07-10-2013, 12:48 AM
This is one of the difficult ones to service and the fault was in the most awkward place it could have been.
It had no output and the output transistors were fine so it was a matter of following the bias signal back into the main PCB and this lead me to a short circuit currant limit transistor that had shunted the bias signal to the output stage to earth.
I fixed it with a new transistor and then there was still very little currant at the output but at least it was producing a tiny bit more than I started with. I checked the currant limit resistor and found no fault with it.
Measuring the feedback circuit was still showing something that was open circuit and it was down to beeping it wire by wire and after it was in bits all over the place I found that the binding post on the front panel was open circuit where it went into a little printed circuit on the panel.
Once that was fixed it all had to go back together again before I could test it and put it in place for normal service in my workshop.
The little TO18 transistor was clearly entitled to go faulty when the poor thing had been trying to supply the load but its failure was a "red herring" and that was what partly caused all the stripping down in order to get to the real fault.
It had no output and the output transistors were fine so it was a matter of following the bias signal back into the main PCB and this lead me to a short circuit currant limit transistor that had shunted the bias signal to the output stage to earth.
I fixed it with a new transistor and then there was still very little currant at the output but at least it was producing a tiny bit more than I started with. I checked the currant limit resistor and found no fault with it.
Measuring the feedback circuit was still showing something that was open circuit and it was down to beeping it wire by wire and after it was in bits all over the place I found that the binding post on the front panel was open circuit where it went into a little printed circuit on the panel.
Once that was fixed it all had to go back together again before I could test it and put it in place for normal service in my workshop.
The little TO18 transistor was clearly entitled to go faulty when the poor thing had been trying to supply the load but its failure was a "red herring" and that was what partly caused all the stripping down in order to get to the real fault.
