mc 250 with hum

If the problem is common to both channels, it is a common mode problem with the fault mode shared by them.
 
Me2
Early McIntosh MC250.
Okay, this thing is messing with my mind. First year model. Bring it up on a variac get a loud 120Hz buzz through both channels on stereo setting at low voltage with vol controls at min which increases to serious jagged waveform at about 30V AC. All wiring to caps appears correct and grounds test as ground. Removed and tested filter caps on nice LC101 tester. Replaced both multisection caps with new as test bad, large 9300uF caps test only 400uA leakage and ESR of .18 ohm and within 5% of each other so using them. After reforming caps, installed and same problem. Scope confirms serious 120Hz on positive rails at bridges (negative rails are smooth). Tried swapping big caps, no difference scope is showing all hum on positive rail. Check bridge diodes with Fluke 77 says OK. Remove power from all but power amp, no change. Remove power from each channel independantly, no change. All four heatsinks have 2 insulators (nylon and paper) each side of main chassis.
If the two big elyticaps and the rectifiers check out, I'd suspect an imbalanced loading. Any excessive heating of the big heat sinks?
 
The front end board should get close attention.
Spot on! Both channels share +/- 12V there so trouble on one side affects both. Impedance +12V into left channel (#10) to ground 2300 ohms Q5 shorted, +12V into right channel (#7) to ground infinite. :)
 

Attachments

  • input.png
    input.png
    207.5 KB · Views: 10
Last edited:
The OP in the solid state forum finally came out with the information that the hum is mechanical....not thru the output but the transformer mechanically humming.

Linear should start a new thread as his problem is different than the OPs

Does sound like a rectifer problem though.

It is possible the OPs issue of the mechanical transformer hum is caused by a failing diode in the bridge.
 
The OP in the solid state forum finally came out with the information that the hum is mechanical....not thru the output but the transformer mechanically humming.

Linear should start a new thread as his problem is different than the OPs

Does sound like a rectifer problem though.

It is possible the OPs issue of the mechanical transformer hum is caused by a failing diode in the bridge.
:blah:
Your advice seems illogical to me since the device in question is IDENTICAL and the symptom is also IDENTICAL, therefore, accumulating a knowledge base in one thread seems the most logical procedure instead of cluttering the forum with a multitude of similar threads! Regardless of whether the OP's symptom was caused by a different fault is not supportive of your argument and, more to the point, enhances the utility of this thread by illustrating the varieties of causes for similar symptoms in the same device. Furthermore; and in futher enhancement of my argument, I will add that MY UNIT also exhibited a mechanical vibration of the power transformer until I replaced one of the shorted multi-section capacitors. Had I chosen to start yet another "MC250 Buzzing" thread, this information would not have been unified.
 
Last edited:
Excessive current draw by whatever fault is the likely issue with buzzing power xformer. Finding the causative fault(s) thru logical troubleshooting is the issue. Leaky rectifier or reservoir/ filter elyticaps and/ or an excessive offset condition being the likely suspects.
 
Last edited:
Excessive current draw by whaever fault is the likely issue with buzzing power xformer. Finding the causative fault(s) thru logical troubleshooting is the issue. Leaky rectifier or reservoir/ filter elyticaps and/ or an excessive offset condition being the likely suspects.
As I stated previously, replacing the filter caps only solved the buzzing transformer but did not cure the stereo 120Hz buzz so, unluckily, further investigation was warrented. With Pio1980's encouragement I proceeded to the input section and discovered a shorted transistor.
 
c_dk is far more the expert on these...
That's too bad, I wish he had been more useful to me but it appears he didn't take the time to read my posts. I'd never have posted on this thread if my situation was not identical. Certainly had no intention to hi-jack. Come from the old school where we all work together to solve a problem. :bigok:
 
Back
Top Bottom