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Console VCA bypass ALMOST working! Please help!
Old 10th February 2020
  #1
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Console VCA bypass ALMOST working! Please help!

*edit: added schems of channel input card showing fader amp circuit.

Hey fellas. Working on a popularly suggested VCA bypass for my Sound Workshop Series 40 console. I've got some Alps Japan 10ka faders which measure around 9.3k with a DMM, so I'm assuming 10ka means 10k impedance audio taper.

Basically, at bottom of fader travel, I'm still getting about 100mVPP with roughly 7vPP @ 1k coming into the fader using mic input.

Talked to a tech who originally built these consoles. He said fader audio boosters and buffers are on a separate card and no extra circuitry should be required to bypass VCA's, but he wasn't sure of correct impedance.

Here's a vid with details

https://youtu.be/k7vxAUihiJ4

Unfortunately I don't currently have schems of the VCA/fader card, but they should be arriving this week. Any idea what's going on? Adding a 4.7k resistor to input cuts signal to fader down to about 4vPP but significantly lowers output ripple at bottom of travel. Doubt this is a tenable solution though. Watcha think? Thanks fellas!
Attached Files
File Type: pdf S40_Input_Module.pdf (310.9 KB, 36 views)

Last edited by jdurango; 10th February 2020 at 07:58 PM..
Old 10th February 2020
  #2
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gyraf's Avatar
 

10KA most often means lin, A10K meaning log (audio). With execptions. Set fader to physically middle position and measure from viper to both ends. 10/90% means log, 50/50% means lin.

/Jakob E.
Old 10th February 2020
  #3
Some faders don't reach the low end of movement due to mechanical issues like a screw that is too long.
Old 10th February 2020
  #4
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Quote:
Originally Posted by Jim Williams View Post
Some faders don't reach the low end of movement due to mechanical issues like a screw that is too long.
Jim, thought it might be a mechanical issue. Removing screws entirely yields same results. Really odd.....maybe something is up with just this test channel....although it was working fine with the VCA.
Old 10th February 2020
  #5
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Just found this old thread with reply from David Kulka;

"
Your original fader has an extra "cut off" tap at the bottom to assure that signal is muted with the fader all the way down. P&G faders are built that way too, maybe you can find one that fits or make one fit. But failing that, yes, a 6-wire fader, as you called it, would work, thhopugh it may not cut off as fully at the bottom."

"Your original doesn't have a switch, just a small ground contact at the bottom of the resistive fader track, that is isolated from that track. The idea is that when the fader wiper contacts it, it is connected only to ground with no possibility of leakage from the resistive track. Most of the better quality faders are built this way including (I believe) all P&G faders. (The small switch at the bottom of some faders was generally used in broadcasting and live sound to activate a cue function, which is similar to solo. You don't want that type of fader.)"

Console Master Fader Wiring

The faders I have are basic 3 wire Alps.
Old 10th February 2020
  #6
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This implies that faders without this feature sometimes don't cut off all the way.
Old 10th February 2020
  #7
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Here are schems for input section showing fader amp, which is still active (different PCB than VCA removed).
Attached Files
File Type: pdf S40_Input_Module.pdf (310.9 KB, 5 views)
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