My Cambridge Audio A300, which I bought in 2001, recently started having an issue where the sound would cut from either or both speakers while moving the volume knob. I could land on spots where the volume was fine for both channels, but there are most spots where one or both channels is attenuated or maybe even amplified.
I re-capped the entire unit, and found three bad filter caps (really low capacitance or no capacitance), as well as two other bad caps (C213 and C214, which were right next to the LM317/337 voltage regulators, so I imagine they got too hot), figuring this would fix the problem. I also used DeoxIT on all the switches and pots multiple times, but the issue remains.
I bought a digital oscilloscope (Hantek DSO2D15) and tried probing around, but I'm having a hard time figuring out how to do that. We only used analog scopes 25 years ago in school, and it was always in low-power situations, so this is a bit new to me. So far I've tried hooking up the signal generator from the scope to the Aux-in left channel (using about 800mV p-p @ 1KHz, but I'm not sure that's right), and was able to hear the tone through speaker. I then figured I should see the signal at the output of U105(B) between R102 and R109, but I just get noise there. It's very possible, if not likely, that I'm just using the scope incorrectly. I tried with the probe at both 1x and 10x, with and without the ground cable attached to chassis ground (I checked that circuit ground is the same as chassis ground), etc.
But I might be looking in the wrong places anyway. Maybe this points to a bad transistor or bad pots? I measured the resistance in both channels of the volume pot in different positions and they matched every time, so I don't think that's it. The transistors in the power amp are MPSA43 and MPSA93. The output transistors are SAP10 darlingtons.
I re-capped the entire unit, and found three bad filter caps (really low capacitance or no capacitance), as well as two other bad caps (C213 and C214, which were right next to the LM317/337 voltage regulators, so I imagine they got too hot), figuring this would fix the problem. I also used DeoxIT on all the switches and pots multiple times, but the issue remains.
I bought a digital oscilloscope (Hantek DSO2D15) and tried probing around, but I'm having a hard time figuring out how to do that. We only used analog scopes 25 years ago in school, and it was always in low-power situations, so this is a bit new to me. So far I've tried hooking up the signal generator from the scope to the Aux-in left channel (using about 800mV p-p @ 1KHz, but I'm not sure that's right), and was able to hear the tone through speaker. I then figured I should see the signal at the output of U105(B) between R102 and R109, but I just get noise there. It's very possible, if not likely, that I'm just using the scope incorrectly. I tried with the probe at both 1x and 10x, with and without the ground cable attached to chassis ground (I checked that circuit ground is the same as chassis ground), etc.
But I might be looking in the wrong places anyway. Maybe this points to a bad transistor or bad pots? I measured the resistance in both channels of the volume pot in different positions and they matched every time, so I don't think that's it. The transistors in the power amp are MPSA43 and MPSA93. The output transistors are SAP10 darlingtons.
