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.

 

I know, seems so simple, he said after hours of searching

 

will look again

 

Try some continuity checks to chassis from circuit points that are supposed to tie to it.

 

The front end board should get close attention.

 

i'm not sure which is intended for me and which is intended for lectrolinear. thanks,

 

The advice is generic troubleshooting, use anything that may apply to your problem.

 

ok, thanks. guess i'll repost and try again.
thanks,
mikeB

 

If the two big elyticaps and the rectifiers check out, I'd suspect an imbalanced loading. Any excessive heating of the big heat sinks?

 

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.

 

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.

 

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.

 

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.

 

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, but nonetheless I'm gratified that my suggestion was useful.

 

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.