Hey all
My first post so please be patient.
My amp blew a Chanel some weeks ago and I’ve dived in with very little knowledge of electronics.
I have managed to get the channel back by replacing several resistors and both the pt’s and ot’s but it has an unpleasant distortion.
I’ve checked almost half the components and they check okay.
If your familiar with this amp any help would be greatly appreciated.
Schematics wise, it is very similar to the a1 with just a couple of component differences.
If this helps, on the pt’s (tr6 and tr8) the collectors show around .5v and 1.5v respectively?? The other channel has them at 2.5 and 4.5v respectively?
Cheers
My first post so please be patient.
My amp blew a Chanel some weeks ago and I’ve dived in with very little knowledge of electronics.
I have managed to get the channel back by replacing several resistors and both the pt’s and ot’s but it has an unpleasant distortion.
I’ve checked almost half the components and they check okay.
If your familiar with this amp any help would be greatly appreciated.
Schematics wise, it is very similar to the a1 with just a couple of component differences.
If this helps, on the pt’s (tr6 and tr8) the collectors show around .5v and 1.5v respectively?? The other channel has them at 2.5 and 4.5v respectively?
Cheers
Welcome to the forum. 'hope you find what you're looking for and stay around to learn and share a little more - maybe improve your system a little too.
It's not unusual for power transistors to take out their drivers too, in a chain of overload events as each transistor fails in rapid sequence, sometimes right back to the input stage. You don't seem to have a lot of similarity between the voltages measured in each channel which suggests that you haven't cleared the original fault(s) yet, either. Take a close look at tr6,8 voltages then tr5,7. I usually start with just Vbe measurement of all bipolar junction transistors and then look at associated circuit voltages for sensible similarity between channels but you will cover those here in any case.
Be sure to use genuine replacement type transistors as you don't need to add instability to the woes by fitting wrong semi types or fakes that may and often do oscillate badly and self-destruct.
It's not unusual for power transistors to take out their drivers too, in a chain of overload events as each transistor fails in rapid sequence, sometimes right back to the input stage. You don't seem to have a lot of similarity between the voltages measured in each channel which suggests that you haven't cleared the original fault(s) yet, either. Take a close look at tr6,8 voltages then tr5,7. I usually start with just Vbe measurement of all bipolar junction transistors and then look at associated circuit voltages for sensible similarity between channels but you will cover those here in any case.
Be sure to use genuine replacement type transistors as you don't need to add instability to the woes by fitting wrong semi types or fakes that may and often do oscillate badly and self-destruct.
Thanks for your reply Ian.
I’ve checked voltages to tr6/8 and are quite similar to tr5/7 at around 35v at emitter and base pins.
It’s the collector voltages that are different.
Reading Hennessy’s pages it seems correct that tr8 is different to tr6 at the collector voltage, which he explains. What isn’t correct is that they are different in each channel.
This is where I draw a blank.
The collectors go through a resistor at 330 ohm to ground so not sure why there is any difference. Unless this is not even the problem?
Again, a newbie but trying no to be...
Cheers
I’ve realised it seems I have lied as it wasn’t my first post.
It seems I have posted in the past regarding ob’s..... ��
I’ve checked voltages to tr6/8 and are quite similar to tr5/7 at around 35v at emitter and base pins.
It’s the collector voltages that are different.
Reading Hennessy’s pages it seems correct that tr8 is different to tr6 at the collector voltage, which he explains. What isn’t correct is that they are different in each channel.
This is where I draw a blank.
The collectors go through a resistor at 330 ohm to ground so not sure why there is any difference. Unless this is not even the problem?
Again, a newbie but trying no to be...
Cheers
I’ve realised it seems I have lied as it wasn’t my first post.
It seems I have posted in the past regarding ob’s..... ��
The collector voltages of TR5/7 will swing up and down with the base voltages as should TR6/8 which follow - its how they amplify and pass signal to the next stage. The transistors operate as DC amplifiers too which means that if there is even a small DC level anomaly further back at the input stage, it will be amplified along with the AC signal and then appear as a larger DC voltage (with much greater current capability) at the output.
So lets trace the problem by measuring the DC voltages at the bases of TR5,TR7 then compare with the other channel and the suggested voltage range on the schematic. Do likewise with the emitter and collector voltages of TR6,8 and compare with the other channel and suggested voltages. It should be apparent where any significant differences or problems arise from and you can easily pull a T092 size transistor to test a suspect properly without doing unnecessary damage to the PCB, assuming its a semi, not a duff resistor, bad solder joint or even an input cap, that has died.
So lets trace the problem by measuring the DC voltages at the bases of TR5,TR7 then compare with the other channel and the suggested voltage range on the schematic. Do likewise with the emitter and collector voltages of TR6,8 and compare with the other channel and suggested voltages. It should be apparent where any significant differences or problems arise from and you can easily pull a T092 size transistor to test a suspect properly without doing unnecessary damage to the PCB, assuming its a semi, not a duff resistor, bad solder joint or even an input cap, that has died.