So I just purchased what was supposed to be a fully functional Alpha Juno 2. It seems to have an issue where the audio out gets loud and distorted erratically. This happens through both left and right audio outputs and the headphone output.
Thus far I’ve disconnected and reconnected all wire harnesses which had no effect. Any suggestions on what to check next?
Remove Q1 and Q2 on the chorus board and check whether it goes away. Those muting transistors are known to die slowly and become more and more distorting.
Hi Fanwander,
So I removed Q1 and Q2 and the distortion remains. Any other suggestions?
Does the distortion still occur when the chorus effect is switched off? If the distortion is still there with the chorus off, then I would take a look at op amps IC1 and IC2.
If the distortion goes away when the chorus is off, then IC 1 and IC2 are good, and something is probably faulty in the chorus circuit.
Hey Sam, yeah the problem persists even if I turn off the chorus. Interestingly, tonight it’s manifesting itself in a different way. Seems to be popping in the signal with momentary loud/distorted signal then back to normal. I’m not doing anything different as near as I can tell. I’ll check IC 1 and IC 2 next.
So you know the chorus circuit is not adding the distortion, which simplifies things somewhat.
The next step would be to do some checking at various points in the circuit to see where the distortion is occurring, and to do that, you really need an oscilloscope. You should be able to see the distortion with the scope at the Juno’s output, so then you would look at the input of the last op amp and see if the distortion is there or not. You just keep tracing your way backward until you see a clean signal, then you know which component is causing it.
If you don’t have an o-scope, you can replace IC1 and/or IC2, and that may do the trick - but it could also be caused by something previous to them.
So I picked up a little hand held o-scope and started to trace the leads on the ICs. IC1, IC2, and IC3 all show the same symptoms on pin 1. The signal will look normal and then seemingly erratically the signal will jump off the scale and a distorted sound will occur. That same presentation occurs when I probe TP1. Should I look at the VCAs?
On another note, if I did decide to replace those C4570C op amps could I use something common like a NE5532?
So I ended up tracing the popping/heavily distorted signal to IC3. I replaced that with an NE5532. After trying the signal again the distortion was almost gone but there was still substantial popping noises so I tried replacing IC1 with another NE5532. That got rid of the popping noises but there is still a very slight distortion in the signal that’s barely noticeable except for when you play multiple notes at once. Sure enough if I check the output of IC 3 there’s a nice clean signal, same with IC2, but IC1 output has clear clipping happening on the output. I’m wondering if I should try replacing IC 2. Any thoughts?