Newb needs help with QSC MX700 Power Amplifier

rogs

Joined Aug 28, 2009
279
@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.
Thanks for that explanation..... at first, it seems a complex way of doing it ..but it doesn't use that many components...so quite cheap to make. Just a question of idiots like me getting my head round the current 'routes' ..:)

But -- if R33 or Z5 are open circuit, and /or C17 is short circuit, them surely you have got the situation where the LED will still be on, but Q11 will also be turned on, the amp will never come out of mute?...

(would also explain the lack of +/- 15V at the op amp....)
 

#12

Joined Nov 30, 2010
18,224
Sometimes you have to account for microamp level budgets. That's why it's called "work".:D

and, it does seem convoluted. I am guessing the protection circuit was an after-thought.

Right now, I'm working from the idea that there is not .6 volts on the other side of D10 and D11. That indicates that the 15 supplies collapsed before they got to the protection circuit.

Of course, the OP only said the 15's are not there. That's why I'm making him measure at the birth place of the 15V supplies. If he comes up with +/- .6V, we're going back to the protection circuit.
 

rogs

Joined Aug 28, 2009
279
Of course, the OP only said the 15's are not there. That's why I'm making him measure at the birth place of the 15V supplies. If he comes up with +/- .6V, we're going back to the protection circuit.
Makes sense to me......although the OP may need to pick a 'P' ground, to get accurate enough readings?
In this case, we know chassis ground isn't even involved.... and maybe the 'A' ground has enough ' ground lift' to mess up DC readings, to be accurate within .6V ??....
 

#12

Joined Nov 30, 2010
18,224
It's all relative. Even if he picks a ground that is off center by 5 volts, the (2 x .6V) will show up in the 4 measurements.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
Okay here is what I have so far. I took the black lead of my voltmeter to the ground terminal on the input terminal strip and got the following readings:
D10a = +14.5 VDC
D10b = +14.8VDC
D11a = -14.8VDC
D11b = -14.6VDC
R20 = +64.8VDC on one side and +14.8VDC on the other
R21 = -64.8VDC on one side and -14.6VDC on the other

On the OP-AMP I have the following:
IC1a Pins 1,2,3,5 and 6 I have 0.00VDC
Pin 4 = -14.8 VDC
Pin 7 = 0.01 VDC
Pin 8 = 14.8 VDC
IC1b Pins 1,2,3,5 and 6 I have 0.00VDC
Pin 4 = -14.6 VDC
Pin 7 = 0.1 VDC
Pin 8 = 14.8 VDC
Quick question. In the automotive world of DC, we do not run into negative voltages. I was using D11a as my ground point before and perhaps that is why I had no reading. Why does it change in this world? We use ground as a zero reference point in automotive. I'm confused.
 

#12

Joined Nov 30, 2010
18,224
That clears up where the 15 volt supplies went...they didn't.

Back to the finger hum test.

I refuse to think about cars right now. That's like stopping in the middle of surgery to figure out why the ceiling lights work.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
Okay so while I was waiting, got out stronger magnifying glass and found few more bad solder joints on board. Resoldered joints, put back together and now this thing is pounding out the tunes. Thank you everybody for your help but sure would like to learn more about this stuff. Anybody have any good websites or books I can read.
#12 - special thanks to you for your patience. You are a good teacher and I respect you for that. You kept it simple and at a level where I could understand you. Still not sure about negative DC and ground though. Perhaps if and when you have time, you could explain.
I appreciate all the feedback. Have a great day
 

#12

Joined Nov 30, 2010
18,224
Now that surgery is over, I am willing to think about cars.

"Ground" or, "common" is merely a convenient place that is defined by the designer or even you. As long as everybody knows where ground is, we can communicate.

Audio waves are both positive and negative compared to their center so most amplifiers provide power for both halves of the wave. It can be done entirely in positive or entirely in negative, but that's inefficient. It's called Class A amplification.

Of course, you could simply declare that -63 volts is ground, but that would make it impossible to do the math in your head while you're troubleshooting the circuit.

Cars only use one polarity because that's all they need.
 

#12

Joined Nov 30, 2010
18,224
Behind every useful voltage measurement there is a brain somewhere in the background. You did the measurements, I did the brain. Keep studying and your brain will catch up with mine.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
Your analogy of positive and negative makes sense to me. That would be why my way of measuring was giving me 2 X voltage readings required. I was using the negative voltage as a zero reference and you guys use it as a negative voltage. Interesting. I do understand the audio wave but I lost it when it got to the bridge rectifier. Then my mind sees only straight DC as we do in cars. Now I have to open the mind and allow it to think in a new light. Any suggestions on websites or anything to get some more knowledge. It may be some time before I come across another amp to repair but I do find it exhilarating to finish the repair and see it work. What would one charge for this type of repair?
 

#12

Joined Nov 30, 2010
18,224
If I could get paid for this, I wouldn't be here giving it away for free.:D

The audio into the rectifiers (B2) is a circuit that is not normally active. It only has any effect when the amplitude is too high, not silent. You don't consider circuits that couldn't possibly be involved in the problem.

This site is based on a, "book" about electronics. Top of the pages, Vol I, Vol II, etc. Still, you're not going to get good at it unless you soak yourself in it for years. It's like learning a foreign language because it is a foreign language. I was taking home a paycheck for doing this for 4 years before the light bulb in my head went on! Now I have 40 more years of experience and book learning. Constant learning! Your head becomes a library of symbols and components and circuits. If you don't love it, you're always going to be an amateur. We even see BSEE graduates come here for advice because they have not had enough time to get a feel for it. Lots of excellent math, but their internal model has not matured yet. It's OK if you don't care enough to devote years to becoming really good at this. You can always check for help on the internet.:rolleyes:

This thread was a good experience. The OP knew how to use a volt meter and followed instructions. He only had to post 6 times before he heard music. There were no bewildered beginners misdirecting the process. The pedants were not ragging on me for using slang. The hyper-intellectuals were not clouding the issue with esoteric minutia. It was a good day.
 

bountyhunter

Joined Sep 7, 2009
2,512
To troubleshoot:

Check all power supply voltages. Do a visual and look for anything discolored from over heat or swollen capacitors. When running, you can lightly finger touch things to see if there is a cooking IC or transistor. If the amp has two channels and one is working, walk through the various voltage nodes checking to see if they track.

Usually best to start at the output and work backwards. The power output is generally what blows when overloaded.
 

rwils139

Joined Dec 27, 2013
6
Would anyone be interested in picking up this thread and lending some direction to the trouble shooting process? I also have a QSC MX 700 stereo amp that's in need of repair.

Alot of good ground work was laid as "bwilliams60" worked his way through the process. It would be a simple matter for me to enter my readings and take off from there.
Not trying to hijack the thread but rather to build on whats been done thus far.

If its the preference of the group that a new thread is started thats fine to. Just thought that if anyone was interested in helping me with the diagnostics this would be an excellent place to start.

Thanks.
 
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