I have been restoring this amplifier for some time now. I did have it in use until I realized it was the cause for taking out tweeters. The amp is becoming parasitic @ clipping.
I've replaced all parts except transformers and the 2N3738 transistors on each channel. the outputs are MJ15025 & MJ15024, the originals were already changed and shorted on one channel due to missing bias diodes that have since been replaced with D1300A diodes.
Being that the new outputs are faster, bias has been an issue. The only way I know of adjusting the bias on this amp is be increasing the R5 resistor in series with the bias diodes to make the outputs turn on harder. Schematic calls for R5 resistance of 10ohms, I'm finding some improvement @ 18ohms, but still seeing some distortion on the scope.
I have also been advised to change capacitor C3 from 47pf to 100pf. 100pf is showing improvement in output but is also showing spikes in the sine wave, I will post pictures. I'm not sure why i'm getting these spikes, if I can eliminate them I think the amp will be 100% along with R5 @ 18ohms. At this point all I can think of is a capacitance between 47pf and 100pf will eliminate the spikes, because 47pf is too low and shows crossover distortion while 100pf does not show crossover distortion but is showing spikes.
I hope you all can follow me here due to my lack of knowledge and if someone has any ideas I would greatly appreciate it.
1st pic: C3 100pf & R5 18ohm
1khz Sine Wave into 8 ohms
2nd pic: C3 100pf & R5 18ohms
20khz sine wave into 8 ohms
3rd pic: C3 47pf & R5 18 ohms
20khz sine wave into 8ohms
4th pic: same signal as 3rd, less volume, enlarged
I've replaced all parts except transformers and the 2N3738 transistors on each channel. the outputs are MJ15025 & MJ15024, the originals were already changed and shorted on one channel due to missing bias diodes that have since been replaced with D1300A diodes.
Being that the new outputs are faster, bias has been an issue. The only way I know of adjusting the bias on this amp is be increasing the R5 resistor in series with the bias diodes to make the outputs turn on harder. Schematic calls for R5 resistance of 10ohms, I'm finding some improvement @ 18ohms, but still seeing some distortion on the scope.
I have also been advised to change capacitor C3 from 47pf to 100pf. 100pf is showing improvement in output but is also showing spikes in the sine wave, I will post pictures. I'm not sure why i'm getting these spikes, if I can eliminate them I think the amp will be 100% along with R5 @ 18ohms. At this point all I can think of is a capacitance between 47pf and 100pf will eliminate the spikes, because 47pf is too low and shows crossover distortion while 100pf does not show crossover distortion but is showing spikes.
I hope you all can follow me here due to my lack of knowledge and if someone has any ideas I would greatly appreciate it.
1st pic: C3 100pf & R5 18ohm
1khz Sine Wave into 8 ohms
2nd pic: C3 100pf & R5 18ohms
20khz sine wave into 8 ohms
3rd pic: C3 47pf & R5 18 ohms
20khz sine wave into 8ohms
4th pic: same signal as 3rd, less volume, enlarged
Attachments
With the level control at maximum, the gain factor at that point approx. is (1 + R3/R2).
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Sounds like the amp is a bit unstable and maybe oscillating if blowing tweeters.
Given the new output transistors have a higher gain the oscillation could be worse.
I would tame the oscillation by making the VAS capacitor a little larger.
Just up it a little at a time until oscillation stops.
I had a similar problem with an old Maplin amp.
It came with no output transistors so I bought new ones.
The amp oscillated badly.
Increasing the VAS capacitor slightly fixed it.
Given the new output transistors have a higher gain the oscillation could be worse.
I would tame the oscillation by making the VAS capacitor a little larger.
Just up it a little at a time until oscillation stops.
I had a similar problem with an old Maplin amp.
It came with no output transistors so I bought new ones.
The amp oscillated badly.
Increasing the VAS capacitor slightly fixed it.
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