Hi all,
I'm fairly inexperienced with transistor circuits (my day job is writing firmware), but about a year ago I got a Yamaha CP-70B which works well except for the amp output being quite low and noisy. I just created an account on here, but this thread has been very helpful in getting a better understanding of how the preamp works and seeing voltages on a working unit. I have isolated the problems to the section before the effects loop, much like the other thread, though the treble control works properly on my unit. I have not made any changes to the circuitry yet, and it sounds like the previous owner never touched it. All of the transistors still seem to be the original SK30A-Ys.
Here are the measurements I've gotten so far of different components:
FET 1:
D - 6.86V
G - 0V
S - 0.581V
FET 2:
D - 5.50V
G - 1.285V
S - 1.855V
FET 3:
D - 12.61V
G - 12.15V
S - 12.37V
When playing the piano, it looks like the signal at the gate of FET 3 has an amplitude of about 80 mV, but the signal at the source of FET 3 (as well as the output from the effects loop) is only about 25 mV. If I'm understanding correctly, a source-follower circuit like that should be acting as a unity-gain buffer, which would result in an output of around the 77.5 mV specified in the operating manual.
The voltages on FETs 1 and 2 look similar to those given in the other thread after the preamp was fixed, but FET 3 seems to be pretty much saturated on my unit, while in the other thread the gate is at 6.4V and the output is at 8.3V. I'm not sure if this is a sign of something being wrong with FET 3, some of the circuitry between FET 2 and FET 3, or issues from earlier in the signal chain cascading to cause problems here.
Does anyone have thoughts on what could be the issue here? I've ordered transistors so that I can replace FET 3 if needed, but I'd prefer to not swap out working components and risk damaging the board more than necessary.
I'm fairly inexperienced with transistor circuits (my day job is writing firmware), but about a year ago I got a Yamaha CP-70B which works well except for the amp output being quite low and noisy. I just created an account on here, but this thread has been very helpful in getting a better understanding of how the preamp works and seeing voltages on a working unit. I have isolated the problems to the section before the effects loop, much like the other thread, though the treble control works properly on my unit. I have not made any changes to the circuitry yet, and it sounds like the previous owner never touched it. All of the transistors still seem to be the original SK30A-Ys.
Here are the measurements I've gotten so far of different components:
FET 1:
D - 6.86V
G - 0V
S - 0.581V
FET 2:
D - 5.50V
G - 1.285V
S - 1.855V
FET 3:
D - 12.61V
G - 12.15V
S - 12.37V
When playing the piano, it looks like the signal at the gate of FET 3 has an amplitude of about 80 mV, but the signal at the source of FET 3 (as well as the output from the effects loop) is only about 25 mV. If I'm understanding correctly, a source-follower circuit like that should be acting as a unity-gain buffer, which would result in an output of around the 77.5 mV specified in the operating manual.
The voltages on FETs 1 and 2 look similar to those given in the other thread after the preamp was fixed, but FET 3 seems to be pretty much saturated on my unit, while in the other thread the gate is at 6.4V and the output is at 8.3V. I'm not sure if this is a sign of something being wrong with FET 3, some of the circuitry between FET 2 and FET 3, or issues from earlier in the signal chain cascading to cause problems here.
Does anyone have thoughts on what could be the issue here? I've ordered transistors so that I can replace FET 3 if needed, but I'd prefer to not swap out working components and risk damaging the board more than necessary.