Please help ID this DBX VCA

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sonicmook56

Well-known member
Joined
Oct 17, 2005
Messages
299
Location
Los Angeles | Echo Park
Well, it's been awhile since I've posted here!  ;)

Can anyone ID this VCA?  It's from a fader in my Sony badged MCI JH 636 built in 1985.  Story below the photos....

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Long story short, I picked up an MCI 600 series console last year which I've been enjoying mixing on.  Aside from replacing a few switches and switch caps, I haven't really done anything to it except clean it up and wire it into my studio.

The major thing that's been off since I got it is the level to the mix bus from the VCA fader.  It's way off!  I've been getting around this the past year by putting everything in a group, then goosing the group master up.

Using a 1K tone at +4 (1.28VAC) to a line input, panned all the way left and the large fader at "0", I get around -10 on the VU meters.  Switching the VCA out of circuit to the rotary channel fader set to "0", I get "0" on the VU meter, which is what I would expect.  Please see short video for more explanation. 

https://youtu.be/cxCobGfRk60

It seems that the overall level of the VCA system is off.  I've calibrated a few things, checked all the rails (twice) but no changes were observed.  I should note that it doesn't matter if the automation PSU is switched on, or off...  It behaves the same.

I'm at the point now where I must assume that someone changed the original VCAs to a type with a different mv/dB sensitivity., but failed to change the surrounding support circuitry.  Crusty flux around the VCA pins, but nowhere else indicate this as well.

So, in hopes to find the correct resistor values, I need to ID these VCAs.  I don't think they are the THAT variety, but maybe they are:

202X
202XT?

I really have no idea!
 
This sounds like a VCA group "feature", not a bug. I'm not a MCI 600 expert by any means, but I bet that those thumb switches next to the bottom of the faders, set to '1' in your photo, assign the channel to a VCA group, perhaps just for muting, but also perhaps for grouped fader control. There may be some master fader somewhere that's adding -10dB to the fader group, but you don't know about it. Note that this type of grouping  is not like a subgroup - a VCA group effectively gangs all of the faders together, so that changing a VCA group master fader will change all assigned channel post fade sends.

Again, I'm not a 600 expert, but I've used VCA groups on larger consoles designed for live sound, they're extremely useful, and act pretty much like what you describe.

Find that VCA master fader!!
 
...or, find the odd bit of circuitry that is adding some constant DC value into the VCA group fader circuitry. Maybe a "zero set" trim that went bad?

Re-reading your post, I now know you know what a VCA fader is - sorry for the waste of time. :)

As a general debugging idea, do all VCA groups behave the same? You're assigned to group 1 now, but try all the rest to see if anything is different.

Aside form that, the only other thing I can think of is some sort of 'zero set' pot on the channels or VCA group faders, or possibly some artifact of an automation system that is not active or installed.
 
I believe one way or another those VCAs are from 'That', either from the DBX times (from which That emerged) or under their own name. There is a detailed pdf on their website where they explain the development of their VCAs and the technical differences, maybe that gives a clue?

Michael
 
Running a bunch of the early IC vcas in parallel delivered lower noise.

A faulty VCA could corrunt the control voltage, if channels are being grouped.

JR
 
Monte McGuire said:
...or, find the odd bit of circuitry that is adding some constant DC value into the VCA group fader circuitry. Maybe a "zero set" trim that went bad?

The control voltage to the VCA comes from one of two places.  The Automation system digitizer cards or locally on the fader.  Switching the automation supply off switches a few FETs in the fader to "local" control, removing a ton of 70's era computer/ logic circuitry, along with the ability to use groups.  I've gone as far as physically removing the group faders in my troubleshooting, but no dice.  All trims and offsets for the VCA system easily calibrate to their published specs.  Yes, the groups all behave the same.  I mix with them often, but I need to start out around +10 instead of "0", which is annoying if I need to push up the drums a bit in a super heavy wall-o-distorted-guitars chorus.


Michael Tibes said:
I believe one way or another those VCAs are from 'That', either from the DBX times (from which That emerged) or under their own name. There is a detailed pdf on their website where they explain the development of their VCAs and the technical differences, maybe that gives a clue?

Yes, there are some very useful white papers THAT published which is a gold mine of information.  My problem is I'm not sure what kind of VCA's I have.  It really comes down to the sensitivity of the control port and the values of resistors surrounding the VCA.  I believe there are three flavors?

6mV/dB
20mV/dB
50mV/ dB

http://www.thatcorp.com/datashts/dn127.pdf
http://www.thatcorp.com/datashts/202data.pdf
 
Here's a pic of a DBX 903 compressor module, isn't that the same vca? If so, this should help you figure out the type. http://vintageking.com/dbx-903-compressor-limiter-used To me it looks like the first 202 replacemant from DBX using 8 parallel ICs instead of the original 202 circuit, I believe I've seen these in SSL 4 & 6ks before - but it's been a while. If this was right, it should have the same characteristics as the 202.

Didn't MCI originally use the very first 'black can' DBX vcas? Maybe you check from the schematics, which vca actually belongs in the console and do a proper replacement following dn127 to see how it behaves?

Hope this helps,

Michael
 
> comes down to the sensitivity of the control

So monitor the control voltage while you measure tone gain and move sliders?
 
I don't know if this is useful information or not, but I have done a fair bit of service work on a local JH636. Just when it was looking like a whole load of issues were going to make the old desk basically a write-off, he managed to score very cheaply a slighly newer version of the same desk that had lead a much easier life, until it was run into by a forklift and the chassis was warped.  We were able to simply replace all the dodgiest cards from the old machine with better ones from the newer machine.

    The older desk had the 'gold can' DBX VCAs on all faders, the newer desk had these modules. Never tried swapping VCA modules between fader cards, but the fader modules were drop-in compatible with each other. Unfortunately it's too far away for me to go check component values on the two versions of the cards, but from memory the PCBs looked identical apart from the VCA module.

While I was typing this I remembered I have one of the old faders on the junk pile here.
 
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