The past few days, I have been occupied with other matters of more immediate importance. Consequently, I have been unable to devote the time needed for troublshooting this amp. I should have those other matters sorted out soon, possibly even today. When this bumpy patch is behind me, I will continue with following the most recent suggestions re this amplifier.
just out of interest, why are you running it at potential full current, you are much better throttling it back a bit while testing and less likely to break something else in circuit while doing so.
Everyone has their own methods
and I do stand by the methods that I've outlined. I've also tried to keep it as simple as possible to try and get a handle on what is going on.The thought process is that if the vbe multiplier is linked out it is taken out of the equation. It can be discounted. With no load attached no current should be able to flow anywhere in the output stage in this state no matter what the front end does. A check of DC offset (if zero which it is) shows that the front end DC conditions are correct. If there is a DC fault causing a sudden offset to a rail then the output stage should still draw no current.
Something else seems to be going on here and a really important question is the failure mode of the output transistors, and that is whether they really are failing short circuit from C to E? hence:
If @62vauxhall could measure them on the diode range and just see if they read very low from C to E that would be informative
Hopefully we'll get to the bottom of this one.
the present shorting between C and E - at least on one of both output devices (maybe of both) is extremely obvious to me, additional even on the driver stage of output buffer. I conclude this from the observed description in post #1 - quote
As per the service manual, the target is 25mv and the left channel bias was pretty close. The right channel was nearly 60mv so I attempted to adjust that channel's bias pot.
As I rotated the pot, both 1.6A fuses blew and I noticed the telltale odor of an overheated component. Even now, I fail to see anything that shows heat damage.
As I mentioned before, this happens due to the deficiency concerning the connecting kind of the pot for BIAS adjustment.
Because not all semiconductors were replaced after this event (output buffer, VA-Stage and Vbe multiplier), the CE shorting of Vbe transistor don't help to find out remaining defective transistors.
As per the service manual, the target is 25mv and the left channel bias was pretty close. The right channel was nearly 60mv so I attempted to adjust that channel's bias pot.
As I rotated the pot, both 1.6A fuses blew and I noticed the telltale odor of an overheated component. Even now, I fail to see anything that shows heat damage.
As I mentioned before, this happens due to the deficiency concerning the connecting kind of the pot for BIAS adjustment.
Because not all semiconductors were replaced after this event (output buffer, VA-Stage and Vbe multiplier), the CE shorting of Vbe transistor don't help to find out remaining defective transistors.
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Wouldnt't the emitter resistors of those be seriously heating up and smoking in that case?
The 0,7V measured some posts above are a bit far from rail voltage.
The problem is that while measuring voltage over the emitter resistors the rail voltages were forgotten...
@tiefbassuebertr we will have to agree to differ in our methods
Confirmation (or otherwise) that the output transistors that appeared to fail when the DBT was in use are in fact reading short circuit could provide a clue. What I'm trying to get at with that is whether the DBT lighting is really caused by a real transistor failure or whether it just appears to be so and the that cause of the bulb lighting is actually something else, maybe even something physical such as the mounting and insulation of the transistor and the act of removing and replacing it seems to provide a (temporary) fix.
Confirmation (or otherwise) that the output transistors that appeared to fail when the DBT was in use are in fact reading short circuit could provide a clue. What I'm trying to get at with that is whether the DBT lighting is really caused by a real transistor failure or whether it just appears to be so and the that cause of the bulb lighting is actually something else, maybe even something physical such as the mounting and insulation of the transistor and the act of removing and replacing it seems to provide a (temporary) fix.