MC275 Power Transformer Buzz

[Bill Ferris]

No sign of tar anywhere, everything looks very clean.
I tried to tight power transformer to the chassis but no difference.

BTW, Do you have any means to measure the amp`s AC mains current draw, either in low range amps or wattage range, and does the power transformer become warm to the touch with full 120 v applied with all the secondary`s disconnected in a short period of time ?

 

[neskor]

I plugged my AC Ampere meter just before thermistor and I got 2.6A reading with secondaries connected and all tubes in place
Didn't notice any warming of the transformer
When the line voltage is bellow 75V there is no buzz.

 

[neskor]

Quick update:
Transformer removed from the case, connected only DMM's on each section of secondary side and I measured 1.6A @120V on primary side.
So definitely I have some short inside the transformer

 

[Bill Ferris]

Quick update:
Transformer removed from the case, connected only DMM's on each section of secondary side and I measured 1.6A @120V on primary side.
So definitely I have some short inside the transformer

Sadly, it would appear so ! , ,

 

[SaturationPt]

If it helps, I believe that the MC75 has the same power transformer as the 275.

 

[dcgillespie]

No, no. That would be the same output transformer that is the same between the MC75 and MC275. The power transformer for the MC275 is a much higher rated transformer versus that of the MC75, since it must support two amplifiers (twice the tubes, twice the current draw).

Dave

 

[Pio1980]

It's possible a generic open xformer of suitable characteristics could be potted into the can, certainly easier to find than a loose OPT.

 

[neskor]

I think it can be rewired again on the same EI transformer body

 

[clinic-audio]

I think it can be rewired again on the same EI transformer body

yes , but you forget the cost of the shipping to USA ( go and back ) because you are in Europe and using laminations and wires which are not from USA is not serious . You will never get a power transformer 100% identical to the original type

A power transformer is not light and shipping charges are important and I don't talk about diameter of the different wires and the varnish ....

 

[clinic-audio]

No, no. That would be the same output transformer that is the same between the MC75 and MC275. The power transformer for the MC275 is a much higher rated transformer versus that of the MC75, since it must support two amplifiers (twice the tubes, twice the current draw).

Dave

100% true !

Dave is right again

 

[neskor]

Does somebody knows what is expected current draw (for this size of transformer) on primary when nothing is connected on secondary taps?

 

[Bill Ferris]

Does somebody knows what is expected current draw (for this size of transformer) on primary when nothing is connected on secondary taps?

A 32nd or much less of 1 amp would be a guess with no connections to any of the secondary wires..

Basically no significant current draw, unless there is a internal problem with the primary, such as shorted turns or some other internal wiring mishap.

 

[62caddy]

My MC275 is for USA market, so 120V Power transformer.
I'm using step down transformer to drop line voltage from 230V to 120V.

Audio Classics is showing PT on their site for MC275 (US version presumably). Expensive but not many alternatives.

https://www.audioclassics.com/detail?detail=043-816&nav=cat

 

[neskor]

Checked all transformer winding and there is no short anywhere.
It looks like transformer Flux/core is going to saturation over 100V on the primary.
Myabe the problem is main voltage 50Hz vs 60Hz.
I found this:
https://www.electricalengineering.x...pens-in-transformer-and-what-are-its-effects/

"Another cause of abnormal transformer core saturation is operation at frequencies lower than normal. For example, if a power transformer designed to operate at 60 Hz is forced to operate at 50 Hz instead, the flux must reach greater peak levels than before in order to produce the same opposing voltage needed to balance against the source voltage. This is true even if the source voltage is the same as before"

Magnetic flux is higher in a transformer core driven by 50 Hz as compared to 60 Hz for the same voltage.

you can use the transformer on 50 Hz if the voltage on 50Hz is lower that the one on 60 Hz in the range of 20% less.ex: if 115V at 60Hz, then 95V at 50Hz. or 140V at 60Hz then 110V at 50Hz.

 

[neskor]

I noticed one more strange thing.
All secondary taps have very small DCR except Bias tap (blue - blue/white wires), I measured around 30 ohms, does it looks normal?

 

[jimimac]

You prolly have dirty power. Transformer hum is quite common over the Pond. The newer Mc75, (don,,,,,,t know much about vintage ),used the same power Transformers as the Mc275 . I potted all Transformers at McIntosh back 20 + years .There was just enough tar in it to hold it in the can.Be careful none of thee wires short to the chasis as that was a problem also. They say over time the actual bolts that hold the tranny together EI type can be loose ???

 

[dcgillespie]

My McIntosh schematic for the MC275 (with M-215 power transformer and voltage selector switch) indicates the amplifier is suitable for operation from 105-130 volts, at 50-60 Hz. From a design standpoint then, it would seem that Mac took into account operation from a 50 Hz AC line.

Dave

 

[neskor]

I think that Mcintosh built that power transformer very tight for required current consumption, so on 50 Hz load is 20% more than on 60HZ and the magnetic core/flux goes to saturation. When I have 80V on primary transformers works normaly and there is no buzz.
My transformer winder told me that HV wire size is good for 1A and heater/coil wire is optimised for only 6A and mc275 needs more than 9A only for heaters.

 

[trainbuftony]

I'd look at this problem as an opportunity.
Boil the old transformer can and dump the old transformer out, clean out the tar, and buy a transformer that will provide the right voltage to the amplifier.
Alternatively it may be cheaper where you're at to have the old core rewound.
Once you have a new transformer making voltage, tie the leads together with a rubber band and pour the can full of hot roofing tar.

Close it up and reattach the leads.

If you're clever, you can fix this for the price of a few tanks of fuel. ($2-300 american)

While you're having it rewound, ask the rewinder person if they can configure the primary for global operation.
Get a 115/125 primary x2 so you can run on 110-130
Or
220-240
Then you can ditch the step down, and know you're set up to play music anywhere there's ac power.
You have the opportunity to take down 2 birds with one stone, fix 2 problems on one operation.

 

[jimimac]

Output or Autoformers are mostly wound in house the power Transformers are Schumacker or Plitron..