QSC 1400

[Techknowman]

Been away for a while, computer crashed. now back, better than ever!
Anyway
I have a QSC 1400 that is eating my lunch.
I have compleatly replaced all outputs and drivers. Replaced all the power caps. basicly rebuilt it.
1 channel is working great the other, not so much.
I'm having a lot of issues with the bias circuit.
I can power up the unit on my variac and get it to full power but the bias circuit is not functioning correct.
the channel begins to clip (led indicator) almost as soon as you put any signal in.
The sound is very distorted and it looks as only the negative side is conducting which is also confirmed by voltage readings at base of drivers Q1 and Q2
Voltage levels are as follows.
Q1 (pos driver)
B .540
C 65.6
E .010

Q2 (neg driver)
B -.954
C -68.9
E -.513

When the unit has no signal the -15vdc supply that powers the bias circute and the op amp is -14.5V
as soon as you place a signal, the voltage starts dropping off a lot, to around -5V, however the +15V side stays relativly stable. +12-13V

voltage levels of D1(B) at D1 D2 connection is +.065. voltage at same point on D1(A) -.092 (bad channel)
voltage levels of D2(B) at Bias pot connection is -.549. voltage at same point on D2(A) -.714 (bad channel)

[Jazz P Bass]

Yeah, those 1980's amps are a treat.
I had a clipping issue with one.
So did this guy.
My Commentary and Technical help: QSC 1400 Amplifier repair
I had the exact same diode bad!

[Techknowman]

I missed one of the emitter resistors on the drivers.
6.8 ohm 1W R16 or 17. When i re-read the service manual it clearly says what to check with the condition i had.
I was a little embarrased

[Enzo]

And for anyone working on the real old QSCs of that era, there is a little trim pot for balance, a couple resistors out to the main rails from it. I learned the hard way to always check that little pot for open as well as the resistors. It can cause grief that is not (at least to me) readily apparent schematically.

[Techknowman]

And for anyone working on the real old QSCs of that era, there is a little trim pot for balance, a couple resistors out to the main rails from it. I learned the hard way to always check that little pot for open as well as the resistors. It can cause grief that is not (at least to me) readily apparent schematically.

Your talking about the null ripple pots?
Mine were 220K or so, 1 was burned up because it was so close to one of the 3K resisters that drop the 65Vrail to 15V it was actually crumbling from the plastic being heated and cooled probably thousands of times.
The bias, current limiter and null pots are all a wierd form factor compared to your average bourns trimmer but mouser has them at under $2 each.
I have done 4 or 5 of these QSC 1400 ilk and i always just replace them. very cheap insurance.
The trick is setting up the unit without a distortion analizer. I usualy just wing it by adjusting the bias manualy. however I can never get the 80mv that the service manual says. usually the lowest limit I can get out of the pot is 95-100ma
Other than that I like the QSC series 1 units they are one of my favorite ones to work on. Peavey PV 8.5C and CS series is another. I have done quite a few of all of them. Very straight forward designs.

[tedmich]

Thanks for the tip on the zener Jazz! The Piher trimmers are LV10's I believe and are usually 100-250ohm for bias, the beefier ones are recommended for later models and the higher wattage ones are cermet at 0.33w and are (I think) red usually; they are true "through hole" trimmers allowing adjustment from the board top OR bottom if the PCB is drilled for it.

re older QSCs "if you don't see red (trimmers) it might soon be dead"

mouser 100ohm 531-PTC10V-100 $0.85
mouser 200ohm 531-PTC10V-200 $0.85

[Techknowman]

The PTC10V's are the ones I use. they are exact fit and function, you can adjust through the PCB, as you said, if drilled.