Alpha Juno 2. Only noise is coming out of the DCO

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Yekuku
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Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Tue Mar 15, 2011 9:40 pm

Hi guys,
I could not resist my GAS so I bought a defective alpha juno 2 hoping that I could repair it easily .
The previous owner was experiencing power interruptions due to a faulty power cable and some weird noise on the outputs.
He took the unit to a technician where it was diagnosed as having a "faulty" main board , so he offered it to me for a fair price.
I fixed the power cable and I noticed this strange behaviour:
Everything works as expected except the sound of the oscillators, only noise is produced.
When I turn all oscillators off and press a key, noise is triggered.By increasing the volume of Pulse or Saw there is no tonal change to the noise sound , but when I increase the volume of Noise OSC the noise gets dirtier.
The noise is passed through the VCF and ENV/LFO which are working ok. By the way I just realized that u can get some pretty cool sounds by using only Noise/VCF/ENV :D
I checked the power supply and it works up to specs.
By checking the schematics, I am thinking that IC5 ( Roland custom DCO) could be faulty.
Also as Pro5 informed me in another thread, the gate array ( IC18) is a common suspect.
Unfortunatelly i dont have an oscilloscope yet, I am using just a multimeter which is PITA.
Any guidance or thoughts about where to start / what to check would be appreciatted.
here are the schematics and a pic of the board:
Image
Image
Thanks in advance

EDIT:
I noticed that IC5(DCO) has six analog ouputts OUT0-OUT5 that feed the 6 seperate VCF sections.
Since filtering is working properly, this should be my starting point. By checking/listening to pins OUT0-OUT5 I should be able to "hear" the audio signal. Is this possible by using only a multimeter ? Can I use a small speaker?
My guessing is that even if I find a way to listen to OUT0-OUT5 , the same noise will be present.
So is it IC5 faulty or could be that IC18 (Gate Array that sends DCO signal to IC5) is malfunctioning and IC5's operation gets affected? Could it be that some other faulty components affect IC5s operation ?
Any help would be appreciated.
thanks
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yunneck » Wed Mar 16, 2011 12:22 am

I dont think i can help you, but you seems know more about Juno's guts than me and maybe you could help me

http://www.vintagesynth.com/forum/viewt ... =5&t=61043

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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Ry-Fi » Wed Mar 16, 2011 2:50 am

Without a scope, you'll actually need to properly listen to the output of the DCO IC. A small speaker driver probably won't do (IMHO). Do you have an instrument amp? A stereo? Boombox? Just wire up a little probe with an RCA or 1/4" or whatever on one end feeding your amp, and on the other end just have two bare wires, one to touch to ground and the other to probe the outputs. This way is guaranteed to let you know if they're outputting properly or not.

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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by sam » Wed Mar 16, 2011 7:03 am

Roland Vp330 mk1.SH101.juno 6.OSCar.ARP odyssey..Tonus 2600..omni.
ms20/50. OBX.
Prophet 5
Wasp.EMS AKS.

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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Wed Mar 16, 2011 9:23 am

Thank you for your help , I really appreciate it :)
Yunneck wrote:I dont think i can help you, but you seems know more about Juno's guts than me and maybe you could help me
viewtopic.php?f=5&t=61043
I am no expert either but as you can see from the schematics midi goes through IC16 & IC17 and then to the CPU IC19. Assuming that u already checked the power supply & the cables , these should be the places to check first.
About your modulation/pitch bend issue , the signal goes through IC8 ( AD Converter) and then to IC18 ( Gate array).
Maybe IC8 had gone bad ? Good Luck !
Ry-Fi wrote:Without a scope, you'll actually need to properly listen to the output of the DCO IC. A small speaker driver probably won't do (IMHO). Do you have an instrument amp? A stereo? Boombox? Just wire up a little probe with an RCA or 1/4" or whatever on one end feeding your amp, and on the other end just have two bare wires, one to touch to ground and the other to probe the outputs. This way is guaranteed to let you know if they're outputting properly or not.
I have some useless powerd pc speakers. I dont mind salvaging one, for testing this.
The IC5 has 2 grounds , digital and analog , shall I use one of these or any ground of the mainboard PCB ?
sam wrote:I made this for such things..

http://www.zen22142.zen.co.uk/Circuits/ ... _trace.htm
Interesting ... Thanks for sharing Sam;)
So do u think that I ll be able to check this using a powered speaker or do i need an audio/signal tracer desperately?
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Wed Mar 16, 2011 8:52 pm

Ok , so here is the update :
I have done some checking concerning the IC5 (DCO).
I have connected a speaker to pins OUT0-OUT5 and they output only noise, as expected.
I measured analog +5V & digital +5V on the same IC at pins 23 & 24 and they get both 4,92V which is normal.
I also checked IC18's ( the gate array ic that sends "DCO chip select" to IC5) VDD +5v and I get 4.92V too.
So is it the DCO or some other component that is affecting DCO's operation ?
I am stuck here, I dont know what else to check since all I got is just a multimeter :(
Any help/guidance would be appreciated ;)
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by sam » Wed Mar 16, 2011 9:42 pm

Have you checked the traces from the gate array to the dco...?

Maybe the dco is faulty...Noise on all those outs is strange.

You could mail Roland, They are usually very good and might have the chip still...
Roland Vp330 mk1.SH101.juno 6.OSCar.ARP odyssey..Tonus 2600..omni.
ms20/50. OBX.
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Wed Mar 16, 2011 10:33 pm

sam wrote:Have you checked the traces from the gate array to the dco...?

Maybe the dco is faulty...Noise on all those outs is strange.

You could mail Roland, They are usually very good and might have the chip still...
There is only one trace coming out from the gate array to the DCO.it is labeled DCO and its function is "DCO chip select". Is this responsible for selecting the correct waveform on the DCO ? Maybe..
If the gate array is malfunctioning and sends wrong waveform information then maybe thats why DCO outputs only noise.
Anyways the problem is that I dont know how to measure this trace and what to expect.
I will certainly email roland, but i want to be sure about which part to ask for...
thanx for your input Sam ;)
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Ry-Fi » Sun Mar 20, 2011 6:47 am

Any update on your problem? I'm sorry that I can't help you at all (I'm just a lowly newb at electronics troubleshooting/repair), but I'm very curious to hear your about progress in attempting to diagnose the real issue here.

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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Mon Mar 21, 2011 12:40 pm

Well, the problem was not as complicated as I expected.
The reason that DCO (IC5 ) was outputting only noise was due to the fact that there was a broken trace on the PCB that connects the main cpu (IC9) to the DCO (IC5 ) and delivers some kind of data (?) to the DCO data input.
The alpha juno 2 is very tricky to disassemble , so I am guessing that the previous owner scratched the pcb while trying to repair the power supply.
Anyways, I used a wire to bridge the broken PCB trace and everything works fine now ;)
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Ry-Fi » Mon Mar 21, 2011 10:35 pm

Fantastic! Glad to hear it. Good job diagnosing and fixing that. :thumbleft:

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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by rhino » Mon Mar 21, 2011 10:54 pm

That's the kind of story that makes us smile.
(picures of nekkid wimmin make us smile MORE)
When the wise man points to the stars, the fool looks at the finger.
- Confucius

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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Tue Mar 22, 2011 9:23 pm

I will post some pics later this week, at the moment I have assembled it for testing purposes but I still have some work to do :
- replace the battery with a battery holder + new battery
- repair the aftertouch, although it is not broken, it needs the power of an elephant in order to activate it
- replace a tantalum cap that looks beaten
- replace some tactile switches
- find and install the latest OS. ( I am interested in being able to control the cutoff via the pedal input)
Thank you all for your support ;)
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Tue Apr 26, 2011 8:14 pm

Finally I found some free time to finish repairing my juno2.
Here is a pic showing the back side of the mainboard where I used 2 copper wires to bridge the cutten traces.
The previous owner had not installed the mainboard properly and while trying to screw the bottom cover , he cut 2 pcb traces.
Image
Since it works I thought that it would be better to leave it like this than scraping the pcb and jumpering the broken traces with solder.

I also removed all keys and cleaned them, cleaned the contacts, replaced 4 tactile switches, applied Deoxit to Alpha Dial, cleaned the volume pot, greased the keys, installed a battery holder and "tried" to fix the aftertouch.
For the aftertouch I followed this info :
http://analog.no/cms/index.php?option=c ... &Itemid=54
I disassembled the aftertouch strip and cleaned it with acetone and then I used a double sided tape to put it back together.
Image
There was no sign of corrosion in either the conductive rubber or the brass conductor so I chose not to use a sandpaper because I was afraid that it could harm the thin brass layers.
After reassembling the whole synth everything works like a charm, except the aftertouch that is.
It seems to have little to no difference compared to its previous condition. It works, but only with a great amount of pressure.
I dont think that I will open it up again, its by far the most trickiest synth I have ever disassembled / reassembled.
I wonder how many people managed to break the mainboard while trying to repair it. :?

Thats all folks !
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Re: Alpha Juno 2. Only noise is coming out of the DCO

Post by Yekuku » Mon May 02, 2011 7:54 pm

Sorry for bringing this up again but after playing a few hours with the juno , I have noticed another problem:
The sound coming from the right channel is lower than the left channel, but this does not happen all the time.
At the lower position of the volume fader till the 3/4 of the faders range, the right channel plays in lower volume compared to the left channel.
When setting the fader to the last 1/4 of the faders range, both channels play in equal volume.
Also when no jacks are connected to the main outputs and only headphones are used both channels play normal.
If a jack is connected to the right main output then the right channel of the headphones drops in volume just like main right output.
Could it be that the volume fader has gone bad ? I have already opened it up and cleaned it but this doesnt seem to be the problem since the fader works as expected when only headphones are used.
Any ideas ?
Thanks in advance
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