I'm still following all this 🙂 Thanks for clarifying the readings earlier.
Can the op tell us whether this amp failed in service or has it been an acquisition with an unknown history? If it is the latter then could any parts have been replaced with incorrect values? Although very unlikely, resistors can occasionally go low in value and R102 dropping in value would possibly give the symptoms reported.
The midpoint adjustment preset I assume is on minimum resistance which translates to trying to increase the mid point voltage to its maximum value.
If TR100 is removed the voltage on what was the emitter (so C101 voltage) should be about 10 volts and is a good sanity check on the biasing of the front end. This also proves R104 and R102 are delivering the correct biasing voltage to the emitter.
The midpoint would fall to a couple of volts in this scenario and crucially the voltage on what would be the base of TR100 should be around 3 volts and should be identical on either side of R108 (the 22k feedback resistor). This is another check that shows nothing is pulling one side down.
The voltage on R111 should have fallen to about 3 to 3.5 volts and will be a little lower on one side than the other (preset side will be the lower)
Can the op tell us whether this amp failed in service or has it been an acquisition with an unknown history? If it is the latter then could any parts have been replaced with incorrect values? Although very unlikely, resistors can occasionally go low in value and R102 dropping in value would possibly give the symptoms reported.
The midpoint adjustment preset I assume is on minimum resistance which translates to trying to increase the mid point voltage to its maximum value.
If TR100 is removed the voltage on what was the emitter (so C101 voltage) should be about 10 volts and is a good sanity check on the biasing of the front end. This also proves R104 and R102 are delivering the correct biasing voltage to the emitter.
The midpoint would fall to a couple of volts in this scenario and crucially the voltage on what would be the base of TR100 should be around 3 volts and should be identical on either side of R108 (the 22k feedback resistor). This is another check that shows nothing is pulling one side down.
The voltage on R111 should have fallen to about 3 to 3.5 volts and will be a little lower on one side than the other (preset side will be the lower)
Well it needs to be stated that the paragraph concerned by JLH is complete nonsense. He utters contradictions in terms such as crossover distortion in Class A, and doesn’t seem to realise that class A is a whole 180 degrees of conduction angle away from class B. You can’t get a 303 into class A at all. Never intended to get anywhere near it. You would need about ten times the heat sinking.
It needs to be stated that JLH was talking about quiescent current - that is the level of operation at the crossover point of the output stage. The point of a class AB amplifier -is to bias the output so the level of output stage current is sufficient to bridge the crossover region. In this case the Class A power level could be measured in milli-watts.
The transconduction plot of transistors is curved between 0.53 and 0.6 volts Vbe. Beyond 0.6 volts this increasingly takes the form of a steep upward slope .i.e. linear as opposed to a curve i.e. non-linear.
The transconduction plot of transistors is curved between 0.53 and 0.6 volts Vbe. Beyond 0.6 volts this increasingly takes the form of a steep upward slope .i.e. linear as opposed to a curve i.e. non-linear.
Thanks guys, currently 2000 miles away but when back will pick this ip again
Re post 17, you may have missed the point of measuring the original R130 out of circuit.
The value of R130 is 150k it should not fail through burnout or go high in value due to overheating in the circumstances here.
The possiblities exist of a discontinuity somewhere in series with the adjustment setting components such as a crack in the traces under the pcb or a cold solder joint.
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There are times a high value resistor simply increases in value, or the end caps fail. But you are correct, you can only be sure you have the right measurement with the component isolated at least with only one lead connected to circuit.
No it isn’t. It is the current drawn when there is no signal.
In the case of the Quad 303 there is no Class A power level whatsoever.
You’re just adding to the confusion. Obviously JLH should have written AB where he wrote A, but that’s what he wrote, and published, and it doesn’t enhance his credibility.
One could try measuring the total resistance (continuity differences) from the top of R130 through intermediate resistors in the adjustment line to earth and do the same for the other channel.
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