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Calibrating Gain Levels in Analog Console
Old 26th July 2020
  #1
Gear Head
 

Calibrating Gain Levels in Analog Console

Hi all,

I've got a Soundcraft Series 600 that I'm slowly recapping

Only problem is the gain levels are inconsistent. Some channels seem to have 2-3 db more gain than the rest. This is happening on channels that I have recapped, cleaned the faders and pots on. All of the anomalous channels seem to have too much gain, rather than not enough.

I was hoping for a trimpot on the channel PCB's that I could use to turn down the gain but there doesn't appear to be one.

Does anyone know any other ways I can calibrate the channels to read the same level?

I'm running my Quantum 4848 straight into the line in's, with the gain knob turned all the way down so there isn't a way to reduce the signal further.

Perhaps the only way would be to solder in a trimmer pot in the place of a resistor?
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Calibrating Gain Levels in Analog Console-screenshot-2020-07-25-21.16.19.jpg  
Old 28th July 2020
  #2
Check resistor values for resistors on U6. Make sure all switch contacts are cleaned first; you might just have tarnished contacts.

It looks like the trim pot VR1 is for balancing out the tail currents
on the differential transistor pair (an off hand guess).
Old 28th July 2020
  #3
Gear Head
 

Quote:
Originally Posted by Philbo King View Post
Check resistor values for resistors on U6. Make sure all switch contacts are cleaned first; you might just have tarnished contacts.

It looks like the trim pot VR1 is for balancing out the tail currents
on the differential transistor pair (an off hand guess).
Thanks Philbo. I think VR1 is just the main gain potentiometer on the channel. It's not a variable trimpot on the PCB that I can calibrate, which is annoying.

I've replaced all of the switches so I think they'll be good.

By U6 do you mean IC6? Do you think the best way to troubleshoot is to measure voltages at the IC's?

Thanks again,
Will
Old 29th July 2020
  #4
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ruffrecords's Avatar
The minimum gain is set by the maximum value of the gain pot. This can vary by as much as 20% which accounts for the 2dB or more variation you observe. There is no real way to fix this unless you hand select pots. Even then you will only set the minimum gains equally. The gain at any other rotation of the pot will vary from pot to pot due to similar manufacturing tolerances. In other words this circuit is not capable of achieving much better than 2dB tolerance on minimum gain. The maximum gain is set principally by the 27 ohm series resistor so this should be more consistent as long as the end resistance of the pot is only a couple of ohms.

Not much else you can do with a simple low cost circuit like this. Professional mixers use a rotary switch to provide repeatable accurate gain steps.

Cheers

Ian
Old 30th July 2020
  #5
Quote:
Originally Posted by kinjack View Post
Thanks Philbo. I think VR1 is just the main gain potentiometer on the channel. It's not a variable trimpot on the PCB that I can calibrate, which is annoying.

I've replaced all of the switches so I think they'll be good.

By U6 do you mean IC6? Do you think the best way to troubleshoot is to measure voltages at the IC's?

Thanks again,
Will
Yes, IC6.

"By U6 do you mean IC6? Do you think the best way to troubleshoot is to measure voltages at the IC's?"

Probably not. Use an ohm meter with the mixer powered off. Measure the same resistor on different channels, comparing readings across channels and looking for discrepancies.
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