The voltage between the emitter of Tr27 and ground, i.e the voltage across R284 which shows a value of 0.82R and measures just about 0.8R is .038v on the channel that still has Ge devices giving a quiescent current of .047A, about 1/2 of what it should be? Although it works the voltage drop for the Si channel is not measurable. I haven't done anything about adjusting the resistors to change the current on the Si side yet, though due to the fading problem with one of the boards. Eventually I intend to change both channels to Si
Have you tried switching the boards over if so does the problem move with the board
How does the silicon side sound with the good board
How does the silicon side sound with the good board
Yes, the fault follows the board.
The silicon side sounds ok with the good board. It's difficult to compare but I think there may be a bit more bass than with the germanium side but there isn't any distortion. I am just listening through headphones though.
The silicon side sounds ok with the good board. It's difficult to compare but I think there may be a bit more bass than with the germanium side but there isn't any distortion. I am just listening through headphones though.
exactly what I would expect !!!
You now need to sort out the bias chain !
Have you changed the drivers yet? The germanium ones are a risk at this age and thermal stability wont be to good I have a unit we discussed that I modded some 40 years ago I will try and get you a picture of how I did mine with the diodes etc
Trev
You now need to sort out the bias chain !
Have you changed the drivers yet? The germanium ones are a risk at this age and thermal stability wont be to good I have a unit we discussed that I modded some 40 years ago I will try and get you a picture of how I did mine with the diodes etc
Trev
Thanks Trevor.
I haven't done any mods yet as the board that fades out took my attention. A picture of how you modified the board would be very useful. In the meantime I will change the speaker output connections to something more modern
I haven't done any mods yet as the board that fades out took my attention. A picture of how you modified the board would be very useful. In the meantime I will change the speaker output connections to something more modern
By the way, did you use BD140s to replace all the transistors, the three AC139s and the AT210?
Also, I am having difficulty in finding 1N1448 diodes. Is there a suitable alternative?
Thanks
Paul
Also, I am having difficulty in finding 1N1448 diodes. Is there a suitable alternative?
Thanks
Paul
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No not at all you need a22k resistor going to two series connected diodes the juncyion of the resistor and diodes are connected via a 4.7 k resistor to the base of the driver transistor and coupling cap
As I say I will try and get a picture re the modules I have converted
As I say I will try and get a picture re the modules I have converted
Maybe adapt the 282 and 283 resistors too, as to compensate for bigger base to emitter voltage drop of the new silicon transistors. On my A15 board I used 150 ohms.