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huy801 11th December 2007 09:10 PM

Mark Levinson ML-2 Problems
I have a pair of old-school Mark Levinson ML-2 power amplifiers. They've worked very well since I've owned them. However, now one of the amps runs much warmer than the other and it automatically turns off after about one minute of use.

Have any of you Class-A amplifier owners come across this before, what was your fix or what should I look at when I open it up? What are the typical culprits of this type of problem. What are the typical culprits that plague the Class-A amps?

I took the amp to a local electronics repair shop and the over the counter estimate was 700-1000 for repair, they didn't even turn it on or hook it up to any equipment . That's out of the question, since they see the name they assume they can charge a whole lot of money.

Any help would be much appreciated.

gmphadte 12th December 2007 03:17 AM

Resolder all of it.

Gajanan Phadte

GRollins 12th December 2007 04:22 AM

First off, does the channel in question still play?
Second, I'd suggest checking some of the basic stuff, like rail voltages, bias, etc. The schematics are, or were, available at if you don't already have them.


Bigred 12th December 2007 04:36 AM

I believe I have ML-2 service info kicking around. Give us a shout if I need to go searching.

huy801 14th December 2007 04:32 PM

Big Red,

Any information would be greatly appreciated, since I am a new member I cannot send you emails. My personal email addy is


RobertS61 29th January 2008 09:05 AM

Hi Huy,

Could be loose/not tightly fitted connectors for the six powermodules.
These are all forks that glide into eachother, like the Regulator- and Frontendcard inside the amp. Sometimes they don't make a good contact and the regulatorcard supplies to high voltages (should be 27 Volts for the Torodial model ML-2 and a tad less 26,7 Volts for the E-model tran. At every powermodule there are thermalsensors that can shut down your amp by triggering the three pole Airpax circuitbreaker (your on/off switch) in case of overheating. If that doesn't work the Crowbar protection on your Regulatorcard (the card closest to the front) will trigger through an overvoltage status, protect the audiocircuit by decoupling it and relay the current to the big black resistor on it and blow the AC fuse. Never put a to high value in the placeholder because it will burn that particurlar resistor.
I have two of these amps, another friend has also a pair and I have an acquitance who has eight of them in a HQD system (Hartley sub & Quad and Decca stacked). We all experience the same problems and you should check them thoroughally by a experienced technician. Mine were repaired by a former Goldmund designer and he soldered 36 silver/teflon wires to get a better connection to the powermodules then the connectors and replaced a couple of dried out little caps. I replaced the original 30 years old blue Sprague's large cans with Cornell Dubilier.
These amps get really hot (400 Watts idling) and need a lot of attention/service to keep them happy. There like old sportscars that are a fun to drive but you have to maintain them. I was never a fan of so called full fledged Class A amps but I'm a believer now thanks to listening to them and the people (like Grey) on this forum.
Have fun with them!

AMV8 29th January 2008 04:07 PM

I have a pair of these as well.

Does it still play music for the one minute it is on. Then I suggesdt as follows;

1 If you have not changed the power supply caps you need to.

2 Check the rail voltages

3 Check the temperature of each output device - carefully. One may be different from thr others.

4 Check all mechanical connections.

5 Look for a damaged or broken soldered joint.

Let us know the above and then we can take it from there. There are a lot of skilled electronics personnel on this web site who will be keen to help on this particular amp.


GRollins 30th January 2008 12:01 AM

I used to be really excited about the idea of plug-in cards. I thought that if they were gold plated they should work forever. They don't. Oh well, another wonderful idea down the drain. It looked so good on paper, though...
These days I solder things.


EDIT: Oh, and before I forget, seems to be up and running at the moment. If you need schematics, now's the time to get them because that site's been erratic for the last month or two.

RobertS61 30th January 2008 01:11 PM

Grey is right. There were never any updates anyway for the ML-2 as far as I know.
The No. 20 it's succesor was updated twice .5 and .6 version.

The ML-2 uses for the powermodules so called Elco pins:
Elco and Edac connectors have forked pins that meet at a right angle and are pushed together when mating. They will wear out if used heavily.
The AP3, 4 en 4 Audio PC cards still used those Elco connectors.

My problems were situated with the powermodules, the six modules that contain the heatsinks (the first two counted from the frontplate are no outputstages but work in tandem with the Regulatorcard inside) and not the two PC cards inside.

My friend with the eight ML-2's bends the connectors lightly together indivually every time he pulls out a module. Do not now if that's a wise practice because they could be breaking from the print or something.

As a improvement the No. 20 uses gas-tight, mil-spec, Varicon edge connectors with the powermodules which disconnect with a 'plop'. The better connector of the two it seems to me.

I had a few products like Rowland, Levinson and Wadia that were modular and the manufacturer always promised as a legitimation for initial higher cost, a cost effective upgrade scheme. In reality they did not upgrade like the ML-2 or these updates were very expensive and yielded always some extra mainboard work.

Maybe someone has better experiences than I have in this compartment.

I have now idea what the real lifespan (without renewing every part inside the amp) of units like the ML-2 or other hotrunners like pure Class A Passdesigns is.

I now replaced the main caps, some small dried out ones on the APcard and the powertransistors in 1993. Maybe Grey or Nelson Pass himself can give a glue/hint what he expects for a lifespan for units like the Threshold SA series or Pass XA series when with normal daily household use and properly serviced without changing hardware like the toroids and mainboard.

GRollins 30th January 2008 02:48 PM

The expected life of a component varies widely. On paper, everything in a solid state amp should last forever. In practice the electrolytic caps tend to die, but even this is highly variable. I've had tube pieces forty and fifty years old that functioned perfectly, even with the original caps.
In the case of the ML-2 you need to provide lots of ventilation. You frequently see pictures of them stacked on top of one another. It looks sexy, but it's not a good idea because it forces the one on top to run at considerably elevated temperatures. This will cause the top amp to age much more quickly than the bottom one. Likewise, don't run them inside a cabinet without forced air cooling. They generate too much heat to even think about doing so.
There's a cheap line of electronics called Audio Source sold locally. A buddy of mine brought over two dead ones that had been stacked inside a cabinet. Just by looking at the circuit boards, I was able to tell him which unit had been on top; the PCBs were burned all over, not just at the output devices. And that's with a class B output stage--not class A. Incidentally, they only lasted about six months from date of purchase in that installation, so that gives you an idea of how quickly you can kill a piece of equipment if you don't treat it right.
Please, be kind to your ML-2s. Let them breathe.


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