Question For EW or others

MCS Guy

The MCS hoarder
This is regarding my MCS 3125

I manage to test the DC offset today. I let the amp settle for about 1 hour, volume all the way down, defeats on, loud off etc.

The right channel is about 15-20MV

The left is the one that is giving me trouble. It fluctuates from -12 or 11 MV to -30 to 35!. What is causing this? Its kinda hard to set it will this kind of fluctuation.

I am hoping that bill or YRLY will know which pot on the amp boards is the DC offset. Is it V201 or V202?
 
MCS Guy, VR-201 is the DC offset pot. I always set the DC offset after it has been on with no signal and volume all the way down for a hour or so. These pots are real "touchy" so be carefull and just barely turn them one way or the other and try to get it as close to zero as possible. Don't worry about the polarity. I would say if you could get a stable voltage reading of 0 to 20 mv you will be OK. If the voltage varies a little, say + or - 10 mv, I would not worry about it. If it varies a large amount then I would say that the input differential amplifier transistors might need replacing. By the way don't touch the VR-202 pots. They are the idle current pots and you could cause the output transistors to over heat if they get to far out of adjustment

Bill
 
That's an annoying level of fluctuation, but nothing show-stopping. If the unit has a way to separate the preamp from the amp (jumpers or a switch), disconnect them and see if it makes a difference.

If not (and it probably won't, but try it anyway) then the old trimpot is likely not making good contact internally, or the differential input pair (if it has them, and it probably does) may need replacing. Also, if there is a DC blocking cap in the feedback path, it may have a bit of DC leakage. A fluctuation of <5mV is what you should end up with.
 
Originally posted by Bill
These pots are real "touchy"
Bill

No kidding! Spent 15 minutes a pot getting it just right.

I manage to get it to fluctuate between 3mv to -3mv. But the left channel does everyone in a while float up to 6 or 7 mv for a few seconds, then comes back down.

Bill, do you know where the LED power meter adjustment pots are? The left channel meter is a little higher than the right.

Thanks to the both of you
 
If you end up replacing the trim pots I would
recommend going to a multi turn type. They
would be easier for you to adjust.
 
MCS Guy, to adjust the power meters:

1. Turn the 3125 on its left side and remove the bottom cover.
2. Hook a VOM to one channel speaker connection. You dont have to have speakers connected. Set the VOM to measure AC.
3. Turn receiver on and set the selector switch to "Rec Level Check"
4. Turn volume up untill yo get a reading on the VOM of 2.83 volts AC.
5. The power meter LED's should read minus 20 db.
6. The power meter adjustment pots are located on the bottom circuit board and are labeled VR501 for the left channel and VR502 for the right channel. They are on the left side of the circuit board and maybe hard to see and might be covered with some wires.
7. If the VOM is still reading 2.83 volts, adjust the pots so that the LED above and below the minus 20 db mark is just lit. make sure your VOM reads 2.83 volts on both channels at the speaker terminals.
8. double check it and you are done.

Bill
 
OK, I now have another problem.

When I got to set the meters, the right channel would be 2.8ish volts while the left side is around 3.1ish volts.

If I set each channel separately, the result is the same, left meter reads higher than the right.

So Should I set the meters off the left side connection, or is there a problem with channel centering (I made sure the balance is in the middle).

Thank you much Bill. Just let me know if you need something.

BTW, what kinda camera are you using? Those pics look pretty good up close
 
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MCS Guy, you have a slight channel imbalance problem but I doubt it will be very audiable. Also power meters are not very accurate to begin with. If you are pretty much satisfied with the receiver's peformance I would just get the power meters to read about the same at the minus 20 db level at 2.8 to 3 volts. You will still be within the factory spec. Turn the volume up and down and check the meters to see if both channels read the same. If they don't, tweak one of the pots till they read the same. I always felt that power meters was a cosmetic feature and not to be taken as an accurite indication of the actual power output. Pretty to watch but don't take to seriously. Just about every receiver or amp I have checked will check slightly different on each channel, nothing audiable but different. Usually this is caused by one section of the volume control's resistance to be slightly different than the other. As for my digital camera, it is a five year old Olympus D-340R. It is only a 1.3 megapixel camera but takes pretty decent shots.

Bill
 
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