QSC model 1700 amplifier

[James Sweet]

Just took in a QSC 1700 stereo power amplifier with a dead channel. This is
a typical large (500W/chan) power amp and some quick checks reveal a bunch
of shorted output transistors. Are there any tricks or quirks to this
particular amp? Also does anyone know the procedure for adjusting the bias?

 

[Bill Renfro]

No real tricks just be sure to change the transistors in pairs. You will
need to remove them to test them. I would also check the driver transistors
Q1&2. If you do not have a distortion analyzer you set the bias by
monitoring the driver transistor bias current. With the amp at room temp.
and no input signal measure the dc voltage across R12 and R13. Adjust TR1
to obtain about 81mv across one of these resistors.
These were really good amplifiers, and could take a lot of abuse.
Good luck,
Bill
Christian Technology
[email protected]

 

[James Sweet]

No real tricks just be sure to change the transistors in pairs. You will
need to remove them to test them. I would also check the driver transistors
Q1&2. If you do not have a distortion analyzer you set the bias by
monitoring the driver transistor bias current. With the amp at room temp.
and no input signal measure the dc voltage across R12 and R13. Adjust TR1
to obtain about 81mv across one of these resistors.
These were really good amplifiers, and could take a lot of abuse.

Thanks, turns out the driver transistors are fine, I checked them out of
circuit. Q1 and Q2 appear to be the only problem, not sure why they popped
but I did notice they had no heatsink compound on them. I'll replace them
and check the bias and see how it does. Seems like a well made amp, build
like a tank and very easy to work on, more power than I'd ever need though.

 

[Bob Urz]

No real tricks just be sure to change the transistors in pairs.

How do you change transistors in "Pairs" on a large pro amplifier?
Answer is you don't. when you have 6,8,10,12 output transistors per
channel they are not really are not paired up. But they all share
current and equalize through the low value emitter resistors. When you
have multiple finals shorted, you should replace them all. If you don't,
chances are the other were damaged and will cause the amp to fail
shortly in the future. Sure, you can change out only the shorted
devices, but do you feel lucky?

You should pay close attention to ANY low value resistors in the output
stages for value. I would use your DMM and check them all for value to
be safe.

Bob



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