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Old 8th February 2008, 04:20 AM   #21
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Quote:
Originally posted by BrianEno
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.
Early Threshold gear at 32 years of age definitely needs
inspection of all the electrolytics, connectors, and potentiometers.
Bias and offset should be checked and possibly adjusted. Only
rarely are semiconductors or resistors a problem. Transformers
will show ther age usually with mechanical noise, but I haven't
seen much of that.

Product which is 15 years old getting toward the borderline, but
is not presumed to be defective due to age. PL product of that
age is not yet showing a high failure rate, so service issues tend
to be random.
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Old 8th February 2008, 08:06 AM   #22
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In mine two ML-2's were allready 12 "new" transistors ST 2N5886/5884 from 1999. So they can't be older then 7 years now.
I have the "E-Core" model so they swapped the transistors after approx. 20 years of service.
My specialist now services a lot of older Krells, Thresholds and Levinson's (he services mine ML-2's from the begining) he now suggests to use Onsemi MJ 21195/21196 transistors.
I have substituded all the caps in the ML-2 (two big cans: Cornell Dubilier: 4 x DCMC154U050DC2B, four smaller ones: 8 x DCMC163U050AB2B, ordered at www.newark.com) , also the small ones (Spragues) on the Regulator- and Audiocard and all the one turn trimmers (12) nad a couple of transistors and resistors.
It costed me 1400 USD for the pair in total.
Do not know how important it is to get matched output- and drivertransistors in a Class A Pushpull design. Maybe grey or Nelson can comment om that. If you do not really need them then the transistors only cost a few bucks per piece.
Find a Levinson/Threshold specialist to do it for you that's my advice.
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Old 8th February 2008, 11:41 AM   #23
AMV8 is offline AMV8  United Kingdom
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Hi

It is important to have matched transistors in the output and driver stages. If not matched then one will probably hog most of the current and starve the others. You can normally tell if this is happening by carefully feeling the heatsink temperature. The temperature on the heatsink around each output device would be the same if they are all matched. If not matched you will find different tempertures at each device - not good and not safe as the one hogging the current will probably blow up eventually.

Don
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Old 8th February 2008, 09:26 PM   #24
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There are numerous factors involved in the matching of transistors. For bipolars (i.e. the majority of the devices in the ML-2), the first thing you would look at is the Beta (aka Hfe) which means current gain. While you can probably get away with using random parts in the front end, it will be more of a problem as you get to the output stage. Unfortunately, these are also the most expensive devices. How many are required to get a good match is open to question. It depends on how close a match you're looking for, whether the transistors are from the same batch, and at how many points you intend to match the devices.
To an extent you can offset current differences with larger and larger emitter resistor values, but that's not an elegant solution. Matching devices not only assures that the transistors share the load equally, but it lowers distortion--something that emitter resistors don't really address.
Incidentally, there's a somewhat more accurate way to check the output devices...measure the voltage drop across the emitter resistor with a meter. The actual voltage will vary with the value of the resistor, the current running through the device, and with the temperature of the amp (has the thing reached its final operating temperature?), but to the extent that you're looking for equal sharing of current you can simply look to see if the voltages are fairly close to one another.

Grey
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Old 13th February 2008, 02:23 PM   #25
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Hallo
I am the source to all the ML-2 schematics at www.marklev.com so I can provide you with the files if necessary. I still have them on a disk somewhere.
I used to own a pair of these amplifiers. I never had problems with these knife contacts, but no doubt others can. Remember to thighten the bolts though.
My problems with these were (besides they heated my apartment):
- A defect input p-channel JFET. Got a new from Madrigal eventually. Before that I had to use an AH5012 from National Semi (http://www.national.com/ds/AH/AH5010.pdf), wired in a provisional way. Borbely used these in his earlist JFET input attempts. It worked well.
- Someone had changed output transistors for the first owner, and these were not good.
- "Someone", se above, had misadjusted bias and also regulator voltages. The voltages for all amplifier stages are regulated, and too much voltage was placed over the amplifier and too little over the pass transistors. And too much bias, making it running very hot.
- Defect Airpax circuit breaker. Be sure to operate it quickly to avoid arching. The former owner of mine thought he could extend their life with careful smooth operation not realising that the opposite was true. I had used my money on the amplifiers then, and had to carefully bend the circuit breakers open to carefully - very carefully - sand the contacts.

RK
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Old 13th February 2008, 03:55 PM   #26
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Default Schematics

Quote:
I am the source to all the ML-2 schematics at www.marklev.com so I can provide you with the files if necessary. I still have them on a disk somewhere.
Have a ML-3 stero-amp in reserve for the ML-2's. Do you have by any chance the schematic for that amp too?

You can still order the Airpax threepole connected magnetic circuitbreaker for the ML-2. It's the partno. 203-222-4984-1.
Onlinecomponents does have couple. Pretty expensive items though.

The real question here is what kind of outputtransistor one should take (all the ML-2 owners and maybe Bendini 25/25 or Hiraga Class). The original in case of a ML-2: 2N5884/5886 or 2N5686/5684 high currentdevices as I recall or maybe "newer" ones like the MJ15025/15026 or 21195/21196 combo somewhat lesser currentcapability but a higher voltagerating. There are probably much more parameters that can make a good choice here the only then Voltage/Current tradeoff.
I'm curious what specialists have to say about their choice of semidevices.
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Old 13th February 2008, 08:37 PM   #27
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Hello
Sorry, no ML-3 schematics, I am afraid! I sold the ML-2s in 1994 or so, so I have no need for the switches.
Good as they were they were too impractical for me. Running too hot. Too little output swing for the ESL-63 really, but perfect for the '57.
I have a bunch of the 2N5884/5886 transistors. Had them in case of a catastrophy. I believe they were the right transistors as not so much voltage was necessary, but they had great current capacity. And slow transistors are not such a big problem when always running in class A. So I would go for those, really. But that is my opinion. Maybe better suited transistors exists, even in TO3 case.

Rolv-Karsten
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Old 13th February 2008, 09:13 PM   #28
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Putting a faster device into a circuit designed for a slower one can give rise to stability problems. Approach this with caution; it may work, or it may not.

Grey
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Old 14th February 2008, 06:46 AM   #29
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This is correct. Something one in the original design thought of as not possible to be the culpritt for problems, might suddenly becode just that.
I would recommend to stick with the original 2N5884/5886s.
RK
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Old 18th February 2008, 04:04 PM   #30
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Hi ,

I have one pair ML-2 with EI transformer inside .

It should use Motorola 2N5684/2N5686 or 2N5884/5886 inside ?

When i bought this unit last year , they had other transistor model inside .

Then i asked a technician to service the units as factory standard (as in service manual on marklev.com) .
Now it already have Motorola 2N5684/2N5686 inside .

Thanks...
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