21-05-2015, 10:41 AM
The hiss is present with the volume at minimum, so it's coming from the amplifier...
It's most likely a transistor, but could be a carbon comp resistor.
In the past, I've had AC128s fail noisy. I'm not sure what the failure mechanism is, but noisy it was. In some cases, it was like an intermittent breakdown between collector and emitter, and it played havoc with the audio!
In view of the available equipment, I would test by substitution. I would also start with the first in the amplifier and work forwards. The first transistor is usually a good bet because any excess noise it makes will be amplified by following stages - the same level of excess noise in a later transistor will obviously have less overall effect.
To make this process easier, it's worth remembering that transistors are basically similar, so the substitution test doesn't need an exact replacement. I would dig out something like an AC128 - perhaps borrow it from your new Hacker? - and use that as your test. The AC128 has high enough current and voltage ratings to work in any position in that set. Of course, once a bad transistor has been identified, then it's nice to find the original type to replace it.
All that said, I've found that the OC types are very reliable indeed. So it might not be a transistor at all! Such is life. But at least you'll *know* that the transistors are OK, and then it'll be on to the next step: resistors.
It's most likely a transistor, but could be a carbon comp resistor.
In the past, I've had AC128s fail noisy. I'm not sure what the failure mechanism is, but noisy it was. In some cases, it was like an intermittent breakdown between collector and emitter, and it played havoc with the audio!
In view of the available equipment, I would test by substitution. I would also start with the first in the amplifier and work forwards. The first transistor is usually a good bet because any excess noise it makes will be amplified by following stages - the same level of excess noise in a later transistor will obviously have less overall effect.
To make this process easier, it's worth remembering that transistors are basically similar, so the substitution test doesn't need an exact replacement. I would dig out something like an AC128 - perhaps borrow it from your new Hacker? - and use that as your test. The AC128 has high enough current and voltage ratings to work in any position in that set. Of course, once a bad transistor has been identified, then it's nice to find the original type to replace it.
All that said, I've found that the OC types are very reliable indeed. So it might not be a transistor at all! Such is life. But at least you'll *know* that the transistors are OK, and then it'll be on to the next step: resistors.
