DBX V8 VCA module teardown

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abbey road d enfer

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squarewave said:
Just so that I'm clear, is the 2181A is the absolute best performing VCA available?
That's correct. Apparently, the V8 does not allow the use of symmetry trimming; hence the use of the 2180.
 

JohnRoberts

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abbey road d enfer said:
That's correct. Apparently, the V8 does not allow the use of symmetry trimming; hence the use of the 2180.
If there is interaction between parallel VCAs that could complicate any trimming process.

JR
 

abbey road d enfer

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JohnRoberts said:
If there is interaction between parallel VCAs that could complicate any trimming process.

JR
Yes. Symmetry trimming can be positive or negative. Improving one chip may detriment another.
 

beatnik

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Thanks everybody for the information.

On the V8 the symmetry pins of the vca ics are left unconnected, except one (U4)

So that means the 2181 trimmable version is used ? And only one vca ic is being trimmed ?
 

emrr

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beatnik said:
Thanks everybody for the information.

On the V8 the symmetry pins of the vca ics are left unconnected, except one (U4)

So that means the 2181 trimmable version is used ? And only one vca ic is being trimmed ?


It's been noted by some people that in some cases the pre-trimmed version in arrays like this can be more accurate than the trimmable version can achieve. 
 

beatnik

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but then I cannot explain why on the first vca pin 4, which is the symmetry, there is actually a connection to the main board.

If it were the non trimmable 2180 then pin 4 should be left unconnected right ?

apologies if I might be missing something obvious
 

beatnik

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Fair enough

I am actually hoping the original ics are still alive and haven't been damaged by the heat.

I will use THAT2180A in case I have to replace any
 

abbey road d enfer

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beatnik said:
but then I cannot explain why on the first vca pin 4, which is the symmetry, there is actually a connection to the main board.

If it were the non trimmable 2180 then pin 4 should be left unconnected right ?
There's no doubt the trimmable version is used, at least in position 1.
Since the symmetry control is akin to an offset, THAT designers have probably found out that only one is necessary.
 

JohnRoberts

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I am too lazy to research this (surely there is advice. ).

It seems logical to put pre-trimmed VCA in the other three slots.

The only question I would raise is wrt termination of trim pins... Do what the app note says for the 3 pre-trimmed ones, and alternately what the trimmed app note says for the one trimmed one.

JR
 

beatnik

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Looking closely at the V2 ics it seems that what I thought were date codes might actually be specific part numbers

There are 3 ics marked 9611-5342-9702 and 1 ic marked 9611-5266-9702

That likely confirms your theory-  first position is a trimmable vca and other three positions are pre trimmed types

I have soaked the parts in acetone to fully clean them and soon will start rebuilding the vca module
 

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GitouMan

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Always wondered if you got it back up running again? I have one that will probably require the same treatment!
 

Rob Flinn

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You might find the last post on this thread, by PorkyC useful to make some comparisons

https://groupdiy.com/index.php?topic=64110.20
 

GitouMan

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Hi Rob,

Thanks for that link. So if I understand correctly all four vca’s could potentially be packaged in one IC. (2164) good to know. I have two dbx sl160 in my rack. I have grown very fond of them over the years, but am becoming paranoia as one vca 8 module has died on me. I’m considering un potting it and repairing it. Both units are the earlier 24 volt models, so I fear the remaining 3 channels will face the same faith. If I can repair the dead one by unpotting, I will probably do the same with the rest as well.

Geert
 

GitouMan

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Hi David,

I did today the same for a dbx m8 potted opamp for the dbx 786 preamp. I was quite successful at it. I went about accordingly:

1 boiled in water for 15 minutes (the removed the top layer of epoxy = bottom side of the pcb

2 boiled for a further 30 minutes (cut with exacto knife on all sides until the bottom of the metal can)

3 boiled further for 10 minutes (pried out the epoxy with a flat screw driver on the width of the metal can)

4 boiled further for 15 minutes I think (was intermittent - took a very wide wood chisel and slowly started to pry upwards the pcb. Did this very gently and several times and on both sides. From a certain moment it will lift a couple of millimeters, I then put it back in and boiled some more)

5 after about another 15 minutes the pcb with all the components on it came loose from the casing. Than continued with further prying away epoxy while quickly submerging it in and out of the boiling water.

In any case the above worked well for me. It makes use of the glass state of epoxy round about the boiling point of water. It becomes very brittle however after a few seconds it cools back down and you need to heat it up again. (Very small process window)

So as you can see it took a lot of time. I also have a dead dbx V8 as well. I think I will do the same as above and include an acetone soak for a day or two to get the board fully clean. I will keep you posted on how that works!

Hope that helps
 

pusher

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I hope someone will clone the V8 module, in the future. Preferably a module that cover both versions  ( was it 16 and 24 Volts ? ) with a jumper arrangement. In the meantime Fairman of denmark made a updated version of the M8  module check out https://www.fairman.dk/p6.htm
I wonder if the 160 S/SL will ever be fashionable in studios again. I never see them around anymore, perhaps because of the vca problem ? also its hard to service the way they put it at the bottom of a folded steel case, no way to get to the bottom of the board without a thorough disassembly.
 

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