Need help with another QSC MX 700 Amp

Thread Starter


Joined 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.



Joined 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.
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Thread Starter


Joined 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.

Thread Starter


Joined 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.


Joined 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.

Thread Starter


Joined 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.



Joined 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.

Thread Starter


Joined 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.