QSC 1200 stereo PA

[punkykatt]

http://qscservice.com/files/7513/6329/3687/1200.pdf
Hey Guys, got this QSC PA on the bench trying to get  going for a friend.  The voltages off both bridges B1B and B1A are a tad low  +50vdc  -50vdc, but what looks like a problem is very low voltages  downstream of D10a, D10b should be +15v actual readings(a=+1,82v,  b= +1.84v) and D11a, D11b should be -15v  actual readings (a=  -1.35v,  b=  -1.38v.  All electrolytic caps have been tested good and no shorts.  all resistors test within spec (ones that were way off were lifted from pcb then tested good). Q11  and Q12 were tested with  the MM set on diode setting and  seem to be normal. All diodes test good.  All pcb traces look to be intact.  What else can I do find why the low voltages.  I am not well versed with SS stuff so any help is much appreciated. Thanks in advance  Punky

[sluckey]

Need more schematic. The +15 and -15 supplies are not on that page.

[shooter]

Need more schematic

+1
Are either LED lit, on the mute board with d10-11?

What else can I do find why the low voltages

If you're not blowing fuses, I do what I call the *knuckle test*, I use my index finger n quickly walk it over IC's and transistors, *looking* for hot spots.  Crude but, works.
IFF the supply will come up disconnected from everybody, you can *simulate* a load with appropriate R and see if the voltages hold.  You'll need to *guess* the current draw of each voltage, like +-15 = 5 ic's and 2 signal type transistors   = 10mA  (just example NOT calculated!)

[PRR]

> +15 and -15 supplies are not on that page.

R20 Z1 R21 Z2 make +/-15V.

> very low voltages  downstream of D10a, D10b

Q11 Q12 are "supposed" to do that, maybe at power-up, certainly at power-down (these amps would thump explosively), and when over-heated.

> Q11  and Q12 were tested

But are there any voltages on them??

I suspect you will find Q11 Base at negative 0.6V. In happy operation, PTC keeps a negative bias small and R33 E9 Z5 give a positive bias to overcome this; R40 adds a little.

> I am not well versed with SS

Maybe you should learn; but I'm not going to write the book. Self and Cordell did, but didn't touch QSC's mute/thermal scheme.

And cut to the end of the story--- these were not that-good amps when new, they failed a LOT. You may find that entire power channels come out on 4 screws for easy exchange; this was QSC's "fix" for frequent failures. I would strongly suspect you have BIG troubles once you get past the shut-down protection. While a general SS background gives a start on most power amp diagnosis, these QSC use a strange floating-supply output stage which requires you to think upside down and inside out. (I know: I invented that topology independently, had it on the chalkboard for a year, and decided it was just too ugly.)

Replace E9 on general suspicion.

If the amp is not-hot, tack 100 Ohms across PTC in case it has gone sour. Don't make it work hard this way, just to see if it un-mutes.

If Q11 Base is *not* negative 0.6V, then Q11 Q12 are suspect. However they "should not" fail unless something else has gone bad.

[punkykatt]

Thanks for the replies Guys.  Found the problem.  When I first tested  PTC I put the probes right on the leads before the insulation sleeves and got a good reading. I just took a reading where PTC is soldered to the pcb, got an open reading, one of the legs were broken inside the insulation.  Its up and running.  Thanks again.  Punky

[sluckey]

R20 Z1 R21 Z2 make +/-15V.

Ahh! Now I see'em.

[Willabe]

 

[PRR]

> Ahh! Now I see'em.

Yeah, well-hidden.

Working in didn't find it, so I worked out from the chip pins. The path led to the +/-54V, via the R+Z network.

Actually, because the supply is floating ground, this can crap-out on large signal swing, so there's another path R22 D4 D5. Too-too-clever these early QSC guys. The company is still around, but I remember a boom/bust period when they introduced good-value amps which then (sometimes later rather than sooner) died and we all grumped at QSC.

Yeah, if the PTC is busted-Open, the protection thinks it is 1000+ degrees and shuts-down. (Like the time my Chevy sensor thought it was always 75 deg BELOW zero and was pouring fuel into a warm engine.)

[shooter]

Ahh! Now I see'em.

same