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Marantz 250M Repair
Marantz 250M Repair
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Old 8th November 2017, 06:38 PM   #61
PB2 is offline PB2  United States
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Quote:
Originally Posted by Burnedfingers View Post
Sir, what I have seen so far is an attempt by you to elevate yourself to a level in which you are trying to make yourself superior to the designer of this amplifier. My last 25 years has had me in the field actually using a majority of the commercial amplifiers that are available in todays market.
I have found none to date that are incapable of some type of damage associated with a full power deliberate short into a dummy load or a shorted speaker load.
Your use of Sir is obviously fake, since the majority of your posts are filled with wise
*** sarcasm for anyone you view as not being in your group.

You claim "is an attempt by you to elevate yourself to a level in which you are trying
to make yourself superior to the designer of this amplifier"
Or I could simply be factually correct. The evidence of many people reporting failures is
in my favor. It is possible that the design is perfect when new but the aged caps have
caused them to be more sensitive to shorts, but in fact the simulations show that this amp
is not designed very well. So sorry for your hero designers.

You claim "I have found none to date that are incapable of some type of damage associated
with a full power deliberate short into a dummy load or a shorted speaker load."
Really? So you actually drive all those amps to full power and short them? Or are you
just making this up as you go, thinking you have superior knowledge of something that
you've never tried? After all you stated a few posts back "Well I guess shear stupidity
would dictate that I attempt such a moronic feat."
It seems that you are making this up as you go and have some need to feel superior.
Why don't you stay out of threads where your only intent is to disrupt members you
don't like, rather than make this crap up.
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Old 8th November 2017, 10:24 PM   #62
Audio1Man is offline Audio1Man  United States
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Marantz 250M Repair
As good engineer it is not easy to judge what happened 30-40 years ago about what other limitations were then, however many others know it all.

I know the engineer that designed the 250s and he is a real professional. In the early 1970s Silicon power devices were work in process for RCA, Motorola and others. I designed power amplifiers also and used several hundred thousand TO3 parts. They were slow and had lots of failures. Isb was in its early understanding and the process of soldering the die to the base of the TO3 needed to be worked out for production. I built a test fixture to check all incoming TO3 parts. It 1.2 amps @ 80 volts the transistor and I measured the case temperature (Mr. Jack Sondermeyer of RCA then said You did what! and I said I dont want the amplifiers to fail in the field.)

The BAD PARTS failed by 40-50 degrees or sooner. They had air pockets under the die and the yields were only 50-60%. As the time went by the die bond was resolved and the Hometaxial were replaced with Epibase.

Many repair guys add new faster parts and never check or understand about phase margin / stability, voltage breakdown, Isb or other parameters.
Duke
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Old 8th November 2017, 11:21 PM   #63
djk is offline djk
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Thank you Audio1Man, a very good post.

"They were slow"

But some of those amplifiers really sounded good, despite their 800Khz outputs (2N6259).

Speaking ill of the dead, if Bongiorno had simply said "As (a) good engineer it is not easy to judge what happened 30-40 years ago about what other limitations were then," All would have been fine. However, his "know it all" attitude told me his basic character.

It would seem that I received the benefit of your 100% Isb strategy when I started buying production quantities of TO-3 parts around 1977 or so, using a 100% Isb tested 60V/3A Motorola part. Thank You.
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Old 9th November 2017, 02:13 AM   #64
anatech is offline anatech  Canada
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Marantz 250M Repair
Guys,
There is nothing wrong with the design of these amps. I have repaired them for literally decades and never had a problem. They don't require modifications or fixes. Just do a proper service and they will be just fine.

Marantz typically over designs the output section by about 25% over what is needed. However, if you have an older amplifier, make sure you replace the decoupling capacitors as they can go open over the years - causing oscillation (big surprise here).

There are some silver/grey axial capacitors used in these amplifiers. Unless they are bad (for sure), do not replace them. They are wet tantalum capacitors and are extremely expensive to replace. They have a higher CV than most capacitors, and they are superior in characteristics to anything else for an electrolytic capacitor. These are about opposite to the solid tantalum capacitors that have horrible characteristics and prone to shorting.

Anyway, take a breath, work off copies of the schematic and board layout and do your work carefully. Do match the diff pair precisely, and match the drivers and outputs best you can. MJ2119x output transistors are the best thing to use.

-Chris
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Old 9th November 2017, 11:16 PM   #65
djk is offline djk
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" causing oscillation (big surprise here)."

I've had the 0.1F Zobel caps go open on old Marantz gear (leading to the destruction of the output/driver stages).
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Old 10th November 2017, 01:37 AM   #66
anatech is offline anatech  Canada
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Marantz 250M Repair
Exactly.

Impressive heat when the amp is a 510. But, the other capacitors are almost as important in keeping a lid on things.

-Chris
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Old 23rd July 2018, 02:03 AM   #67
murillollirum is offline murillollirum  Brazil
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Just thought useful to share. https://www.google.com.br/url?sa=t&s...f0kAcSbUQ5s5ej
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Old 23rd July 2018, 03:09 AM   #68
anatech is offline anatech  Canada
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Marantz 250M Repair
Hi murillollirum,
Excellent! Somewhere I have an original of that bulletin. This held true for all their amplifiers, you were expected to install matched transistors. Installing transistors you matched in-house was okay as long as you could show they agreed with the factory matching. Another thing they desired was replacing the outputs on that one channel as a set, rather than just the couple that actually failed completely. Always replace until you hit good transistors, then one level back from that. This was to replace parts that were over stressed but had not completely failed - yet.

I had really good teachers at Marantz. The same could be said for a few other brands like Yamaha and Denon. The engineering staff at Carver and McIntosh were also very giving with information. It's really too bad that those days seem to be over. We really need that industry support these days.

-Chris
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