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MCI JH-600 2 channel module makes noise

Discussion in 'Pro Audio Equipment' started by PhantomBox, Dec 29, 2008.

  1. PhantomBox

    PhantomBox Active Member

    MC and HNY to everyone.
    Some time ago I delivered a 2 channel-strip module from a MCI-made (now Sony) JH-600 console. It worked really well until the owner lend it to someone else for a few recordings. When he he got it back it did nothing at all. So he called me again to have a look at it.
    Turns out that a multiconnector got loose from the power supply to the mainboard, so I soldered it back in place.
    Now here's the problem: each channel alone connected to the mainboard sounds perfectly. But as soon as I connect the other channel, an anoying noise appears that is not a "hum" nor a "hiss" but more like a "dsss". It becomes less audible the more I turn up the fader (rotary or linear) but it does not disappear.
    When I disconnect any one of the channels from the mainboard this noise is suddenly gone, only to reappear when I reconnect the previously removed channel.
    I realy hope you could follow me.
    ... any idea?

    PhantmBox
     
  2. RemyRAD

    RemyRAD Well-Known Member

    Not sure why, my 84-year-old mother got into all my MCI manuals and removed & threw out the manuals from the looseleaf folders. She liked the looseleaf folders. She thought I didn't use them any more. So my entire collection is in one huge garbage bag. I can't put my fingers right on the channel strip module. But let's see here?

    So you have a pair of these channel strips. And you are indicating something about having them plugged in to "a motherboard"? Um, this must be some kind of custom motherboard since they only offered 18 inputs as a minimum frame. So this must be a custom motherboard? Is this mounted in a special enclosure with proper supports? Your noise sounds as if something has been inadvertently changed when you reconnected to the power supply. But you may also be having some kind of interaction problems between the two units were one of the 5534's may be oscillating? And it may be a result of the connector wiring? Slight differences in the impedance of the power supply cable could cause this as well as the length of the power supply cable. There may be some decoupling capacitors, which are frequently electrolytic, then may have failed? That could cause the problems you are describing. The decoupling capacitor is generally a large electrolytic that helps to filter & calm down the power on the channel strip circuit boards. And since electrolytic capacitors are a little bit like batteries, they have a limited useful life. You may need to replace all of them?

    And you did not indicate if you had any thing plugged into the channel strips to load down their inputs with proper sources. So it sounds to me like power supply decoupling may need to be replaced? Especially since you indicate there is no problem with each unit plugged in individually? So it's an interactive problem between them which usually means a decoupling problem. This might be in that connect you repaired? Could be on the channel strips? Could be on the "mainboard" whatever that means? Because you're not talking a whole console now are you?

    Spinning in circles in the dark
    Ms. Remy Ann David
     
  3. PhantomBox

    PhantomBox Active Member

    Hi RemyRAD
    Actually, that particular console had 36 channels. Im talking about the lower motherboard from which I cut off a section with 2 receiving connectors. That's my motherboard for these 2 channel strips.
    It is mounted in a special enclosure with proper mountings and its own internal power-supply (a huge case, but not as huge as for example 2 channels of a Harrison MR3 console, which I also made).
    And I do have the manuals for both the MCI JH600 and the Harrison MR3 consoles.
    Since the connection from power supply to mainboard worked OK before I didn't modify the length of these cables and just resoldered it.
    There are lots of 5534's (and/or 2003's in this case) on each channel board, so which one are you talking about in particular?
    Connecting it to a Mackie 1202 console this noise is present with or without mics connected.
    As for the huge electrolytic capacitors used for decoupling, do you mean they should be connected between the +18V and ground rails, and between the ground and -18V rails on the motherboard?
    I think I remember that on the Harrison MR3 console that I mentioned before, there were 100uF/25V electrolitic caps soldered between those rails on the PS (+15V/gnd/-15V) for the VU/PPM meter section. I can't remember if the MCI console also had 2 caps connected between the +18/GND/-18 rails before entering the motherboard since it wasn't me who took the whole thing apart, but it is quite possible, now that you mentioned it.

    PhantomBox
     
  4. Boswell

    Boswell Moderator Distinguished Member

    You will have to use trial-and-error to find out which joint was not making contact before you re-soldered them all, and then de-solder it again.

    Seriously, though, the diagnostic here is that the noise decreases as you turn the fader up. That points to either a ground loop or an ultrasonic oscillation being conducted through the ground wires.

    You need to get hold of an oscilloscope and look at the character of the noise as seen at the output. If it's periodic (e.g. a 100KHz sinewave) with an amplitude varying with fader position, you need to apply more decoupling to the circuit. What is probably happening is that the extra power taken by the second strip is causing the power supply to sag sufficiently to push the operating point of some amplifier into an unstable region. You should also use a digital voltmeter to measure the power rails with 0, 1 and 2 strips inserted.
     
  5. PhantomBox

    PhantomBox Active Member

    Hi Boswell.
    I checked today all joints from the PS to the motherboard and resoldered them. They are all tight now and OK. Then I connected the unit to my Mackie 1202 and the noise ("dsss") remained.
    Then I put 2 pcs 100uF/63V electrolitic capacitors on the motherboard itself, one between -18V and gnd and the other between +18V and gnd. I checked it again on the Mackie and yep, the noise was still there.
    There is something to the noise: when the faders are completely turned down the noise is at its strongest. When I turn up one fader it decreases until +/- 0dB. When I turn up the other fader it decreases even more. When one or the other fader passes 0dB it increases again. There is a ballance point when the noise is almost unaudible, but its still there at any fader setting.
    Another thing is that this noise has a high and low point that repeats (oscillates) at an interval of more or less 800 to 900 ms.
    Any idea of what that might be?
    BTW, the +18V and -18V are about 8mV apart. That's as close as I can adjust them. And I don't have an oscilloscope.
     
  6. Boswell

    Boswell Moderator Distinguished Member

    From your further description, I think this is likely to be a stability problem. It's ironic that your re-soldering of the power connections probably has reduced the ground impedance to a level that is now causing trouble with circulating currents.

    The null at around 0dB on the faders is probably a balance point between the ground noise being injected through the low end of the fader and lower-amplitude anti-phase noise coming in on the top end.

    An oscilloscope is the diagnostic instrument of choice for this type of problem, but since you do not have one to hand, you may be able to use the Mackie 1202 and a computer DAW to make some quantitative measurements. For example, do you get the same amplitude noise from both channels? Is it the same noise from the two channels or random (use a phase plot)? As you take one channel through the 0dB fader position, does the phase of the noise invert relative to the other channel?

    I can't picture the mechanical setup you have with the two strips, but is the problem only present when you have both strips connected, or is it there if both are inserted in the motherboard but only one in use?

    BTW, I hope the +/-18V rails are actually 36V apart, but I suppose you mean that their numerical values differ by 8mV. That's fine. They could be 1V different and it would not affect the operation apart from giving asymmetrical clipping.
     
  7. PhantomBox

    PhantomBox Active Member

    What is a "computer DAW"?
    And the problem is only present with both strips connected. As soon as I remove one strip the noise disappears.
    It (almost) disappears also with one fader completely up and the other completely down. Previously I wrongly said that passed the 0dB point on one fader it increases again. Also, it doesn't oscilate in an 800 to 900ms intervall anymore (?!).
     
  8. Boswell

    Boswell Moderator Distinguished Member

    Digital Audio Workstation - this is your computer with audio interface attached running audio recording and editing software.
    I still can't tell whether by "remove one strip" you mean physically remove the strip from the motherboard or leave it in situ and simply not connect it.
     
  9. PhantomBox

    PhantomBox Active Member

    I don't have an DAW on my computer.
    And I mean physically removing the strip. There is no "on/off" switch on one board.
    BTW, I will get my hands on an oscilloscope around the 20th of this month.
    What should I be testing exactly? (To be honest, I don't know much about using an oscilloscope).
     

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