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Old 12th April 2009, 06:57 PM   #21
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hey-Hey!!!,
What did Heyboer quote you for the job?
cheers,
Douglas
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Old 14th April 2009, 05:28 AM   #22
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Location: SAO PAULO - SP
Quote:
Originally posted by cchean
No, he has not sold them. He's waiting for a less expensive repair quote or a replacement for this output transformer. You know anyone?
Hi cchean ,

Thanks for your reply . I only asked you about the solution
that you had adopted , because just by now I have in my
service bench , a pair of McIntosh MI-75 , and one of them
has the output transformer with a short-circuit between the
negative feedback winding and the primary winding .

Such thing , rarely happens . I did consult Audio Classics
in NYC , too , and the reply was the same USD$ 600 for
the service , I did consult the brazilian exclusive dealer
of McIntosh equipments , and again the same reply ,
direct from McIntosh factory in NY , USD $ 600 .
And in my case , I have to pay the shipping and customs
fees . My client will need a truck of money !!!!

Here in Sao Paulo city , we have 3 or 4 transformers manufacturers , that can do the job and rewind an exact
replacement for the stuff , and the price is aprox. the same
( about R$ 1200 = USD$ 560 ) BUT I am afraid that the final
sound will not be exactly the same , because a Mc out transformer
has aprox. 55 ( yes !! : fifty five ) independent windings , that
are connected together in a such special way .

The only thing I can say , and perhaps it will help , is that
the MI-75 out transformer , is the same used in the
MC-60 , MC-75 and MC-275 . No relevant changes among them.

Regards ,

Carlos
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Old 3rd May 2009, 08:26 PM   #23
bobd53 is offline bobd53  United States
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Quote:
Originally posted by refference

The only thing I can say , and perhaps it will help , is that
the MI-75 out transformer , is the same used in the
MC-60 , MC-75 and MC-275 . No relevant changes among them.

I'm in the process of restoring an MI-75 and have no schematic for it so I was using one from an MC-275 as a guide during ohmmeter checks before hooking it up to the variac for the first time. I discovered one difference which, depending on application, could be significant. The MI-75's output transformer has an additional, isolated winding for the 70, 115, and 230 volt outputs instead of using the winding that's between the cathodes of the 6550s. The MI-75 specifications mention that all outputs are ungrounded, something which wouldn't be true of the MC-series amplifiers. While that's not likely to be a problem in a home system, it's something to be aware of when making the substitution.

Strangely, the search function here didn't find this thread when looking for MI-75. I found it with Google.
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Old 4th May 2009, 01:01 AM   #24
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[QUOTE]Originally posted by refference
[B]
The only thing I can say , and perhaps it will help , is that
the MI-75 out transformer , is the same used in the
MC-60 , MC-75 and MC-275 . No relevant changes among them.

Hi all ,

I apologize to all , my big mistake , the MC-60 out transformer
is different , not the same as MC-75 and MC-275 . My fault .

The MC-75 and MI-75 out transformer , has an additional primary
winding , that is connected to the 12AT7's plates .

Quote:
Originally posted by bobd53

The MI-75's output transformer has an additional, isolated winding for the 70, 115, and 230 volt outputs instead of using the winding that's between the cathodes of the 6550s. The MI-75 specifications mention that all outputs are ungrounded, something which wouldn't be true of the MC-series amplifiers. While that's not likely to be a problem in a home system, it's something to be aware of when making the substitution.
You are correct , but that is not going to be an unsolved problem

Regards ,

Carlos
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Old 30th September 2010, 11:28 PM   #25
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So, what happened?
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Old 6th October 2010, 06:12 PM   #26
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Nothing !!
Until now the problem remains unsolved !!!
The MI 75 remains a useless " lot of iron "

Carlos
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Old 6th October 2010, 07:55 PM   #27
TheGimp is offline TheGimp  United States
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Have youcontacted the original manufacturer? They are still in business.
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Old 6th October 2010, 07:59 PM   #28
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MC(2)75 output transformers seem to suffer from shorts more often.
Actually pretty strange, because generally it seldom happens with output transformers (unless it is caused by tube failure, negative bias failure or related things).
A friend of mine is a MacIntosh geek; he has a nice collection of the older types, and two MC275's. One of the MC275 is an older type, the other is a nowadays production type.
The modern one has a shorted output transformer. For a long time he has been trying to find a surplus transformer somewhere but did not find one.
As a transformer winder I offered him to look into it, also because the non standard winding technique of MacIntosh output transformers is an interesting subject (and object) in itself.
For some time my friend hesitated, very understandable, because any action would make the MacIntosh less original. Not able to find a surplus transformer however he finally gave me the green light, and one Sunday afternoon we started to dismantle the transformer by removing the tar out of the can by melting.
I knew that McInt used c-core transformers before but we dismantled an EI core transformer.
After taking the transformer home I removed the EI laminations until I was left with the coil. My plan was to remove the windings from the coil, at the same time checking how the coil was wound (primary sections, secondary sections, interleaving, winding ratios, wire gauges, a.s.o.), also in order to be able to wind a new coil resembling the dismantled one as much as possible.
It is known that McInt uses bifilar winding techniques in the primaries, which means that much is asked from wire isolation because anode (plate) windings and cathode windings lay side by side (over 400 VDC next to almost zero VDC).
In itself this can be done well when using proper wire (which McInt does), but, and this is the cause of the problem I think, the winding of the coil turned out to be a real mess.
You can start winding (bifilarly) the first layer, but in order to wind a second, third a.s.o. layer exactly the same, you must apply foils between layers. When not doing this every next layer will be wound worse (and therefore also less bifilar) because the winding plain gets worse and worse for every next layer.
I found out that there is NO ISOLATION/INTERLEAVING AT ALL in this MacIntosh output transformer, so also, and this is really very dangerous, NOT BETWEEN PRIMARY AND SECONDARY SECTIONS, so there is a voltage differential of over 400 VDC between loudspeaker connections and transformer windings ONLY ISOLATED BY THE THIN WIRE ENAMEL. I know that this isolation can withstand high voltage differentials, but in this untidy wound transformer resonances will occur because there is room between wires. In the long term the resonances can damage the wire isolation, a situation often the case with shorted (bad quality) power supply transformers. Besides, the transformer was not impregnated so there is nothing to prevent it from resonating.
At the end I knew how the transformer was wound, but when winding a new one I will apply interleaving a.s.o. for safety reasons, and also use c-cores.

Pieter
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Old 6th October 2010, 08:38 PM   #29
cerrem is offline cerrem  United States
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I have wound the MC-75 output transformers.....
They are TRI-Filar wound not the BI-Filar type found in other Macs........
This is to lower the B+ to the bootstrapped driver....
The biggest pain is removing the transformers from the canister......
There are many ways to do it.....
The proper wire needed to do the job is not an "off the shelf" type of wire....It's expensive wire and is a special order and the min buy is HUGE...
By weary of some rewinders....I have seen some winders cheat by using regular wire and some interleaving added that is not in the original design... They figure once it's re-potted who will ever figure know...

Best Regards
Chris
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Old 6th October 2010, 09:16 PM   #30
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Chris,

The transformer I dismantled was bifilar wound, no separate winding for a driver (but wether it is bifilar wound or trifilar wound does not make a difference as far as safety is concerned...).
For magnet wire there are two readily available qualities, grade 1 and grade 2.
Grade 1 has a thinner and solderable enamel, grade 2 has a thicker enamel which is very high temperature resistant (used for motor coils a.s.o.) and can only be removed by scraping.
The grade 2 is suited for bifilar winding with high voltage potentials.
Interleaving or not interleaving is not a point of discussion for me; it is merely a matter of safety.
I would not be surprised if the older output transformers (with c-cores) are interleaved. The transformer I dismantled was "cheaply" wound and not in line with MacIntosh's reputation; actually a real shame, maybe the stuff comes from some Chinese supplier since MacIntosh is not a US business anymore (when tested by some consumer safety board the amplifier would fail because the output transformer does not meet standard safety precautions).

Pieter
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