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MA 2400 level problem


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#1 Dave C

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Posted 26 September 2006 - 05:16 PM

Hi,  
I have a number of MA 2400s of varying vintage, plus some K2 amplifiers, and the oldest MA 2400 is giving me some trouble. Hopefully someone will have seen this problem before and might have a solution, or can suggest where to start with it.

The amp in question has the date code 8069 (so from 1989?) and is branded an amcron MA2400.

The problem is that the level on channel 1 is consistently around 6dB lower than that of channel 2, regardless of sensitivity switch setting. The sensitivity switch has 2 settings; .775 and 2.2 volts.

The problem exists even if both input attenuators are set at full/12 o clock position/ etc.

The problem also still exists regardless of whether the XLRs on the pip module are used or the TRS inputs are used. (so it's not the soldering of the XLRs or the pip connections I assume)

When turning the amp on, the green signal led on the affected channel fades out noticeably quicker than on the good channel, and when switching the amp off, there is a very noticeable turn off transient (like a deflating or discharging noise possibly) from the bad channel.

I have already re-soldered the switch contacts for the sensitivity switch and the ground lift switch, and worked them both as the amp was quite dusty inside. It is from a generation that came without the foam at the air intake, just a metal grid.

So, if anyone has any ideas, I would love to hear them. I'm getting quite fast at macrotech disassembly at this stage...

Many Thanks and Best Regards,

Dave C.

#2 MChapman

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Posted 29 September 2006 - 11:43 AM

I assume (and we all know where that can get you smile.gif )you have switched the input signals from channel to channel, and that they are both balanced inputs.

Well, it could be a dirty contact on the sensitivity switch, dirty CMR pot on that channel, or the CMR is way out of adjustment. Lastly, it is maybe even a bad input IC, but this is very rare. Another very long shot would be a bad input pot.
<span style='color:blue'>J. Mark Chapman
Crown Application Support Technician</span>

#3 Dave C

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Posted 29 September 2006 - 02:15 PM

QUOTE(MChapman @ Sep 29 2006, 04:43 PM)
I assume (and we all know where that can get you smile.gif )you have switched the input signals from channel to channel, and that they are both balanced inputs.

Well, it could be a dirty contact on the sensitivity switch, dirty CMR pot on that channel, or the CMR is way out of adjustment. Lastly, it is maybe even a bad input IC, but this is very rare. Another very long shot would be a bad input pot.
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Hi Mark, and thanks a mil. Never thought of CMR adjustment.... I have the service manual, but is there a component number that I should look for on the board?
I assume its on the input/main board of the amp somewhere, right?

Re: sensitivity switch, can I replace this with a generic component, or does it use unusual pin spacing?

Thanks again for your advice, I'll report back to let you know how I get on over the weekend.

Thanks and Regards,

Dave C.