Newb needs help with QSC MX700 Power Amplifier

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,317
Good Morning,
I am looking for someone who doesn't mind sharing advice and leading a newbie through a repair on a QSC MX700 Power Amplifier. I teach DC electrical to automotive apprentices and find it quite satisfying. I have pretty good knowledge of components and can follow a schematic quite well. What I need is someone to guide me through the process of testing this amp and leading me to the promise land. For me, this will be a challenge. I have done car audio amps and have had good success. This is green territory for me but more than willing to go at it step by step. Please help.
Here is the problem. Switch on, unit hums, green light only on in front but no sound at all. Have it hooked up to Pioneer SX-5 receiver and two 8 ohm speakers. Is this right so far? When you are done laughing, please provide some insight. :)
 

#12

Joined Nov 30, 2010
18,210
Start with the obvious. Make sure no plugs are in the input jacks and stick your finger on each input screw, one at a time, to see if you can get it to amplify finger hum. Notice how many channels are not working. Measure for +/- 63 volts DC and +/- 15 VDC. Switch your meter to AC and see if you can measure amplified finger hum at the junction of C12 and C13.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,317
Okay when I touch my fingers on each input screw, there is no response. I haven't quite narrowed down my terminals yet for 63V measurement but I have 132 VDC, 67VDC and 64VDC depending on combination of terminals I use. I would assume (but I will confirm) that 64VDC is the correct combination. On the 15V side, I have nothing. In the diagram, it shows PTC1 and PTC2. Can I assume these are positive temperature coefficient thermistors and if so, what do they look like. I have this board out and I don't see anything that is marked PTC1 and PTC2 and nothing that resembles a thermistor.I have located the components around it but nothing. Where do I put me leads to measure that junction. Will one be on ground and the other at the junction? Thank you for your help. I appreciate it
 

#12

Joined Nov 30, 2010
18,210
The PTC thingys are on the heat sink feeling the temperature of the output transistors. Do you see a red LED lit up?
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,317
Okay #12, I had no LED so I located the PTC's and they were in the center of the board tucked back in under the heat sink (nice place) and I found a bad solder joint on the one PTC. They measured 61 and 48 ohms respectively. Once I repaired the joint, the red LED came on. Still no reaction with finger on input and still no 15VDC coming out of D10 or D11.

R!f@@ - What was the purpose of your comment?
 

#12

Joined Nov 30, 2010
18,210
OK. Red is good. The LED indicates that the PTC's are dumping the error signal so Q12 does not collapse the +/- 15V supplies.

Now we're looking for the birth place of the +/- 15V supplies. At the top center of the drawing find R20, 2.4K 2W. That feeds +63 to Z1 (15V). Find out what voltage is on Z1 then look at the mirror of that circuit and find Z2...or just find pins 4 and 8 of the op-amp and see if the +/- 15V is there. But I'd be happier if you found R20 and R21 and measured both sides of them so I can double check you.

R20 and R21 might get hot enough to crystallized their solder joints, but I doubt both of them failed at once. Odds are the op-amp shorted and collapsed the +/- 15 supplies.

Of course, I'm assuming all the inter-connecting wiring is still connected. If it isn't, the PTC circuit will not have the 15 volt supplies arriving at D10 and D11.
 
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#12

Joined Nov 30, 2010
18,210
Thanks. I've been staring at the schematic for 20 minutes trying to figure that out. Bottom line: the secondaries of the power transformer float. Hard to break old habits, like assuming there is a hard ground somewhere near the transformer.

Still, I think I've got this puppy whipped. Probably going to have to unsolder something to see which part is collapsing the 15 supplies. I guess I have to wait until the OP wakes up to continue.

Y'all feel free to continue if I'm at church when the OP gets back.;)
 

rogs

Joined Aug 28, 2009
279
The GND is at output stage emitters
QSC ground the output device collectors, not the emitters...
It's an unusual configuration, they AC couple the output by using the main DC supply decoupling capacitors as output AC coupling caps as well.

Inherent protection against DC faults.....(unless one of the main caps fails!)

Probably more reliable protection than some of the DC output protection circuits which use relays, that I've seen over the years.....
 

Dodgydave

Joined Jun 22, 2012
9,169
Ok lets start by measuing the supply voltages on the Op amp +/- 15 v on pins 4, 8, also pin 7, and the voltages on the output transistors +/- 63v
 

rogs

Joined Aug 28, 2009
279
It's not very common, but can be very effective at protecting against DC fault conditions.

Useful - especially if you are using expensive loudspeaker drivers!
 
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rogs

Joined Aug 28, 2009
279
OK. Red is good. The LED indicates that the PTC's are dumping the error signal so Q12 does not collapse the +/- 15V supplies.
I can't see how this works..... if the LED (LD3) is on, then its cathode at the junction of R34 and R35 must surely be a couple of volts negative to the power ground (P) to allow the LED to turn on?....

If that's true, then surely Q11 must also be turned on?....
As its emitter is also connected to the 'P' gnd , the base will be at a lower potential, and base current via R40, R34 and R35, will flow.
If Q11 is turned on, then so is Q12.... and the + and - 15 volt rails will be collapsed.....

Unless I've misunderstood somewhere?......
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,317
Okay I just want to make sure I am getting this right. So I will place my black lead of my voltmeter on R21 and my red lead on R20 and I should read 63VDC. Then I can go to R20/R13 junction with my red lead and R12/R21 junction and I should read 15VDC. Is that correct? If so, I think my readings are way off. I will check them again but please confirm I am doing this correctly. Thank you
 

#12

Joined Nov 30, 2010
18,210
@rogs

Q11 is the key. PNP, emitter grounded, must have negative supply to start conduction.
The negative supply is coming from R35 (47K). If the PTC branch does not dump that current to ground, it will turn on Q11.

But there is still a connection through R34 (6.8K). Yes, and 2 volts plus 200 ohms at 1.16 ma is 2.32 volts negative. 341 ua is still to be accounted for.

R33 (150K) is bringing 328 ua and R40 (100K) is bringing 138 ua. Thus, Q11 is off if the LED is on.

If the PTC string goes to high resistance, the 1.16 ma through R35 (47K) will pull the base of Q11 negative and turn on Q12.
 

#12

Joined Nov 30, 2010
18,210
Pick a ground for the black lead. Red at 4 places defined by the ends of R20 and R21. You are looking for +63, +15, -15, -63.

What do you find?
 
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