keeney123]

Re: It just keeps blowing...

This is my last post. Promise!

[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.

[petehall347]

Re: It just keeps blowing...

yes a transistor can behave like that .

[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?

[keeney123]

Re: It just keeps blowing...

The voltage on the cap will dump and charge so this is a oscillation.


This part of my statement is incorrect. Once the cap charges it will stay charge because of the DC source. With that said I realize to give an accurate analysis of all the detail of this circuitry as BudM I would have to do a detail study. My choice is to do other things that I enjoy like art and helping people what I can. At this time I feel my contribution would do more harm than good so with that I bid BadCaps Forum farewell.

[rievax_60]

Re: It just keeps blowing...

Post 9 explain what post 3 is.
This one is better:
https://cdn.badcaps-static.com/pdfs/...8f9437ce2d.pdf

http://www.tedpavlic.com/teaching/osu/

I had missed post #9. Post #3 is very misleading though.

[budm]

Re: It just keeps blowing...

Post 9 explain what post 3 is.
This one is better:
https://cdn.badcaps-static.com/pdfs/...8f9437ce2d.pdf

http://www.tedpavlic.com/teaching/osu/

[rievax_60]

Re: It just keeps blowing...

What capacitor and what Q2?

The capacitor with the reference "C" and the transistor with the reference "Q2" in the diagram in post #3 in this link, http://www.edaboard.com/thread237660.html

[budm]

Re: It just keeps blowing...

In post #3 in that link, the diagram and explanation are incorrect. The capacitor needs to connect to the Emitter of Q2.

What capacitor and what Q2?

[rievax_60]

Re: It just keeps blowing...

When that 100 Ohms is in place then you will have understand how that will affect the bias scheme of all the transistors after that, I do not have the time to explain. All you have to do is to calculate what the Vdrops will be on each components.
C524 conducting, what do mean by that? It is part of the Boostrap circuit, its jobs has nothing to do with DC Voltage when the failed transistor has 100 Ohms is in place of the transistor, it has to do with the impedance of the circuit, that cap can be left out and the circuit will still go out of balance.

bootstrapping principle.

http://www.edaboard.com/thread237660.html

In post #3 in that link, the diagram and explanation are incorrect. The capacitor needs to connect to the Emitter of Q2.

[keeney123]

Re: It just keeps blowing...

When that 100 Ohms is in place then you will have understand how that will affect the bias scheme of all the transistors after that, I do not have the time to explain. All you have to do is to calculate what the Vdrops will be on each components.
C524 conducting, what do mean by that? It is part of the Boostrap circuit, its jobs has nothing to do with DC Voltage when the failed transistor has 100 Ohms is in place of the transistor, it has to do with the impedance of the circuit, that cap can be left out and the circuit will still go out of balance.

bootstrapping principle.

http://www.edaboard.com/thread237660.html

As I said I was exploring your idea. C524 conducts on power up with your leaky cap. We are not talking about a different circuit then the one we have. We are only talking about one moment in time when the components fail. I am not asking to explain in detail the circuitry. I don't have the time to figure that all out as there are many things happening at the same time and many things happen in the future time with this leaky transistor circuit. If I had nothing to do I would figure it out. I am sorry I wasted your time. I will not ask you any more questions.

[keeney123]

Re: It just keeps blowing...

here you go this should help anyone learning ....

I am very unconventional how I learn. I learn abstractly. It is the easiest way for me to learn. Thank you your time in providing this and I am sure any technicians and future technicians will find this very useful.

[rievax_60]

Re: It just keeps blowing...

NB = non-burning ( which seems not to be always the case )
R620 - Carbon film 330 ohm 1/4W(NB)
R622,624 - Carbon film 1 ohm 1/4W(NB)


I have not checked any of the resistors with e multimeter... just the transistors. I checked the resistors only visually and cut R620 so that I can close the hood.
When the parts arrive, I will disassemble the boards and test everything.

By the way, could faulty emitter resistors cause this damage?
Now I remember that I got good ohm readings on them and decided not to change them.

There are many possible faults that can be present in the idle current setting circuit that will cause the output stage to be damaged at power on. The idle current setting circuit can be checked independently or use the precaution mentioned in post 7.

[budm]

Re: It just keeps blowing...

When that 100 Ohms is in place then you will have understand how that will affect the bias scheme of all the transistors after that, I do not have the time to explain. All you have to do is to calculate what the Vdrops will be on each components.
C524 conducting, what do mean by that? It is part of the Boostrap circuit, its jobs has nothing to do with DC Voltage when the failed transistor has 100 Ohms is in place of the transistor, it has to do with the impedance of the circuit, that cap can be left out and the circuit will still go out of balance.

bootstrapping principle.

http://www.edaboard.com/thread237660.html

[keeney123]

Re: It just keeps blowing...

TR528 is part of the active Bias circuit for the output section, and it is conducting already to maintain bias.
What do you that what that TR528 is for?
I guess you did not read those links I provided.
More:
http://www.eetimes.com/document.asp?doc_id=1272214

It is very obvious that it is a bias circuit I already knew that. We are talking about your theory of TR828 leaking, not a normally operating circuit. We now have a 100 ohm transistor with C524 conducting so the bias circuit is a mute point as a pond power up it is getting shorted out by the capacitor. At some time when enough voltage is built up of the cap. for the bias to take effect the resistor bias and zener and diodes conduct. The voltage on the cap will dump and charge so this is a oscillation. I have not tried to figure out the exact voltage when all of this will happen if TR828 is indeed leaking because I would not be able to figure out exactly what resistance it would have.
All I was asking was if you could draw a path for the leakage current which you had suggested and then I asked would not C542 cap be the major path of conduction with the leaky transistor as the power was applied.I will not be working on this amplifier equipment so going into a detailed understanding of every part of every type of amplifier circuits would not be worth it as I would forget that knowledge in a very short time. I find somethings interesting and wanted to explore your theory. I know how these device work, meaning how each component works and understanding how they work together is a matter of study. For me I have a very difficult time at looking at all these crossing lines that do not connect or things that are omitted and the schematic broken in pieces. That is something I was born with. Instead of me trying to go through all that trying to see the leakage path I ask you as you had already saw it. I also thought in the process of you showing me this you would also be showing anyone that was interested in this thread. Also, your links are available for any young technician wishing to learn the operation of these circuits.

[petehall347]

Re: It just keeps blowing...

here you go this should help anyone learning ....