Alpha Juno 1 repair

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juanpi_salmo95
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Alpha Juno 1 repair

Post by juanpi_salmo95 » Wed Jun 26, 2019 5:07 pm

Hey guys! I bought a Roland Alpha Juno 1 some weeks ago, and even though it is in decent conditions and everything is more or less working it has some issues that are kinda annoying. The problems with it are basically the followig:

1. There’s always a cyclic clicking noise in the output when a key is kept pressed. It sounds like some clock noise which frequency seems to depend on the preset.
You can hear it in the first file from the attached link.
2. In the second audio file you can hear a frequency sweep of the low-pass filter. It seems that the cutoff at first decreases, then at a certain point there’s a jump and it goes back higher, finally starts decreasing again. It wouldn’t be a big problem, but it everytime that a patch has an envelope on the VFC it completely messes it up.
3. The chorus sounds a little bit noisy, still has that clicking sound than it’s in some way amplified by the chorus.
4. There’s sometimes a background sound even without any key pressed. Every 6 pressed keys the pitch of the background noise changes, which make me thing that there’s something wrong with one of the voices. This problem is NOT always present, but it’s not given by anything heating up because it’s present immediately after the synth is turned on.
5. The output level looks a little bit too weak in my opinion, but I still have to follow the Service manual procedure to verify this issue.

I hope that someone of you could help me out with a diagnosis for this beast that I really want to fix. Thanks guys!

AUDIO FILES:

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meatballfulton
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Re: Alpha Juno 1 repair

Post by meatballfulton » Wed Jun 26, 2019 9:39 pm

juanpi_salmo95 wrote:
Wed Jun 26, 2019 5:07 pm

In the second audio file you can hear a frequency sweep of the low-pass filter. It seems that the cutoff at first decreases, then at a certain point there’s a jump and it goes back higher, finally starts decreasing again. It wouldn’t be a big problem, but it everytime that a patch has an envelope on the VFC it completely messes it up.
Check the patch EG parameters. The Juno EG is not an ADSR, it has five stages and can create more complex shapes than an ADSR. It allows a shape like you described.
3. The chorus sounds a little bit noisy, still has that clicking sound than it’s in some way amplified by the chorus.
That's normal.
I listened to Hatfield and the North at Rainbow. They were very wonderful and they made my heart a prisoner.

juanpi_salmo95
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Re: Alpha Juno 1 repair

Post by juanpi_salmo95 » Sun Jun 30, 2019 10:39 am

meatballfulton wrote:
Wed Jun 26, 2019 9:39 pm


Check the patch EG parameters. The Juno EG is not an ADSR, it has five stages and can create more complex shapes than an ADSR. It allows a shape like you described.

That's normal.
Thanks for your reply! Nope, it's not about the envelope. The sweep you can it's done by manually changing the cutoff frequency of the low pass filter (from 127 to 0), the envelope is turned off.

juanpi_salmo95
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Re: Alpha Juno 1 repair

Post by juanpi_salmo95 » Sun Jun 30, 2019 1:20 pm

Update: I checked with a voltmeter the input at the FREQ pin on the 3R05 when running the synth in test mode. As expected starting with the cutoff frequency paramenter set to zero and increasing it the voltage at the pin decreases. When the "jump" occurs the voltage suddenly goes back to its maximum value, then starts decreasing again. Do you think that it could be some capacitor? Or it's just a CPU problem (hope not :c). Thank guys.

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meatballfulton
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Re: Alpha Juno 1 repair

Post by meatballfulton » Sun Jun 30, 2019 1:35 pm

It's hard to tell since I can't open your attachment. How are you doing the sweep...mod wheel, alpha dial, some other controller?
I listened to Hatfield and the North at Rainbow. They were very wonderful and they made my heart a prisoner.

juanpi_salmo95
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Re: Alpha Juno 1 repair

Post by juanpi_salmo95 » Sun Jun 30, 2019 2:30 pm

meatballfulton wrote:
Sun Jun 30, 2019 1:35 pm
It's hard to tell since I can't open your attachment. How are you doing the sweep...mod wheel, alpha dial, some other controller?
With the alpha dial, running the synth in test mode. I've just found out that the output from the S/H which goes to the filter chips and controls the VFC cutoff is behaving in the same way, doing that "jump". Next step in the path is the gate array.. and things become difficult. :?

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Re: Alpha Juno 1 repair

Post by juanpi_salmo95 » Sun Jun 30, 2019 2:32 pm

Anyway, don't know if this is clear but the numers in the display are decresing/increasing correclty from 0 to 127, no jumps.

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