Let me start off by saying that I am not an electronics student or engineer. I am just a guy with limited knowledge and a voltmeter. I am here to grovel for some help and/or insight. I am hoping that someone here will be gracious enough to expend time and energy to help me out.

I have a QSC Audio Amplifier MX 700.

Here is the manual with wiring diagram:
http://qscservice.com/files/9013/6329/5856/mx700man.pdf

It looks like it was manufactured in January 1996. It was given to me this weekend. Upon hooking it up:
1. Green LED (LD2) comes on. Fan comes on.
2. Channel 2 works as it should.
3. Channel 1 has no output.

Taking the lid of the case,
4. Fuse (F1A 10A) for channel 1 was blown.

Replaced fuse and proceeded to try again.
5. Fuse lit the room like a kodak flash cube from the 70s. I think I saw my own skeleton for a moment.

I checked the voltage across J2-5 and J2-6 (right before the fuse) and it's about 100V AC. That's the same as the voltage across J2-7 and J2-8 (the good channel 2). So the transformer is good.

A quick continuity check at the fuse holder showed that something in channel 1 is grounded (dead short). I cannot find any visible signs of arcing...black marks, cracks, deterioration.

I suspected that the four large capacitors (C12a, C13a, C14a, and C15a) might be bad (based on a similar thread), however, after removing each from the board and testing, they are all good.

What is the best, most methodical procedure to narrow down a short? Is there anything obvious (such as transistors or the gain knob switches for example) that inherently fail and subsequently ground out?

Again, I appreciate any help. I would be grateful to take direction on what to test and then report back with findings. Thanks again.

 

Welcome to AAC.

5. Fuse lit the room like a kodak flash cube from the 70s. I think I saw my own skeleton for a moment

Never replace fuse without checking for shorts.

I will look in to it.

 

OK...find a 22Ω to 47Ω 2W resistor.
Disconnect power and use the Resistor to short out the PSU Capacitors.

You need to show the inside of the amp to show you if you dunno what I am talking abt.

Discharge all the caps. Use DVM to measure voltage across caps to make sure they discharged. If any low voltage is there you can short the pins with a wire or use the Resistor to discharge them completely. But at first do not dead short them immediately after disconnection from mains. If so you will get another flash and this time it might be dangerous.

Now put the DVM in diode check mode and check all the power transistor ( 2SA1302's & 2SC3281's) All 12 of them. Check from pin to pin. Note down which are showing dead shorts and post back.

 

I can pick one of these up at radio shack on the way home this afternoon. Acceptable?
http://www.radioshack.com/nte-2w027---2w-27-ohms--2%/55078463.html#start=15&q=2w+resistor&sz=12

Here is a picture that I got off the internet. Identical to mine, except I am not missing transistors at the back right side of the board.


The capacitors do not hold a charge for very long on the board...due to their being a dead short somewhere. But I will make sure they are all completely discharged before testing the transistors.

I can check the transistors while they are on the board, yes?

 

You can always check for dead shorts on board. If found shorted take it out and confirm.
Do not be hasty in replacing it, if you found a shorted one. There are other things that needs to be checked.
Complete discharging is needed to check transistors on board

Oooops ! I made a boo boo. You would need a 5W one not a 2W one. I use 22ohm 5W one.

 

i have used corn starch( very small amounts) to help highlight any cracks in transistors. I wpuld bet money if a transistor has 'blown', it will have a fine crack in its plastic body. sometimes they are easy to find as there will be a large crater where a chunk has blasted out it.
as for locating the short, with paralled power devices, if one is shorted, they will all test shorted while in the board. hence my suggestion to examine them visually too.

 

I can check the transistors while they are on the board, yes?

As Kermit2 says.. if one is shorted, they will all look short while still connected on the board. Might be quicker to unsolder one end of each of the 0.22R emitter resistors (R23,24,25 --R27,28,29) rather than the transistors themselves. That should help isolate the offender.

 

Still an easier way to isolate without removing the transistor is just removing the emitter resistor associated with that transistor.
The white cement one's connected to each power transistor.
By removing the emitter R you isolate the transistor. There will be a base resistor. So you might get low base to collector readings but no short if the Tr is OK.

 

rogs is fast typist.
But in this case there are no base R's so need to remove the transistor if BC junction short is found.

I was looking at the Amp and the design looks familiar as in used by Peavey

 

OK...I think I get it.

I'm going to discharge the caps. Then, I'll test each transistor from B to E, B to C, and E to C, using the diode function on my DVM. On board.

If I find a short (which I assume is "OL" on my DVM), then I will probably find one on every transistor (since they are parallel). If its a B to C short, then I will have to remove all six transistors and check.

However, if the short is B to E OR E to C, then I will just remove one end of the emitter resistors from the board, and recheck the six transistors to find the bad one(s).

Am I tracking you guys?

 

Take your readings B to E, B to C and E to C and then reverse your leads (after discharging caps). Test just as you would a diode. These are like two diodes back to back. You will get a reading on the diode test of around 0.4 - 0.7 if okay. A short will indicate 0.00 and an open will indicate O.L.

 

Ok. Got the six transistors removed. Q4a is dead shorted on all pins in all directions. The data on the other 5 are comparable to each other. Around .540 or open depending on test lead orientation.

 

In post 5, r!f@@ says to not be hasty to replace. There are other things to check.

Very excited to move on to next step.

 

OK ! Good Job.
You found one culprit. Let's assume it is the only one.
You say you got all of them removed. So follow post#11 as bwilliams60 suggested.
Test all of them if they are desoldered so we can be sure.
The 2SC ones are NPN's & 2SA ones are PNP's.

Both PNP and NPN, Meter should give you an 'OL' Reading between CE on diode check mode, forward and reverse. If you get a reading then it is faulty

For the BE and BC junctions
The NPN's will give a reading only if the Positive probe is on the Base respective to C and E. Negative probe on base should not give you a reading.
The PNP's will give a reading only if the Negative probe is on the Base respective to C and E. Positive probe on base should not give you a reading.
If you get any thing different concerning the above reading you cannot use it.

Next is to check all the White cement 3w Resistors...."The Emitter Resistor" readings. Diode or resistance check will show you Very low or shorted reading if they are OK.
Then check the Drivers of the bad channel. That will be Q1a & Q2a according to diagram.
One is a NPN and other is a PNP. Take them out and test them as above. If they are OK solder them back.

Post ur results

 

Ok, you are either way out on the east coast, or you get up really early.

Btw, thank you so much for your time.

Here are the results of my testing.

 

All of the product data sheets I have found show pins 1, 2, and 3, as the base, collector, and emitter respectively. However, I did read somewhere that the 2SA and the 2SC are supposed to be complimentary to each other, which is supported by my results.

Anyways, you can see that the Q4a is definitely the odd one out.

 

Yup !
You got Just one dead guy there. You are lucky. When I get these things to repair I get more that just one shorted.
So how's the Driver Tr and the Resistors

BTW I am here at Capital City

 

I'll have to check the emitter resistors and the drivers when I get home this afternoon. It's 8:30am here right now, and I won't be home until 4 or 5 this afternoon. BTW, it's 18 F degrees here (-8 C)...

Is there anything else I should check after that?

Also, for my own knowledge and future reference, what lead you to suspect the transistors? Is that the common failure? I just want to understand the trouble shooting logic here, for my own benefit.

 

My Experience in the field leads me.
And common Logic. A huge flash and fuse blows...A dead short.
So first check the power components.
Then there associated components.

You at -8... Coldest I get was 16 and in my room near AC

 

If the Drivers are faulty we have to widen the search.
The Emitter resistor might blow due to faulty Tr.
If the drivers are OK then I believe search is over.

I will tell you a way to test the amp without expensive damage. But later