MC 250 Oscillation

basil13

Active Member
Hi All,

I have been working on a stubborn MC 250 with some help from c_dk and thought I was successfully done with it. This has been my first MC 250 to work on and has been an adventure.

After a few days of play testing, I noticed some distortion on one channel. I put it on the scope and sure enough there is some oscillation on the positive half cycle (see picture).

It only happens under load, it is clean otherwise.

The channel is using the original drivers and output transistors. The class A board still has its original transistor as well. However, I had to essentially rebuild the class A board with many new parts.

The 0.56 ohm 5 watt wirewound resistor was replaced previously with a NTE, I have read that Mac does use NTE parts, is this a Mac replacement (see picture)? If not, could it be the source of my problem?

Any other parts I should look at troubleshooting?

An oscillation problem is a new experience, so any other advice would be appreciated.

Regards
Bob
 

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Is that R56? Mac used Rockwood or later Cober wirewounds that were inductive. Not much but just enough to keep the circuit stable.

When Rockwood changed their manufacturing process for .15s as used in the MC2205 Mac had to change the circuit to work with the lower inductance.

PM me again
 
I already get this trouble and the answer is not really good .
Can you try to disconnect the grey wire (output of power amplifier who goes to the input of output transformer ) and connect it to a oscilloscope input . See if the signal stay clean or not and this at all levels .
If yes , good news !
Can you make high precision measurement of this resistor ? Is it inductive or not ?
Waiting to know more
 
Can you try to disconnect the grey wire (output of power amplifier who goes to the input of output transformer ) and connect it to a oscilloscope input . See if the signal stay clean or not and this at all levels .

Hi Patrice

Just to clarify, I assume you want me to check the output transformer by feeding the signal into it by connecting my signal generator to the gray wire?

I did that with the dummy load connected and got a clean signal measured across the load. 2 volts is the max signal it will send which should be fine because I was seeing the oscillation at low gain.

Unfortunately I don't have a precision tester to measure the resistor, I will take c_dk up on his offer though.
 

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Will the Mounties let a poor old wirewound into the country or will I have to get it a passport?

The last time I went into Canada they were concerned I wanted to stay.....of coarse that was in the early 70s and well you know there was a small emigration of draft age men......

PM me a address and I will mail you one and we can settle this issue.....
 
Will the Mounties let a poor old wirewound into the country or will I have to get it a passport?

The last time I went into Canada they were concerned I wanted to stay.....of coarse that was in the early 70s and well you know there was a small emigration of draft age men......

PM me a address and I will mail you one and we can settle this issue.....

Considering how many ''refugees'' are illegally flooding across the Canadian border (especially in my neck of the woods) from the US presently due to your governments policies,perhaps WE need to build a wall and send Trump the bill.

Don't wise-ass my country,I really don't appreciate it or tolerate it. And as far as your draft-dodging hippies go,I would have had them returned to you. No amnesty.In my world,your deserters and traitors would have been shot,not welcomed.

Best regards,

Art
 
I would say that those young men would have called themselves resistors also......'twas nothing but a play on words....

We were going up to Agawa Bay to go camping and rock climbing.....the young Canadian border official took issue with how much food we brought with us for a 4 night stay. The seasoned veteran official looked in our cooler and predicated we being 19 years old would be out of food in two days.....so sent us on our way.

We had a great time and went up a few times more.
 
I received the resistor from c_dk yesterday and installed it today. The resistor has taken care of the oscillation issue so far.

The amp has provided so many twists and turns I am apprehensive to say its resolved. I will monitor it next week and report back if the oscillation returns.
 
I am glad that you have so far been able to confirm my memory from 30 years ago......I am sure this tidbit was shared by DOB during a amp clinic.

I am sure there are WWs currently available from suppliers with the needed inductance, can't be much, but without a scope and test bench facilities for a repaired amp how would you know......
 
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