Help with Denon Power AMP needed!

[dezihh]

Hi mates,

may I ask you for help?

I am having trouble getting my Denon AVR-4311 to work again. The power amps on the left are defective.
In the meantime I've replaced the both Darlington transistors, a terminal resistor and some diodes. I have marked the parts that have already been replaced with a red circle.

To me it looks like the working point at the base is shifted.
At the base of the PNP Darlington (DHCT-A3) there is a voltage of -4.8V
At the base of the NPN Darlington (DHCT-C3) there is a voltage of -2V
I do not fully understand the circuit and would appreciate a good hint as to where I can look further.

Small note: The board is on the workbench and is operated directly on connectors N502 and N503. "-FILTERB" is connected to "-HIGHB". "RE +" and "Thermal" are not connected.

[ambrosia heart]

Q508 Q509 should be replaced in pair even 1 damaged.
R524 and R525 0.27ohm resistor should be replaced in
pair even only 1 damaged

constant current source and current mirror
may be normal depending on what type of
protect your amp detected.   If DC detected,
they must be checked.    If ASO, NOT needed. 

[madires]

Have the parts you've replaced failed or are they OK? When you have no idea about the cause of the issue then follow the input signal and check each stage step by step.

[mzacharias]

Watch out for those diodes and zener diodes across the bases of the output devices. The commonly go bad when the amp fails - sometimes only one or two of them.

[dezihh]

Hi together,

thank you very much for your hints! It took some time, now defektive transistors, diodes and resistors  are replaced.
However, I had still problems, with DC on output and I had no input amplification.
Today I found my (last) problem: By checking the Diodes with a DVM I found a shortcut for the Diodes D101 and D102.
For testing purposes I replaced them with two 1N4007.
Now I found out, that this was a bad idea.  The determined shortcut for those 2 Diodes  seem to be working as designed
After veryfing this by doing back a shotcut instead of using a 1N4007 for D101 and D102, DC went away from output and amplification is back again, too!
In other words: Amp is running again 
But there is one problem left: I have thrown away the original diodes, because I thougt the where defective.
Now I have to find an replacement for the original D101 and 102. In documentation, there is written the type "RM73B--0R0KT" for this diode.
This diode is mentiond in the net, but it I don't know exactly the funtion, even I can not find a replcement type.
Could you help me here again, with an idea, how to replace it right?

Thank you for your help!

[madires]

RM73B--0R0KT seems to be a 0 Ohms SMD resistor.

[dezihh]

Oh, would make sense! But why it is inside the diagram a diode?

[madires]

What's printed on the PCB?

[dezihh]

Good morning!

Regarding your question, attached you find the pcb. For my point of view, I still find no reason, why it is designed as is. You see on the pcb this resistor has no "bridge" function.
Any idea?

[madires]

Both are in series. So the original design intended a voltage drop, e.g. for bias, but later it was changed to a direct connection. Maybe the schematic can give you a hint.