cchean said:
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
refference said:
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.
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
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
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
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
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
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
If you go to the official Mcintosh website and choose the mc2000 mono amps you will see a nice large picture of the Mac transformer before potting. I immediately made note that there was NO interleaving insulation. You can also see the hurried haphazard disoriented windings. Any DIY or refurbisher would choose to take much more time, there free time, and neatly lay each layer on the winder bobbin and even stress as you go.
I have wound several huge commercial transformers and motors. I know that to make money you must let the winding machine go and just record the number of turns. On large motors the windings are insulated on each turn with mica and nomex and each layer coated.
I can not see why you would have any real problem if you have a current unit to pattern from.
I also thought that the transformers were 4-5 filar, one for each output ohm value of 2,4,8,16, something like 50 for commercial applications or telephony.
Like was stated earlier please publish any data you record from the transformer for some of us in the future. Also the number and thickness of laminations and deminsions.
Have fun it is a worthwhile endeavor Tad
I have wound several huge commercial transformers and motors. I know that to make money you must let the winding machine go and just record the number of turns. On large motors the windings are insulated on each turn with mica and nomex and each layer coated.
I can not see why you would have any real problem if you have a current unit to pattern from.
I also thought that the transformers were 4-5 filar, one for each output ohm value of 2,4,8,16, something like 50 for commercial applications or telephony.
Like was stated earlier please publish any data you record from the transformer for some of us in the future. Also the number and thickness of laminations and deminsions.
Have fun it is a worthwhile endeavor Tad
The older original MC-75's used C-core transformers made by Westinghouse and were not interleaved between the primary and secondaries. The original MC-75's were Tri-Filar wound....There is a technique to wind these quickly and clean layered...This is to keep the leakage inductance at a low level needed for the bandwidth...as for the winding capacitance,this does not present a major issue with such low plate load used in this circuit, the effective plate load being 1/4 the full windings ...
There is no a safety concern with the output transformer winding since the secondary winding has a common referenced to chassis Ground....just like in any traditional transformer if it were to short Primary to Secondary,this would pass to ground and pop the fuses...
The grade of the wire enamel is not the issue...it must be a certain thickness to meet the agency requirements of the day....thats why the original transformers used QUAD FORMVAR enamel coating.... It would still pass for todays agency requirements..I know since I consult in this industry for these matters for worldwide distribution as well doing EMC testing and certifications..
The only time the secondary was sometimes lifted "floated" off of ground is when the amp was being used as a driver for the cutting heads output on the octal plug typically...and the ground terminated at the cutter end or not grounded at all...depending on the set-up...If secondary is floated..it presents no voltage potential for breakdown...but could still fail due to over-current buring of the wire...
The C-core have a much different inductance curve than the EI laminations...the later re-issue amps used the EI core made by an outside vendor from Chicago and I agree with you they are not impressive...thats why they are for sale all the time...
I am hesistant to put on the internet any of my data and findings....I use to share a lot at one time...the problem is the few A-holes who use this info to profit from and start companies using my R&D work....
Chris
There is no a safety concern with the output transformer winding since the secondary winding has a common referenced to chassis Ground....just like in any traditional transformer if it were to short Primary to Secondary,this would pass to ground and pop the fuses...
The grade of the wire enamel is not the issue...it must be a certain thickness to meet the agency requirements of the day....thats why the original transformers used QUAD FORMVAR enamel coating.... It would still pass for todays agency requirements..I know since I consult in this industry for these matters for worldwide distribution as well doing EMC testing and certifications..
The only time the secondary was sometimes lifted "floated" off of ground is when the amp was being used as a driver for the cutting heads output on the octal plug typically...and the ground terminated at the cutter end or not grounded at all...depending on the set-up...If secondary is floated..it presents no voltage potential for breakdown...but could still fail due to over-current buring of the wire...
The C-core have a much different inductance curve than the EI laminations...the later re-issue amps used the EI core made by an outside vendor from Chicago and I agree with you they are not impressive...thats why they are for sale all the time...
I am hesistant to put on the internet any of my data and findings....I use to share a lot at one time...the problem is the few A-holes who use this info to profit from and start companies using my R&D work....
Chris
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Chris,
Agreed that a grounded secondary will be safe by popping the fuses in case of a short.
However I feel that the transformers fall short by having improper isolation. The transformer should not fail during normal use; with improved isolation by applying interleaving and clean layering these shorts would never occur as long as there are no "outside" causes like mentioned before. Transformers are very reliable components when wound properly.
Pieter
Agreed that a grounded secondary will be safe by popping the fuses in case of a short.
However I feel that the transformers fall short by having improper isolation. The transformer should not fail during normal use; with improved isolation by applying interleaving and clean layering these shorts would never occur as long as there are no "outside" causes like mentioned before. Transformers are very reliable components when wound properly.
Pieter
The McIntosh design is also known as "Twin Coupled", where the cathodes are fed through a transformer winding as well. Under most circumstances, this would heat a transformer, so it would need to be huge. You may want to Google around for Norman Crowhurst's Twin Coupled design information, and Audio x press covered it as well.
Oh yes , of course !! The sales man and McIntosh's distributor in Brasil is my client and a good friend , we know almost everybody on the Mc Intosh factory .
This is not the problem , the problem is cost , the xformer is very heavy and the
shipping Brasil - USA - Brasil + customs fees + duty taxes , make things unfeasible .
We are looking for a used MI - 75 , or we are thinking about the possibility to make a domestic clone , what is very dificult , because even you take care on every detail , the sound will not be the same , for shure .
Mc Intosh output xformers are prone to short circuit between windings , because they do not have insulation between layers .
I beg to differ... Every bad Mac transformer that I ever got into (power, output) that had shorted windings was faulty because the terminations where the enameled wire was soldered to the cloth wires were shorted. IE. the solder terminations on 2 or more wires were placed too close together and through movement or coming loose, shorted. I have never seen a Mac transformer have shorts because of inside the actual windings themselves.
Of course, this doesn't cover transformers that have been abused; weather, or output tube shorts, etc.
Nothing special about the winding technique of old Mac unity coupled, except that a lot of labor was required. That's why the new Mac stuff is not wound like the classic Mac tube stuff... It would be cost prohibitive.
As far as I know, all the classic Mac unity coupled transformers were wound like the old original Williamson spec output transformers on C-cores. IE. They had two bobbins side by side and each bobbin made up one side of the push-pull circuit.
I have a couple of MI350 outputs and they are unity coupled penta-filar wound. Each primary is 150 ohms impedence end to end and is interleaved 10 times. Again, two bobbins side by side. Each bobbin is layered as follows: pri, sec, pri, sec, pri, sec, pri, sec, pri. With a 150 ohm primary this is not that many turns per section.
There was a poster on here around a year or so ago that lived in Great Brittan (?) who was going to make a pair of these MI350 outputs. Is he still here? If so, how is it coming along?
Daniel
Actually, it is "Unity Coupled" not "Twin Coupled". And having a cathode winding in a Unity coupled configuration will not heat a transformer more than what a standard output transformer will heat up, as the signals through both windings are wired so they are in phase with each other.
Mac unity coupled transformers are huge because they were way over-engineered. I know of people who have taken a MC-30 and taken out the tube rectifier and put in silicon diodes for the B+ supply to raise the B+ to around 460 volts and get around 60 watts RMS without any more distortion at that power level.
Since Mcintosh long ago quit making replacement transformers for some of the late 50's and early 60's units it would seem they would be glad to give out any information they had so there old items could remain useful. Mac is very proud of their heritage and it seems they have more 25 year old products still producing sound than any other manufacturer on the planet.
I still believe that a DIYer could, very carefully, and with adequate research produce a rewinding or replica of superior quality to the original. There winding facility which you can see on many old plant tour videos is nothing anywhere near high tech. Up until a few years ago some of the original female employees who wound the original 60's and 70's outputs were still working in the winding facility. These were the good paying jobs and required a certain amount of finesse. I think much is outsourced today with the cost of New York labor.
Even today at the motor plant I worked at for years the winders are the highest paid technicians in the plant. No one figures core to wire specs everything is dimantled and counted, measured for gauge and wound back to the original with modern insulated wire. There is a reason the windings are not interleaved insulated. It affects the frequency characteristics and winding to winding coupling. Not that it is a big thing, it just does.
If you have the old one it is not hard to rewind. The messy part is burning out the potting and wire insulation. Once it is dryed it will unwind just like it was wound. It helps to unstack the laminations and clean and revarnish.
Tad
I still believe that a DIYer could, very carefully, and with adequate research produce a rewinding or replica of superior quality to the original. There winding facility which you can see on many old plant tour videos is nothing anywhere near high tech. Up until a few years ago some of the original female employees who wound the original 60's and 70's outputs were still working in the winding facility. These were the good paying jobs and required a certain amount of finesse. I think much is outsourced today with the cost of New York labor.
Even today at the motor plant I worked at for years the winders are the highest paid technicians in the plant. No one figures core to wire specs everything is dimantled and counted, measured for gauge and wound back to the original with modern insulated wire. There is a reason the windings are not interleaved insulated. It affects the frequency characteristics and winding to winding coupling. Not that it is a big thing, it just does.
If you have the old one it is not hard to rewind. The messy part is burning out the potting and wire insulation. Once it is dryed it will unwind just like it was wound. It helps to unstack the laminations and clean and revarnish.
Tad
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