Drawmer 1960 Tech Anyone?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Ian MacGregor

Well-known member
Joined
Jun 3, 2004
Messages
280
Location
Echo Park, Los Angeles, CA, USA
Has anyone had any experience tech-ing one of these units? I'm seeing a lot of spurious signal output, even without the input being driven. It appears that the problem is in the compressor/line input section as the mic pre appears to work ok. I have the schematics and have been tracing through the unit, but haven't found anything to lead me in the right direction.

I'm getting the same issue in both channels, but it's not like each channel is acting identically, which is interesting. Anyone have any info about the potted "VCA" module?

Looking for anyone with some experience here, maybe there's a known issue/problem here.

Ian 
 
The first version of the potted VCA burned up and died pretty regularly.  The first board layout was also different, and the current replacement VCA's won't fit it.  That's the only thing I've heard about, and experienced. 

Do you know the age, and have you contacted Drawmer?
 
Hi,
Not sure about age and haven't contacted Drawmer as I got the unit second hand (for very cheap...). I suppose that if it is the VCA, getting assistance through Drawmer may be my only hope.

I find it interesting that both the channels have failed...

Now to contact Drawmer, thanks for your help.
Ian
 
It appears that I´m joining the party late.... but I actually have a 1960 on the bench, trying to wrap my head around that thing. In case your fault appears on both channels then I´d suspect the chip which drives the insert. It´s a NE5532 shared for left and right channel at the same time.

The 1960 here has no potted black box, only a bunch of TL072 and 5532 which don´t appear in the schematic I have (with the black box). No chip VCA around how hearsay suggests it to be the case. Is there someone who knows what´s truly going on? From what I see I´d guess the potted thingy was used only to hide the tube driver circuit. If that´s the case then 1960s seems to be a vari Mu type compressor, using ECC83s for remote cutoff mode.
 
Isn't it a optical comp? Of so and if they used optocouplers for it, then those are suspicious as well. They are known to fail after some years.

Btw, is yours the one that just sold on ebay? Great find!

Maarten
 
From what I remember when trying to make qualified guesses to what the blackbox contained (a very long time ago) - I think it's a vari-mu like differential long-tailed pair, but with a (semiconductor) variable current source common to the cathodes in stead of controlling the grids like in "real" vari-mu.

Much like an OTA, IIRC.

Jakob E.
 
OTA=Operational Transconductance Amplifier
something like LM13600/LM13700/CA3080/...
maybe useful (sorry, lost original source to give credit or refer to direct link) ota-tutorial
 
For what it's worth , I recapped one with the elna electrolytics
and made a noticable difference , not brighter like i may have thought
but less muddy , so overall a little more clear & tighter .
Did one ch first and compared them

Gar Gilles [ gtr amp builder ] while he was still alive , helped me build
a gtr preamp design of his that controled gain from the cathode ,
that may have been too wide a range for a micpre but gtr would
quickly get into overdrive
 
Hi Ian.
Had same problem with a unit i had to fix. Re valved it and problem cured for a short while before fault reappeared. Checked the unit in situ and found that the sidechain wiring was open at the patchbay. Rectified this and put more new tubes - problem solved!
Check your wiring if using sidechain jacks. If not - continuity check between sidechain tip and ring - if this isn't showing low resistance clean the jack contacts or temp solder across the contacts and check.
I concluded that the sidechain return on the faulty unit was acting like an ariel and this was causing oscillations around the valve stages. Once the valves had been stressed they needed replacing.
This was a repair i did 4 to 5 years ago and the unit has been in daily use since then without problems.
Hope this helps - the symptons seem to be identical to the unit i had to repair!
 
The non potted vca 1960 schematic is posted at the gmail site...for educational purposes only, obviously:)
 
Some pots are between DC coupled opamps. Not a good design practice with non-precision opamps and carbon pots. I swapped many TL072s with low offset chips to adress that problem.
 
topcat said:
The non potted vca 1960 schematic is posted at the gmail site...for educational purposes only, obviously:)

hmmm, if the controll voltage controlls the FET siting between, and controlling the gain of the two 5532's just before the tubes( mid page, far right) doesnt that make this a FET compressor? I allways thought this was a vari-mu comp.
J
 
Johan said:
topcat said:
The non potted vca 1960 schematic is posted at the gmail site...for educational purposes only, obviously:)

hmmm, if the controll voltage controlls the FET siting between, and controlling the gain of the two 5532's just before the tubes( mid page, far right) doesnt that make this a FET compressor? I allways thought this was a vari-mu comp.
J


Hence the epoxy block?

I'n the 80's I x-rayed, cut up or disolved a few epoxy modules that promised the moon but the contents were always a bit disappointing ..

tc
 

Latest posts

Back
Top