Welcome, Guest. Please login or register.
May 24, 2019, 05:53:32 PM
Home Help Login Register
News: Please post general conversations, videos, music, and similar stuff in the "Tips, Tricks and General MCI conversations" section, use the Misc section for any product not listed in the categories, parts or custom projects. Thanks!

+  MCI Console and Tape Machine Forum
|-+  MCI Consoles
| |-+  JH-600 Series Consoles (Moderator: Westrek)
| | |-+  VCA "levels"
« previous next »
Pages: [1] Print
Author Topic: VCA "levels"  (Read 3334 times)
rev_mook
MCI Member
*
Offline Offline

Posts: 3


« on: May 09, 2016, 06:11:11 PM »

Hi Gang,

I am the relatively new owner of a 636 with the JH50 automation package (it works!).  What I'm having trouble with is the calibration "level" of the VCA system.  Hopefully I can explain this...

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 fader set to "0", I get "0" on the VU meter, which is what I would expect.

It seems that the overall level of the VCA system is off.  I've calibrated a few things but no changes were observed.

In addition, group 1 sets the gain much higher than any other group (or no group) without a fader designated as a master.

Any ideas?



 
Logged
Harrison
MCI Master
MCI Member
*****
Offline Offline

Posts: 305


« Reply #1 on: May 13, 2016, 12:48:13 PM »

With reference to the overall VCA level, is this with the VCA group switch set to local?
Does it change when moving to Master. (Forget VCA groups for the moment).
(We'll deal with group 1 later)
PC
Logged
revmook
MCI Member
*
Offline Offline

Posts: 6


« Reply #2 on: February 02, 2017, 05:15:53 PM »

Wow, It's been a minute since I've thought about this issue on my MCI.  What I've been doing to work around this issue is put all the faders in a group, then trim up the entire console.  It's kinda annoying to do it that way though.....

Harrison, to answer your question, yes, all faders group select switches are set to local control when I get this level error. 

I've followed the procedure in the manual on adjusting the automated master fader with no change in actual audio level.  Ideas?
Logged
revmook
MCI Member
*
Offline Offline

Posts: 6


« Reply #3 on: February 03, 2017, 04:40:35 PM »

Made a youtube video for clarity.  Sorry for holding the camera in portrait mode, I didn't know any better!

https://youtu.be/cxCobGfRk60
Logged
Harrison
MCI Master
MCI Member
*****
Offline Offline

Posts: 305


« Reply #4 on: February 06, 2017, 06:38:04 PM »

Firstly, there is no calibration as such, and if there was, it's not going to calibrate out a 10dB error.

What happens if you switch off the automation +5v? (Outside of the muting and soloing going away).
Have you checked the 15v on the auto cards and it coming to the fader pack. Each auto card has its own 15v regulators.

Turning off the auto +5 allows the fader DC VCA circuitry to work by itself with no external modifiers (groups/master/muting).
The channel VCA voltage is normally digitized and all these modifiers are "added" to the 8-bit word that becomes the fader DC voltage coming back from the digitizer card. No auto +5v means that the fader DC goes straight to the VCA control port and does not go via the digitizer card.

There is one other thing. What VCAs have you got and do they look original? Take a picture, please.

PC
Logged
revmook
MCI Member
*
Offline Offline

Posts: 6


« Reply #5 on: February 11, 2017, 08:54:38 PM »

Thanks for your input so far.  To answer your questions -

My faders have the THAT module stuffed.  Bi-polar rails on the fader are slightly off +17.7V, -17.5V; measuring from J2, pin 7 as 0V reference.

Pin 2 on J2 has 11.3V?  Not sure what this should be.

Turning off the automation supply does not change the level from the VCA fader - problem remains.

I took a look at the voltage going into pin 7 of the VCA.  When the fader is all the way down, it reads 5V.  All the way up, it reads 0V.  Muting the fader pulls the VCA control pin up to around 5.5V.

I'm guessing this is a low rail somewhere since this problem is global and the problem remains when the automation supply is turned off.  Thoughts?
Logged
Harrison
MCI Master
MCI Member
*****
Offline Offline

Posts: 305


« Reply #6 on: February 14, 2017, 03:52:35 PM »

Picture please. That there is no change Auto On/Off is good. 17v on VCA is good.

It's probably a fader law problem associated with the change of VCA, especially if black 202s were taken out. (Good thing)

The black 202 VCA has a different dB/mV change rate compared with later VCAs.
For example, changing black 202 for gold 202C VCAs requires the feed resistor to the control port on the VCA to be changed. On the 500 this is on the VCA daughter board. It will be near the VCA on a 600.

I am going to work on an early JH636 tomorrow in fact, so more later, but it is not faulty rails.
The reason for a picture is I want to see the That VCA board. There are a number of 3rd party retrofits available or are these homemade? That's the question. And if there is any documentation of this retrofit, a scan would be nice.

Another test.
VCAs were/are usually run as VCAtts. ie, they are configured as attenuaters(?), because the distortion figures go through the roof when you try and put gain into them. MCI does the same, the VCA is run at unity with the fader at the top, followed by a +12dB buffer.
So, same test as 0VU in to hard L/R mix, no EQ. But send -8dBu. You should get something approaching +4dBu on the mix buss output. ie the VCA is at unity (and essentially having no effect) and you are seeing the +12dB gain buffer only.
Please confirm Y/N.

PC

Logged
Harrison
MCI Master
MCI Member
*****
Offline Offline

Posts: 305


« Reply #7 on: February 14, 2017, 05:35:55 PM »

The 12dB gain element for the VCA is the VCA inverter on the main module. (10K/39K)
Therefore, at fader full up, the in and out of the fader module should be the same (pretty much).
Logged
Harrison
MCI Master
MCI Member
*****
Offline Offline

Posts: 305


« Reply #8 on: February 18, 2017, 04:39:05 PM »

The console I've been looking at (#015) has 202Cs and has a -3dB error channel fader to VCA.
R200 on the fader pack is 1K05. If it's 3K9, then your console used to have black 202s. It is this resistor that resolves the DC voltage vs the requisite mV/dB.

If you are using the Gyraf SSL 202 clone board, there is additional scaling on that. This is a L pad before the IC that feeds the control port.
Logged
revmook
MCI Member
*
Offline Offline

Posts: 6


« Reply #9 on: February 21, 2017, 10:24:31 PM »

Harrison,

Thanks for the replies!  My console is a late model Sony badged 600 series, serial number 714.  The packs have flux around the VCA pins and the big Molex connectors, so either the VCA was changed and the Molex pins re-flowed at the same time, or the heavy pins wouldn't wave solder correctly, so they were done in a secondary assembly process.

Checked a few of my packs R200 value which are indeed 1K05.  I got some spare faders on eBay which have the gold can (202C VCA) and put one in a known good channel, but still got the same gain error.  Being curious I soldered a trim pot in place of R200, adjusted it for 3K9, plugged the pack into the console, put the fader at "0" and the output to mix was within .5dB of where I would expect it to be.

So, if these THAT VCA's need a different resistor value in R200, does the rest of the notes in the schematic apply?  This would have been a HUGE oversight from the tech that did the original VCA swapping.  It's hard to believe it wasn't noticed or corrected until now, which is why I still have a sneaking su****ion this is a global problem.  Thoughts?  



Photos of my packs below below.




« Last Edit: February 21, 2017, 10:35:58 PM by revmook » Logged
revmook
MCI Member
*
Offline Offline

Posts: 6


« Reply #10 on: February 21, 2017, 11:13:31 PM »

I popped one of the VCAs out of circuit to see if I could find any identifiers.  Nothing stands out except the DBX logo which is in the copper.  I don't think these are from THAT.

edit: These appear to be DBX 202X.


« Last Edit: February 21, 2017, 11:21:22 PM by revmook » Logged
Rob
MCI Member
***
Offline Offline

Posts: 215


« Reply #11 on: February 22, 2017, 11:27:41 AM »

Don't know if you have seen these That Corp tech documents

http://www.thatcorp.com/datashts/dn137.pdf
http://www.thatcorp.com/datashts/dn124.pdf

I'm sure there was one which related to subbing them for DBX versions as well but can't seem to find it
Logged
revmook
MCI Member
*
Offline Offline

Posts: 6


« Reply #12 on: February 22, 2017, 12:34:53 PM »

Hi Rob, Yes, the document I think you are referring to is here.
http://www.thatcorp.com/datashts/dn127.pdf

edit: Also this data sheet has useful info
http://www.thatcorp.com/datashts/202data.pdf

« Last Edit: February 22, 2017, 12:55:21 PM by revmook » Logged
Harrison
MCI Master
MCI Member
*****
Offline Offline

Posts: 305


« Reply #13 on: February 22, 2017, 05:00:19 PM »

We could  "he could've should've would've" all day, but the main thing is it's working.
The 202XL packs that you have are configured as 202 replacements not 202C replacements, hence the need for the old 3K9 feed resistor.

By the way, those 202XL packs are good, so this has been worth the effort. Let's hope he set the SMPTE IMD.

There is a discussion as to whether you set the best IMD at fader 0  (ie where you use the fader most) or VCA unity gain zero.  ie you can get 2 different results.
PC
Logged
Harrison
MCI Master
MCI Member
*****
Offline Offline

Posts: 305


« Reply #14 on: February 22, 2017, 05:19:05 PM »

They are probably 202R. This was the 8-wide open frame designed to replace the black 202.
It does therefore, make you wonder what was taken out!!
OR... this is how it came from the factory and it's always been like it.
PC
Logged
Pages: [1] Print 
« previous next »
Jump to:  


Login with username, password and session length

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!