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Faulty C-Audio Pulse 4x300
Faulty C-Audio Pulse 4x300
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Old 2nd September 2009, 02:11 AM   #1
oliverwestby is offline oliverwestby  United Kingdom
diyAudio Member
Join Date: Apr 2006
Location: Oxfordshire
Default Faulty C-Audio Pulse 4x300

I have a C-audio (now crown audio) Pulse 4x300 PA amplifier that has one channel faulty (channel 3)
Having spent probably 10 hours attempting to find the fault, I am stuck for what to do next.

The schematic for the amp is here on the 6th page down (sheet 3)

Now for what faults I have. firstly and I have no output on channel 3, all other channels work.

The clip light for channel 3 lights up at very low volume setting despite no output.

There is signal at the base of TR28, there is no signal at either the collector or emitter of TR28.

DC voltage on TR28: Base = 0, emitter = -4, collector = -73
Other channels DC voltage: Base = 0, emitter = +2, collector = -73

Collector of TR22 has DC voltage of +73 - other channels have +17

Measuring the DC on TR22's collector with circuit board earthed to plug causes the amp to shut down. If earth disconnected then noise can be heard from the speaker but does not shut down amp.

DC voltage on TR33 is 0.4 on every pin with earth disconnected. with earth connected then it shuts the amp down.
DC voltage on other channels equivalent to TR33 there is Base = 0.6, Collector = -1, emitter = +1.1

I have replaced TR52, TR42, TR41, TR26, TR28, TR40, TR39, TR22, TR21, TR12 and TR47
I have exchanged TR33 for another channel and problem stayed with channel 3.

I am now completely stuck so any help is greatly appreciated

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