Mark Levinson 20.6 Failing

Beirut is a big city. There must be competent technicians there that can troubleshoot and fix electronics! Of the pair is one still working ok? That would be a big help in repairing without a schematic the defective one!

"Beirut is a big city..."

This is really helpful to someone who lives there, but that aside, the chassis in those amps are quite packed and you still need to disassemble a good bit to even do a proper visual inspection. Even then, you'll need some test equipment to determine the root cause of the issue. Not a DIY job, in my opinion. Those amps are definitely deserve a proper diagnostics and repair. Contact ML and see if they recommend a service center in your general vicinity. 

I believe Richard Kreymeyer is still at Mark Levinson in Service?  He can tell you exactly what’s wrong or what you’re possibly going to need, but my guess would be capacitors.  The fact that the amps weren’t used extensively prior to you getting them is actually worse. Had they been left on all the time and used more frequently, they would likely have gone on and worked just fine.

Call Levinson as the prior post suggested and see if Richard is still there, but anyone there can help.  All in all, plan on a costly repair.

When I look at robertjermans comment, what I seem him saying is that there shouldn't be a need so ship the amps anywhere to have them fixed.  It is not like there is some magic black box inside that needs to be decoded, but rather an amplifier, the function of which is well understood by a competent tech, and even without a circuit diagram or operating specs, with a second working amplifier a good tech should be able to isolate and repair the non-working amplifier.  

Given Beirut has 1-2 million people and is a modern major metropolitan area, with major universities, there should be at least dozens of techs who could easily diagnose and make the repairs.  It is not as if the OP lives in a town of 50 with no electricity, running water, or techs.  

I have had the 20.6’s for over twenty years and I had the exact same problem.

If the amp is on and the LED is red, that means the voltage rails have power. Cold heat sinks mean the current source from the positive rail through the driver stage has been cut off and the power resistors are thereby starved and do not conduct the bias current.

If you are confident with electronics, remove the AP-5 card. On the upper left of the rear board, and snugged next to a red capacitor, you will see a small transistor Q115 with a white spacer on the center pin. If that transistor is defective -- or it has a bad solder connection -- it cuts the power through the rail because its emitter is connected to the common bases of the two silver can transistors next to it, which are the constant current source transistors, and pulls them on to turn on the current source.

In order to access the solder side of that board you have to remove the rear heat sink off the AP-5 card (and you will need new mica insulators before reinstalling).

Try first to touch up the solder joints of the transistor Q115, especially the base (center) pad. Be careful, those pads are small. Put the heatsink back on the board, and reinstall the card. If the amp works, remove the AP-5 card and replace the mica insulators with new ones and goop. Or you can order a replacement transistor (2N5401) from Mouser and replace the original since you have the board open anyway.

Good luck.

hello to all : me even possessor of mark levinson 20.6, (card AP5.5) the concern is hardly solvable ! indeed we know almost all, that the capacitors are not a problem if we accept all their limits and especially their limit of life. The most worrying point it seems to me, is of course the Teflon card on which is soldered the components! Indeed the tracks over time with the heat expand, and end up providing micro cut, those of course cause cuts in the sound. So what is the reliable solution? if it is not to agree that there is not, if it is not to accept the relative now of these marvellous devices! yes marvellous devices I say well, because it does not seem to me to find an equivalence to approximately 5000 or 6000 dollars. There is well the last cards AP6, which are not in Teflon, but on the one hand they are extremely rare and apparently of other concerns also come to be grafted.
Frankly, it's a pity that Mark Levinson didn't look more deeply into the problem ! because these devices are still worth it even now
cordially to the brotherhood