Question about Focusrite ISA428 Pre Pack quad microphone preamp (with )

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My Focusrite ISA428 has some consistent issues where channel 1 buzzes, and channel 4 is unusable for 2 hours after power up. Channel 4's needle goes right up to the meter and stutters there for an hour, then settles at a high db reading, then after a following hour falls at the 0db position where the channel is only now usable. Makes my ISA428 a usable 2 channel unit most sessions.

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Does this happen with no mics AND outputs connected? If it doesn't, you have a severe ground loop problem. Make sure ALL interconnected equipment is powered from teh same receptacle or power source. Use ONLY balanced lines, XLR or TRS cables. A possible problem MIGHT be supersonic feedback that you can't hear but is saturating the amp. Thats is why we disconnect all inouts and outputs and observe if symtoms are still there.

Posted on Apr 06, 2011

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  • danodeniel74 Apr 06, 2011

    Well, I did mention that it CONSISTENTLY reproduces these symptoms at every power up, for the same amount of time. It does this with nothing connected. I'm almost a slave to power up 2 hours before a recording session. But I will ensure the other advisories that you gave.

  • Fred Yearian Apr 06, 2011

    OK next test... sometimes it takes several. This MAY be a thermal issue. Thermal issues are fairly easy to resolve as we use hair driers to warm and circuit cooler to vary the temp of suspect chips. If you can, remove the cover of the unit. ALWAYS look for obvious things... common things are bad solder ESPECIALLY at potentiometer leads. Then verify problem exists with the cover off. If it does, from a cold start that would take two hours, instead warm the insides of it using a hair drier... does it come on sooner? Next from a cold start wiggle areas of the board, especially on the side or area that has the pegged meter. If you have the thing immediately coming on when wiggling, you likely have either a bad solder joint OR a crack in the circuit board. Use a bright LED flashlight and "candle" the board looking for cracks. The above are the procedures I would use in my shop. Don't be surprised if removing the cover fixes the problem... it will be back unless you find the smoking gun...
    Now for suspected components assuming there are no cracks or bad soldering, the LIKELY component is one of the can electrolytic caps. If hair drier brings thing on right away, cool each down with circuit cooler. Give at least 5 seconds each after spraying for cold to "soak" in.
    Chips and transistors are next suspects and resistors are far down the list statistically.

  • danodeniel74 Apr 07, 2011

    wow detailed test. I havent been able to go at these lengths as yet, but I should mention that yesterday when for curiosity i tapped the area directly above the troubled channel 4, the meters responded to the very light impact- moth needle and LED's! there's definitely something going on in there. I could reproduce the LED's responding by lightly tapping the area on top, or on the side of the channel 4 everytime!

  • Fred Yearian Apr 07, 2011

    I have confidence you will find something when opening... LIKELY a bad solder joint at least for the channel that pegs the meter. We call these problems "morning sickness" as the devices recover after they warm up. An experienced eye will spot bad solder as a tiny grey crack around where leads come through the board. Overnight these broken joints oxidize and as they warm or slight current flows through teh oxide, the joint re-establishes contact... till it cools down and starts the cycle over.


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