QSC Power Amp gets hot

[gerryt]

I have a QSC Series One Model 1100 Power amp. It is working normally, however on channel two the output transistors and heat sink get very hot. Wondering if this is a problem with too much current getting to them or if the transistors are failing? Channel 1 runs cool.

 

[WHONOES]

Do both channels work?

 

[gerryt]

Yes, both channels are working normally, channel two gets really hot, channel one does not

 

[Martaine2005]

Perhaps the heatsink compound has dried or is non existent?.

Martin

 

[gerryt]

A bit more information that may or may not be relevant. I tested the DC supply to both channel one and channel two output transistors. The reading going to channel one output transistor was 1.18 and reading to channel two output transistor was 2.77.

 

[Audioguru]

Your numbers 1.18 and 2.77 cannot be the supply voltage of the stereo amplifiers that produce 50W into 8 ohms. Their supply will be at least 60V or more than plus 30V and minus 30V.

 

[davenn]

A bit more information that may or may not be relevant. I tested the DC supply to both channel one and channel two output transistors. The reading going to channel one output transistor was 1.18 and reading to channel two output transistor was 2.77.

Your numbers 1.18 and 2.77 cannot be the supply voltage of the stereo amplifiers that produce 50W into 8 ohms. Their supply will be at least 60V or more than plus 30V and minus 30V.

Indeed,
@gerryt , I think you had better tell us how and where you did your tests ?

 

[gerryt]

What I tested was the two wires (blue and white see the attached photo) that go directly to the output transistor on the heat sink. I disconnected the wires from the output transistor and then turned the amp on and connected each wire to my multimeter on the auto DC voltage setting. Channel one, which does not get hot, had the reading of 1.18, channel two, which gets hot, had the reading of 2.77.

 

[davenn]

What I tested was the two wires (blue and white see the attached photo) that go directly to the output transistor on the heat sink. I disconnected the wires from the output transistor and then turned the amp on and connected each wire to my multimeter on the auto DC voltage setting. Channel one, which does not get hot, had the reading of 1.18, channel two, which gets hot, had the reading of 2.77.

Yeah, we cannot see from the photo where the 2 blue wires come from or go to. So at this point, those 2 voltage reading dont mean much

Have you looked online for a service manual or at minimum a schematic ?

 

[davenn]

OK I found the service manual and it of course has a schematic

QSC One 1100 Technical & Service Manual (Page 6 of 59) | ManualsLib

Service manuals really help when trying to diagnose faults

 

[davenn]

OK, the PSU section first
you need to check that those + and - 37V DC rails ( on the right of the diagram) are there and correct for each of the channels

 

[davenn]

Here's the main amplifier section ......




The voltages shown should be referenced to the 0V rail ... that should be the ground shown as a downward pointing arrow

Where your 2 blue wires fit into there I dont know

 

[WHONOES]

Can you check the Base - Emitter voltage on each of the output transistors in its quiescent state i.e. with no amplification taking place.
Also, check the d.c voltage on the output of each amplifier across their output terminals under the same conditions.

 

[gerryt]

Thank you for all of your input, I will do some more testing. I have also discovered that Q2 in the schematic also gets hot. Thinking I might need to replace Q1 and Q2 as a matched set. Or, wondering if there is a bias problem causing Q4 to do most of the work.

 

[WHONOES]

Can you measure voltages as requested.

 

[gerryt]

Can you measure voltages as requested.

By each amplifier do you mean channel and channel two?

 

[davenn]

By each amplifier do you mean channel and channel two?

as per the schematic in post #11
it shows one channel, do it for both

 

[gerryt]

OK, the PSU section first
you need to check that those + and - 37V DC rails ( on the right of the diagram) are there and correct for each of the channels

View attachment 50869

Please bear with me, I am just learning about how to do this. I have found the ground rails on my circuit board, but I do not know where to put the leads from my voltage meter.

 

[gerryt]

Please bear with me, I am just learning about how to do this. I have found the ground rails on my circuit board, but I do not know where to put the leads from my voltage meter.

Ok, figured how to test the + and -37V DC rails. They are both running at -35 and +35 DCV

 

[gerryt]

Can you measure voltages as requested.

Voltage for power transistors in quiessent state:
Channel one -34.00 and +34.00 DCV
Channel two -33.00 and +32.00 DCV (this is the channel that gets very hot)

DC Voltage for amplifier output in quiessent state:
Channel 1 -1.2 DCmV
Channel 2 1.0 DCmV