I'll have to leave it for a while shortly.
I think you'll find that voltage (across Qe7) varies smoothly with the bias preset, the problem is that its just not going quite high enough to turn the outputs on. That is because of the slightly different characteristics of the new drivers.
Assuming that the voltage does vary smoothly this is what we do. And we use the bulb tester while sorting this bias issue.
1/ Firstly make sure that the good channel is turned back down to zero bias.
2/ Turn the repaired channels bias preset back so that it is as a 1 k resistor. Remember that maximum resistance equals minimum bias current.
3/ Look at the circuit. Do you see Re27, a 1.8k that is in series with the bias preset. We need to reduce the value of that resistor by a small amount. Now going down to a 1.5k may be to much of a jump so I would suggest adding a 22k across it (making 1.66k) and again seeing if the bias now adjusts. That 1.9 volts you measured should now go a little higher.
Its trial and error. The bulb tester and variac make it safe for the amp so keep reducing the value of that 1.8k either by adding resistors across it or if it gets to say 1.5k then by replacing it.
I think you will need to see around 2.4 to 2.5 volts across Qe7 before the output transistors start to conduct.
I know you want to try the amp. Providing it is just a low bias issue we have at the moment then the amp should work correctly. You could try it with headphones and if that was OK a speaker BUT KEEP THE LEVEL VERY LOW because the bulb will limit current and cause the rails to collapse which could cause the output to suddenly latch to one or other rail. Not good
And connect the speaker AFTER the amp is powered up because again the bulb will limit current and not allow the amp to stabilise.
I think you'll find that voltage (across Qe7) varies smoothly with the bias preset, the problem is that its just not going quite high enough to turn the outputs on. That is because of the slightly different characteristics of the new drivers.
Assuming that the voltage does vary smoothly this is what we do. And we use the bulb tester while sorting this bias issue.
1/ Firstly make sure that the good channel is turned back down to zero bias.
2/ Turn the repaired channels bias preset back so that it is as a 1 k resistor. Remember that maximum resistance equals minimum bias current.
3/ Look at the circuit. Do you see Re27, a 1.8k that is in series with the bias preset. We need to reduce the value of that resistor by a small amount. Now going down to a 1.5k may be to much of a jump so I would suggest adding a 22k across it (making 1.66k) and again seeing if the bias now adjusts. That 1.9 volts you measured should now go a little higher.
Its trial and error. The bulb tester and variac make it safe for the amp so keep reducing the value of that 1.8k either by adding resistors across it or if it gets to say 1.5k then by replacing it.
I think you will need to see around 2.4 to 2.5 volts across Qe7 before the output transistors start to conduct.
I know you want to try the amp. Providing it is just a low bias issue we have at the moment then the amp should work correctly. You could try it with headphones and if that was OK a speaker BUT KEEP THE LEVEL VERY LOW because the bulb will limit current and cause the rails to collapse which could cause the output to suddenly latch to one or other rail. Not good
And connect the speaker AFTER the amp is powered up because again the bulb will limit current and not allow the amp to stabilise.
That's surprising.
Lets jump in at the deep end and test the amp with heaphones/speaker as I outlined above.
Lets see where its all at.
Excellent
Now unless something is drastically wrong its distorted simply because there is no bias, you are hearing crossover distortion.
2.3 volts I think is right on the edge of where it will set up and adjust so turn it all off and turn the bias back down again (always start with it on mimimum) and try fractionally reducing that 1K8 resistor.
Go back to having NO speakers attached again and then see if it now adjusts.
Now unless something is drastically wrong its distorted simply because there is no bias, you are hearing crossover distortion.
2.3 volts I think is right on the edge of where it will set up and adjust so turn it all off and turn the bias back down again (always start with it on mimimum) and try fractionally reducing that 1K8 resistor.
Go back to having NO speakers attached again and then see if it now adjusts.
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