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TAC Scorpion II noise on channel LINE IN

Discussion in 'Vintage Analog Gear' started by PhantomBox, Sep 4, 2011.

  1. PhantomBox

    PhantomBox Active Member

    To anyone:
    On my TAC Scorpion II XRB32/8/2(K) console made in 1990, when I use the line in option on channel 1 or 2 and assign this channel to group 1 & 2 with PAN at center and nothing connected to the "IN" socket on the patch-bay (only way to use a insert a line on this desk, no separate LINE connectors on the back of the desk), a loud melodic "bsss" noise comes of the console. PAN to the left or right and "bsss" on group 1 or 2.
    Same happens on channel 3 or 4 and group 3 & 4, and so on up to channel 7 or 8 and groups 7 & 8.
    From channel 9 up all seam to be OK.

    Any ideas on what's going on there?

    Phantombox
     
  2. JoeH

    JoeH Well-Known Member

    Not having a schematic in front of me, it's just a wild guess that there's an IC in there with mulitple outputs, some feeding 1-8, then 9 and up are from other outputs of a different IC (or an IC with multiple outs). I'm not real clear on the type of noise you're describing. If the ICs are socket mounted, you may be able to remove, reseat and possibly even swap them out with a nearby channel strip. Could also be a bad cap, but too touch to tell from your description. Are all over channels ok in this regard? (Have you checked power supply points for this as well?) Also check for "Cold" solder joints and such. Could always be something as simple as a bad ground.
     
  3. PhantomBox

    PhantomBox Active Member

    Hi Joe.
    The noise is not a hiss or a hum. Its more like a melodic "bsss", somehow pitch- but definitely volume-dependent on how many of channels 1 to 8 are assigned to the same group(s).
    There are only socket mounted TL072's in this console. IIRC, there are 2 larger ICs (not socket mounted) on the 4001 module but this module only serves for main VOL on L/R out, AUX out, and TAPE out 1 and 2, and has the oscilloscope.
    Do you have private mail? I could send you the schematics if you want.

    Phantombox
     
  4. JoeH

    JoeH Well-Known Member

    Probably wouldn't do much good to send me schematics; I don't work on these kinds of things a lot anymore. I mostly meant that I don't know if it's separate, swappable modules, or ribbon cables underneath, connecting everything on one big motherboard, etc. If these are separate modules that have an edge connector going into a slot, you may want to try swapping it out with another. If it's a multipair ribbon cable, you may want to gently remove it and reseat it, maybe even using some Caig De-oxit cleaner or equivalent. (Clean the card's edge too, if that's the case.)

    If you're comfortable with removing and swapping ICs, you could (if you're careful) swap the ICs out from another module. With older stuff like this, the best approach is usually more detective work, checking voltages, looking for cold solder joints, dried out or leaky caps, dirty/corroded contacts, etc. From the sound of what you're describing, I'm still thinking it's a bad ground or cap opened up. Just a hunch, of course.
     
  5. PhantomBox

    PhantomBox Active Member

    The TAC Scorpion is a modular design with swappable modules, an edge connector on each module and several ribbon connectors:
    on the INPUT modules its a ribbon for the line in, send /return and direct out as well as one for the fader
    on the GROUP (dual) module its one for the VUs, send/return and outputs and another fader
    on the MASTER module it are several for the different outputs, VU L/R and PFL and OCS. And once agian 2 faders for L/R

    I checked all of the ribbons as well as the edge connectors and modules and on the rails and they are OK.

    I just noticed that the gain controls on ALL modules were nearly all the way up. I turned them down to about 11 o'clock and - voila - noise is gone unless I turn them up again.
    I'm not sure, but, is this normal behavior for a console?

    Phantombox
     
  6. JoeH

    JoeH Well-Known Member

    Sounds like you have some oscillation going on with the inputs opened up full like that (are they unloaded? nothing attached/plugged in to them?) Could also be some kind of RF coming in as well, and with so many set at full-open, it may be a cumulative effect. Best to keep 'em all set at full off unless you need them.

    It could also just be poor circuit design, but again, I'd keep anything not being used set at full off or muted until you need it. Could also be issues with the power supply, maybe something's flagging when all circuits are full on, using full gain on the op amps, etc. Sounds like you can work around it well enough, but it may be worth looking into by a qualified tech in the long run.

    OTOH, for the $$ you may spend, it may be time for something newer and more reliable if it keeps happening.
     
  7. RemyRAD

    RemyRAD Well-Known Member

    This sounds like another situation regarding proper gain staging & proper input loading. Many amplification circuits frequently become oscillating circuits when integrated circuit chip operational amplifiers are not connected to things properly. TLO70 series chips are extremely high input impedance JFET inputs. They have an input impedance on the order of a couple of million ohms and expect to see at least a 10,000 ohm load down at the input. Without that, they go crazy. And it will make you crazy too. Obviously you are being quite successful in that department. Those were nice little consoles and I think still quite good. A beautiful lower cost analog desk. If you have your gain trims turned up all the way with nothing feeding those modules you are usually bound to hear something bad.

    Hearing only good music in my head
    Mx. Remy Ann David
     

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