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Old 28th February 2004, 06:29 AM   #1
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Default Please help my MC240

Hi. I just got my MC240. Is all AB Resistor and MIT cpas. And it sound no problem. Then I replace some Dale Resistor and Jensen Oil caps. Then I play it. I hear a lot of distortion(not hum or hiss) . What was the problem?? Those Jensen and Dale are brand new and I did test them. I check many time and dont know what is wrong. Just hope someone is in NYC area that could be help me. Thanks a lot.
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Old 28th February 2004, 01:22 PM   #2
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Default Mc240 help

Go thru the amplifier slowly and carefully. Most likely you have a cold solder joint, incorrect connection or incorrect part value somewhere.

Over the years I've learned to check each part with a meter before installing and make only a few changes at a time.

If both channels are distorting, look at the power supply. Also you may have a wrong part value and repeated the problem on both channels.

If you need a schematic for the Mc240 give a yell.

- Gary
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Old 28th February 2004, 01:46 PM   #3
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Hi,

Quote:
If you need a schematic for the Mc240 give a yell.
Check it for mistakes: most of the copies I found on the web have a mistake in the PS section. A cap with reversed polarity IIRC.

BTW, what's the point, or better what do you expect to gain, by replacing MIT caps and Holcos with Dales and Jensens?

FWIW, the MITs can be too revealing of problems elsewhere, the Holcos are rather darkish sounding.
The Dales are a tad on the harsh side whereas the Jensen can be a little too lush...

Net result of the effort seems to be a similar balance as before...

Where the amp benefits most IMO is in replacing the PSU caps and cathode bypass of the 12AX7A for BGs and good sounding input tubes...Also caps C1 and C11 can be ditched if you're sure you don't have any DC offset from your source(s).
Just my experience with the MC240.

Comments?

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Old 28th February 2004, 03:38 PM   #4
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Hi, Thanks for all help. I also think about the problem is from the PS. But I did not change the PS parts. But I will again tomorrow.
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Old 29th February 2004, 09:02 PM   #5
AV8R is offline AV8R  United Kingdom
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Default MC240 and Jensen caps

I wonder why everybody loves these paper and oil caps.

Measure the leakage current at elevated temperatures.
They at 80 deg C have a leakage of several tens of meg Ohms
and you find as coupling caps, the bias of the following tube creeps up so warm sound. Check with say EL84 in small amps and the grid volts has gone up from near zero to several volts when the cap is heated. Its a fuction of impurities in the paper and not being a perfect hermetic seal.
The old Vitamin Q caps dont have this problem.
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Old 29th February 2004, 09:54 PM   #6
SY is offline SY  United States
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Quote:
I wonder why everybody loves these paper and oil caps.
Fashion. Remember back in the '70s when everyone was replacing them with film caps? Plus ça change, plus le même chose.
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Old 29th February 2004, 11:11 PM   #7
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Default A LA MODE....

Hi,

Just as not all film caps sound alike so do PIO sound different.

The only ones I heard so far that are darn neutral happen to be the Jensen.
Oh, they don't leak either as far as I can tell...Then again I don't know of anyone running their oilers close to 80 degrees Celcius either.

Instead of dismissing things before hand why not try them first?

Tube amps are pretty fashionable too, aren't they?
Or does one need to run a Stupid Ended one to be a dedicated follower of fashion?

Cheers,
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Old 1st March 2004, 12:22 AM   #8
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To be truly fashionable, yes. What I do is the equivalent of bell-bottoms, Nehru jackets, and lava lamps.
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Old 1st March 2004, 12:37 AM   #9
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Hi,

ROTFLMAO....

Quote:
What I do is the equivalent of bell-bottoms, Nehru jackets, and lava lamps.
No wonder Schwarzenegger got elected...

Cheers,
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Old 1st March 2004, 01:34 PM   #10
AV8R is offline AV8R  United Kingdom
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We are a lot of spiky creatures that love put downs.

Good night all
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