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Scott 222c/lk48 volume imbalance

shelby1420

shelby1420

Enjoying the music.....
Hey all, I am noticing an imbalance in my Scott and have ( I think) narrowed it down to the volume pott, A) how would I confirm this and B) where can I get a replacement and what value would the replacement have to be?? Thanks all
 
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Hi,

Just for clarification: are all the resistors been changed and if so, are they matched within 1% on both sides?

I had a channel imbalance on my Scott LK-72A until I replaced (for the second time) all the resistors, being sure their values were within 1% between each channels and against schematic.

If the resistors are well matched, it could be also a tube (phase splitters or 12AX7's) weaker on one side. Don't forget the feedback stage also (usually a resistor and a cap paralleled on the 16 ohms tap).

If all this is ok, the volume pot could be worn, especially between 9 o'clock and 12 o'clock.

All my tube gear (except the Scott and a japanese Monacor) have mistracking between channels due to old pots.

Usually, I feed a tone (440Hz) via a cd player and measure at the speaker screws (you must have speakers attached...) in AC range.

Then, I adjust this way for almost perfect balance: I solder a resistor (you'll have to experiment, usually between 100k and 1M) going from wiper lug to ground lug on the louder side of the pot.

It's a trick given by a generous member of this board a few months ago: http://www.audiokarma.org/forums/showpost.php?p=3249139&postcount=3

Patrice

*As for replacement of the pot, you'll have to source a donor unit or the piece alone on the auction site. This pot is particular as it sports a loudness tap and a power switch...
 
About the AC testing on speaker screws.

I use this calculator to achieve the best balance: http://www.sengpielaudio.com/calculator-amplification.htm

I enter the AC voltages for each channel and it gives the difference in dB. My target is under 1.0 dB, ideally .5dB.

Over 1dB, I can hear the imbalance.

To create the test tone, one could use the free software Audacity and then burn the file on CD (or feed the computer line out directly to the amplifier, assuming you have a good soundcard. I guess a cheaper soundcard could mess the test as it could have bad balance between channels too...).
 
shelby1420 said:
Hey all, I am noticing an imbalance in my Scott and have ( I think) narrowed it down to the volume pott, A) how would I confirm this and B) where can I get a replacement and what value would the replacement have to be?? Thanks all
Click to expand...

Hi,

Take AC voltage readings off of the grid of the first stage of the tone control
section of the amp. You will normally find something like a .001uf capacitor connecting to the grid of one half of each 12AX7 in the tone control stage. See if the voltages are essentially the same with both channels driven via a signal source of equal amplitude per channel. If the voltages are different, there is a good chance it is in the volume control circuit (which includes the loudness control, the high, and the low filters (some don't have both). If you look on the wiring diagram, you will see the volume control, balance control, low and high filters and loudness switch all between the line level inputs and the grid of the first stage of the tone control network. This area does have issues. Inaccurate tracking of the volume control, channel to channel, is common in Scott amps. By this I mean that you may be a little heavy in the left channel at low volumes but as you crank the control up, that may change to either being balanced or heavy in the right channel. No real fix other then to replace the control or to just use the balance control.
Bad components in the loudness circuit can also cause one channel to be weak. If you notice that the problem is not in this area, suspect the tone control network. Bad capacitors and resistors in there can cause all kinds of issues. You will just have to trace out the AC voltages stage by stage until you find where the voltage becomes lower in one channel then in the other.
remember, we are looking at the AC signal voltages. They will probably range from .25 volt AC to about 2 volts AC. Once you find the point where the voltages are unequal, just trouble shoot the stage preceding the voltage drop.
 
I will be having a look under the hood this week for sure, might be a few night before I get to it tho..... thanks for the help!!!
 
I was listening to it today and after about an hour noticed some distortion in that same channel, no matter what the level was at still distortion, what does this mean???
 
shelby1420 said:
I was listening to it today and after about an hour noticed some distortion in that same channel, no matter what the level was at still distortion, what does this mean???
Click to expand...

Rick, I remember after the restore at the end of April you had this issue (channel imbalance). It sounds as though there could be issues in one of the circuits that was rebuilt.

Have you checked voltages between the left and right channel circuits and made notes of the differences?

. . Falcon
 
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