Need help with another QSC MX 700 Amp

Discussion in 'Technical Repair' started by rwils139, Dec 27, 2013.

  1. rwils139

    Thread Starter New Member

    Dec 27, 2013
    Anyone interested in sharing their experience and education in trouble shooting and repair?

    If you have some time and a little patience I need to diagnose the reason as to the failure of the #1 channel of my QSC 700 amp.

    I am no expert! I can however follow schematics and am competent with a multimeter. I have work experience with a soldering station but there's a HUGE difference between being a parts changer and being able to trouble shoot a circuit.

    Following bwilliams60's thread almost got me to the solution, however he was able to resolve his problem by reflowing some solder joints. Good for him, bad for me.

    Anyway, if someone would like to throw in here I would greatly appreciate it.

  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Here is the manual, input a signal into both channels at the same time,and compare the good channel with the bad using a scope and Voltmeter.
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
    This amp has separate floating supplies for each channel.
    So, this amp can power up even with blown/shorted output transistors without blowing fuses/breakers.
    EDIT: Even if the power amp is blown it won't output high DC voltage into a speaker load due to the floating supply topology.

    Quick checks:
    1) Make sure the power supply +/- 63 volt rails are present and correct. (Measured across the main PS capacitors)

    2) Make sure the op-amp +/- 15 volt supply's are correct.
    Last edited: Dec 28, 2013
  4. rwils139

    Thread Starter New Member

    Dec 27, 2013
    Thanks for the link Dodgydave. tubeguy the current statis of the unit is...

    1. Unit powers on. Fan is on. Grn led ( LD2 ) is on. #2 channel seems to function as it should.
    2. No output on channel #1. Red led ( LD1 ) Clip 1 is on. Red led ( LD3b ) is on.
    3. There's residue on the board from fuse F1A blowing out at some time although obviously replaced since.

    OP AMP IC 1a (channel One)

    pin 1 1.31 vdc
    pin 2 0.98 vdc
    pin 3 0.35 vdc
    pin 4 0
    pin 5 -0.29
    pin 6 0
    pin 7 3.76 vdc
    pin 8 4.61 vdc

    OP AMP IC 1b (channel Two)

    pin 1 0
    pin 2 -.14vdc
    pin 3 0
    pin 4 -14.57vdc
    pin 5 0
    pin 6 0
    pin 7 .15vdc
    pin 8 14.48vdc

    Other measurements :

    D6a 31.3 vdc D6b 64.1 vdc
    D7a -99.6 vdc D7b -64.3 vdc

    D10a 4.62 / 13.9 vdc D10b 13.98 / 14.48 vdc
    D11a 0 / -14.2 vdc D11b -14.2 / -14.5 vdc

    R20a 4.61 / 31.18 vdc R20b 63.6 / 14.4 vdc
    R21a 0 / -98.0 vdc R21b -63.7 / -14.5 vdc

    Ideas on how to proceed from here would be greatly appreciated.
  5. MrChips


    Oct 2, 2009
    Unsolder capacitor C9a, 100/35V and check if this is shorting.
  6. rwils139

    Thread Starter New Member

    Dec 27, 2013
    Desoldered the C9a 100uF/35V cap as you suggested.
    There is continuity between the leads.
    So yes sir, it is shorted indeed.

    I'll have to aquire a replacement cap. I have nothing here thats even close in value. Any suggestions beyond that point?

    Would it be wise to replace all 4 of these caps? To replace all the electrolytic capacitors?

    Thank you much for your help. Your opinion is appreciated.
  7. MrChips


    Oct 2, 2009
    I normally would only replace the single bad one.
    It would be a good idea to order spares if you have to place an order.
    Post a photo or give us the dimensions.
    Depending on your location I can send you some via mail. Send me your address via PM.

    For testing, what do you have that is 25V or greater? You can temporarily attach a cap in place and see if it works.
  8. rwils139

    Thread Starter New Member

    Dec 27, 2013
    These particular capacitors are manufactured by PACCOM.

    The values are 100uF/35V 10% 85 Deg.

    They measure 10.5mm dia x 13 mm long. The lead spacing is approx. 5.2 mm.

    I have a Nichicon Fine Gold audio cap thats 470uF / 25V thats new. Will that work to test with?

    Most of my other caps that are new are higher voltage / lower capacitance that I use in speaker crossovers.

  9. MrChips


    Oct 2, 2009
    That will do for testing. The physical size does not have to match.
    Solder the 470uF in place paying attention to polarity and see if it works.
  10. rwils139

    Thread Starter New Member

    Dec 27, 2013
    MrChips, We have success!!! Awesome.

    Voltage measurements are comparable between the channels now as they should be.

    Used same imput to test both channels. Equal output with same amount of drive for both channels. Decent fidelity both channels.

    My friends band uses a couple of these amps for their floor monitors.
    They're playing new years eve at the Argosy here so they are all very anxious to get everything working 100%.

    I appreciate you're taking the time to help out a beginner.

    Thank You.