OK so the measures are the following
Q13 voltage
Bias at minimum value
E to C 2.1v
E to B 0.6v
C to B 1.5v
R73 and R75 3mv
At medium value ( the original value)
E to C 2.3v
E to B 0.6v
C to B 1.6v
R73 and R75 7mv
At full value
E to c 2.5v
E to B 0.6v
C to B 1.8v
R73 and R75 140 mv ( the same as the other chanel at middle value of por )
The other thing that I notice is that Q19 gets warm you can tuch it but the others are cool
So I think that there is something wrong there
Q13 voltage
Bias at minimum value
E to C 2.1v
E to B 0.6v
C to B 1.5v
R73 and R75 3mv
At medium value ( the original value)
E to C 2.3v
E to B 0.6v
C to B 1.6v
R73 and R75 7mv
At full value
E to c 2.5v
E to B 0.6v
C to B 1.8v
R73 and R75 140 mv ( the same as the other chanel at middle value of por )
The other thing that I notice is that Q19 gets warm you can tuch it but the others are cool
So I think that there is something wrong there
The voltage increasing across C to E of Qm13 as you turn up the bias look good.
The voltages across (you are measuring across and not to ground) the 0.47 ohm resistors suggest a current of 6ma to 0.3 amp.
There should be no distortion at those current and so something else is going on. If Qm19 is getting warm (and it will at the highest settings) then Qm21 should also be getting just as hot. That is an important check to make.
If only Qm19 is getting warm then also make sure that resistor R3 (near the speaker sockets on the diagram) is cold. That resistor should be cold at all times.
Based on the results so far (and assuming the DC offset is low at all times) then I would say the amplifier is unstable and oscillating. That is where an oscilloscope comes into its own.
The voltages across (you are measuring across and not to ground) the 0.47 ohm resistors suggest a current of 6ma to 0.3 amp.
There should be no distortion at those current and so something else is going on. If Qm19 is getting warm (and it will at the highest settings) then Qm21 should also be getting just as hot. That is an important check to make.
If only Qm19 is getting warm then also make sure that resistor R3 (near the speaker sockets on the diagram) is cold. That resistor should be cold at all times.
Based on the results so far (and assuming the DC offset is low at all times) then I would say the amplifier is unstable and oscillating. That is where an oscilloscope comes into its own.
OK I have checked and both transistors gets hot when the bias is at full 2.5v across Q13 and R3 is always cold, the DC offset is near 30 mv no matter the position of the bias preset, so as you said it should be oscillating in the audio aspect you get a base humm at zero volume and when you turn it louder it starts to tap ( like tac tac tac ) or sometimes it makes a faster tap.
OK so assuming that it is oscillating and knowing that I don't have an osciloscope what component could theorically make that oscillation the components are the same ones that before make it work I mean the same model but different units so I assume that after the short circuit one or more of the components went wrong but because I replace them we can discard the transistors the electrolytic capacitors and the resistors R63 R49 R51 R53 R57 and R55
So I will wait for your advice
OK so assuming that it is oscillating and knowing that I don't have an osciloscope what component could theorically make that oscillation the components are the same ones that before make it work I mean the same model but different units so I assume that after the short circuit one or more of the components went wrong but because I replace them we can discard the transistors the electrolytic capacitors and the resistors R63 R49 R51 R53 R57 and R55
So I will wait for your advice
Real fault-finding is a bit more involved than that I'm afraid 🙂 The oscilloscope is the number #1 diagnostic tool to find problems like this.
This type of circuit can be quite critical on the semiconductors used.
Is the problem affected at all by altering the bass and treble controls ? These are unusual in that they operate on the feedback network of the power amplifier. So does altering them change the noise.
We can only discount the transistors if they are of the correct identical type (that is to say new old stock) and not some generic replacement. There are also a lot of fake parts around from auction sites.
While it is certainly possible to replace the transistors with modern parts, it is not guaranteed that they will work 'out of the box', you may need to make small adjustments to the amplifiers stability and compensation networks... and that again becomes a job that requires the correct test gear.
This type of circuit can be quite critical on the semiconductors used.
Is the problem affected at all by altering the bass and treble controls ? These are unusual in that they operate on the feedback network of the power amplifier. So does altering them change the noise.
We can only discount the transistors if they are of the correct identical type (that is to say new old stock) and not some generic replacement. There are also a lot of fake parts around from auction sites.
While it is certainly possible to replace the transistors with modern parts, it is not guaranteed that they will work 'out of the box', you may need to make small adjustments to the amplifiers stability and compensation networks... and that again becomes a job that requires the correct test gear.
OK I understand that the osciloscope is the right tool for the job I got a friend that has one but he is on vacations
These are the transistors that I'm using
Q19 2sd525 Q21 2sb595 Q13 2sa992 Q15 BC640 Q17 BC639 Q11 and Q23 2sc1815 Q7 and Q9 2sa1015
This configuration of transistors is working on the right channel perfectly
When you move the bass or the treble it makes the hum louder but I think it just add bass or treble to it
Just for knowing what is the theory behind the oscillation problem?
These are the transistors that I'm using
Q19 2sd525 Q21 2sb595 Q13 2sa992 Q15 BC640 Q17 BC639 Q11 and Q23 2sc1815 Q7 and Q9 2sa1015
This configuration of transistors is working on the right channel perfectly
When you move the bass or the treble it makes the hum louder but I think it just add bass or treble to it
Just for knowing what is the theory behind the oscillation problem?
Oscillation and the reasons behind it are a vast subject, however put simply it occurs when two conditions are met, one is that the loop gain has to be greater than one (1) and the other is that the signal at that point has to be 'in phase' (positive feedback) rather than out of phase (as in negative feedback).
Those conditions can occur when devices of different specification have been use, and when the original design depended heavily on those characteristics to form part of the overall stability of the design.
If you want to experiment then I would suggest first of all that you use a DBT (dim bulb tester) as that will protect both the amplifier and speakers.
Its very hard to guess what is going on without seeing the problem on a scope.
There should not be any hum with the volume on minimum. Are you certain that there is no damaged or open print from having worked on the amplifier and having replaced parts ?
Those conditions can occur when devices of different specification have been use, and when the original design depended heavily on those characteristics to form part of the overall stability of the design.
If you want to experiment then I would suggest first of all that you use a DBT (dim bulb tester) as that will protect both the amplifier and speakers.
Its very hard to guess what is going on without seeing the problem on a scope.
There should not be any hum with the volume on minimum. Are you certain that there is no damaged or open print from having worked on the amplifier and having replaced parts ?
The coper side is a disaster the pads of some transistors are gone but I kind of fixed it soldering over the trace it seems like the one that repaired before me overheated the pads so there could be a problem there if you suspect of that I will check it again the other problem is that the entire PCB is covered with something like flux or god knows what so I can't clearly see the traces path
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