Alpha Juno 2 VERY noisy chorus needs repair help

[trondl]

Recently i bought this unit cheap knowing a few things needed repairing (skippy dial fixed by resetting the synth, uneven velocity fixed by cleaning, presumably weak battery but works ok) .
All of them fixed easy but I created a new one:
A noisy chorus circuit!

The noise can be described as digital noise/silence shaped with a square wave 50% / 50% timeframe (drrrrrt / silence / drrrrt / silence...). The noise increases with CSR RATE, but goes away if bellow ca 70 value.
If I sett CSR RATE value low, play a note and increase CSR RATE it plays correctly. But if i play a new note with the now high setting it will distort the sound.

VCF and VCA LFO is not touched by this error.

The unit is fairly clean and shows no obvious signs of shorts or defect parts.

Any help here?

I have the 20 page service manual available if requested.

 

[gain wire]

hi there trondl,
my initial response was going to be check the connections, clean the potentiometers, but I see you've already done that.
I am starting to wonder if maybe it's a capacitor problem, an old electrolytic might be leaky, something like that.

You know what? post this on Gearslutz.com in the geekslutz forum. I think you'll have a good time there and better luck.

cheers,

gain-wire

 

[trondl]

thanks gain-wire.
now posted on gearslutz

 

[Enzo]

If the chorus is adding this, get the scope on it. Chorus is generated in a variety of ways, but it involved phase shifting. SO if there is a bad stage in a bucket brogadee, or whatever, it should be visible. What does this distortion look like on a screen?

 

[trondl]

Don't have access to a scope but this is how it sounds on CSR RATE: 127 and dropping towards 0 value after 5 seconds. Note that this is recorded at very high volume and is not very audible when played at normal levels.

 

[Peckerwood]

Have you ever heard any effect that was NOT noisy? By noisy I mean compared to having nothing between the instrument and the amp. If you have I would like to buy it.