Right...
All resistors measured fine.
Collectors match rails
Jct 0.22 measured - 43....more to follow
Pin 13 - 39.5
Pin 9 - 47
Pin 12 - 43
Pin - 43
So while testing the middle pin on. 22 resistor I did notice voltage starting to drop away after a few seconds and q136)(pre driver that was previously cold) started to heat up. So I went over some solder joints again to be sure.
So checked the junction again and could see after a few seconds the voltage started to drop, still not right as the food channel sits at 0v on switch on. So I waited to see what happened, it did get to 0v so it looks like the feedback to the ic is working in some way. Unfortunately q136 got so hot it went poof...
So a968 on order now.
This is with all the outputs still disconnected at this stage
All resistors measured fine.
Collectors match rails
Jct 0.22 measured - 43....more to follow
Pin 13 - 39.5
Pin 9 - 47
Pin 12 - 43
Pin - 43
So while testing the middle pin on. 22 resistor I did notice voltage starting to drop away after a few seconds and q136)(pre driver that was previously cold) started to heat up. So I went over some solder joints again to be sure.
So checked the junction again and could see after a few seconds the voltage started to drop, still not right as the food channel sits at 0v on switch on. So I waited to see what happened, it did get to 0v so it looks like the feedback to the ic is working in some way. Unfortunately q136 got so hot it went poof...
So a968 on order now.
This is with all the outputs still disconnected at this stage
Thanks.
OK, so the output is stuck close to the negative rail. The other voltage readings don't really tie up, for example -47 on pin 9 would see (approximately) around -46.3 on the emitter of Q132, - 45.6 on the emitter of Q136 and finally - 44.9 on the emitter of Q4.
So the volt drops we should be seeing across those transistors don't tally with what would be expected.
There are two things to consider here.
1/ A DC fault giving a high offset, and where the fault is not in the output stage, should not overheat any of the transistors.
2/ Q136 getting hot means that current is flowing somewhere it shouldn't. This points to an output problem but as is so often the case there may be more than one problem.
Without having this is front of me. here is what I would do.
1/ Force a zero bias condition. The bias spreader is an unknown quantity and may even be faulty and so we should try linking it out by connecting pin 14 to pin 8.
That should force a no bias state meaning that all the transistors are off.
2/ In this condition the output offset should be zero. Assuming it is not, we can then start to fault find normally without anything going up in smoke due to excess bias. If any of the transistors got hot in this zero bias state then it would suggest a problem with one or more of the transistors.
OK, so the output is stuck close to the negative rail. The other voltage readings don't really tie up, for example -47 on pin 9 would see (approximately) around -46.3 on the emitter of Q132, - 45.6 on the emitter of Q136 and finally - 44.9 on the emitter of Q4.
So the volt drops we should be seeing across those transistors don't tally with what would be expected.
There are two things to consider here.
1/ A DC fault giving a high offset, and where the fault is not in the output stage, should not overheat any of the transistors.
2/ Q136 getting hot means that current is flowing somewhere it shouldn't. This points to an output problem but as is so often the case there may be more than one problem.
Without having this is front of me. here is what I would do.
1/ Force a zero bias condition. The bias spreader is an unknown quantity and may even be faulty and so we should try linking it out by connecting pin 14 to pin 8.
That should force a no bias state meaning that all the transistors are off.
2/ In this condition the output offset should be zero. Assuming it is not, we can then start to fault find normally without anything going up in smoke due to excess bias. If any of the transistors got hot in this zero bias state then it would suggest a problem with one or more of the transistors.
Ok, ill wait for the transistor to turn up and link the ic pins as advised. I did swap the ics and every single transistor over from good channel to bad, checking the fault condition at each stage and nothing has changed. Every single driver gets hot, but i only had it powered long enough to take measurements. Q136 going poof was a result of leaving it powered too long and i imagine the others wouldn't be far behind.
I would have binned it by now but where is the fun in that?
My money is on a bad trace somewhere sending something somewhere it shouldn't.
Thanks for helping so far
I would have binned it by now but where is the fun in that?
My money is on a bad trace somewhere sending something somewhere it shouldn't.
Thanks for helping so far
Long time since I got round to this again. Replaced q136 and reattached outputs. Found one resistor open and another reading low value.
I suspect the low value resistor was causing the offset, which the 0016 ic was trying to correct. Having no outputs connected caused q136 to overheat and possibly the resistor to go open.
Dim bulb tested and all looks good.
Thanks for everyone's guidance!
Steve
I suspect the low value resistor was causing the offset, which the 0016 ic was trying to correct. Having no outputs connected caused q136 to overheat and possibly the resistor to go open.
Dim bulb tested and all looks good.
Thanks for everyone's guidance!
Steve
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