QSC Powerlight 3.4 troubleshooting

F

Franz Francis

Freshman
Apr 17, 2011
30
1
8
Any component level troubleshooting Guru here?

I am troubleshooting an old QSC Powerlight 3.4 amplifier with a power supply problem on both channels.

The design of this amplifier is each channel has its own independent power supply.

The channel’s power supply output design is for two independent supplies on the secondary side of the switching transformer.

One is the supply for the output rail transistors which are all testing fine, plus and minus 48, 96 & 144 volts.

The other is the low voltage supply feeding a half wave rectifier design powering the op amps and other biasing components of plus and minus 14 volts. This supply is completely dead. The winding is not reading open with an ohm meter and the half wave rectifier supply is not overloaded which would be associated with a short or suspected faulty component drawing too much current.

The basic schematic on QSC’s website is what it is, very basic with limited information about the switching power supply. I am able to get the channel working by using an external D.C supply.

The other channel of this amplifier is completely dead because of a faulty Pulse width modulator chip therefore I cannot compare readings.

My question, will the low voltage supply on the secondary of this switching design disappear if for some reason the switching frequency or pulse width is incorrect. Thanks

Franz
 
Last edited:
Re: QSC Powerlight 3.4 troubleshooting

Hi Franz,

So is the +- 15V ok just after the half have rectifier? If so then it could be that the series diode - D738 (1N4004) - is dead. It's also worth checking the other series diode D741 (1N4148) and their series resistors R748 and R747. There are so few components in this part of the supply, it should be fairly quick to check them all, even if you have to unsolder the odd leg or two to isolate a component.

Cheers,
Michael


Franz Francis said:
The other is the low voltage supply feeding a half wave rectifier design powering the op amps and other biasing components of plus and minus 14 volts. This supply is completely dead. The winding is not reading open with an ohm meter and the half wave rectifier supply is not overloaded which would be associated with a short or suspected faulty component drawing too much current.

The basic schematic on QSC’s website is what it is, very basic with limited information about the switching power supply. I am able to get the channel working by using an external D.C supply.
Click to expand...
 
Re: QSC Powerlight 3.4 troubleshooting

Thanks Michael for the info.

I was able to repair the power supply. There was an open trace between the transformer A.C supply legs feeding the two half wave rectifier diodes.

My previous troubleshooting was in my friend’s warehouse with his D.M.M which was limited to 20khz and the reason why I was not able to read any A.C.

Back in my lab my oscilloscope painted a very clear picture.

The transformer secondary legs measured 115 kHz, 40 volts peak to peak which was not getting to the rectifying diodes (D707 & D705). Thanks


Franz
 
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