related question: the pcb for the VCO in a 2600 is super small.
is this because its a very simple circuit, or is a portion of its function displaced to another place in a 2600? (wondering about a 2600-or even better a 2500-vco clone....)
Joined: Feb 16, 2007 Posts: 347 Location: Manchester UK
Audio files: 6
Posted: Fri Aug 14, 2009 3:44 pm Post subject:
rjd2 wrote:
related question: the pcb for the VCO in a 2600 is super small.
is this because its a very simple circuit, or is a portion of its function displaced to another place in a 2600? (wondering about a 2600-or even better a 2500-vco clone....)
The module is small because it's only the core sawtooth oscillator - VCO 1 & 3 has the Square converter on the main board, and VCO 2 has the Square/PWM, Triangle and Sine on a seperate "stand off" board.
It's a great oscillator to fiddle about with though! _________________ Kronos 2-88, Kronos 61, Studiologic Sledge V2/SL, Broadwave ARP 2600EX, Broadwave 18U ARP based Eurorack Modular, Broadwave Minimoog Clone, GEM S2 Turbo.
I have a problem with the Arp Dual VCOs: they don't oscillate. The only schematic I found (earlier in this thread, and also in an service manual) is quite different from the circuit on Andys PCB. Can anybody point me to the right schematic?
PS: I made a schematic with KiCAD, but it ist still very raw and I'm not sure yet if it's correct... it is placed here:
Joined: Feb 16, 2007 Posts: 347 Location: Manchester UK
Audio files: 6
Posted: Sun Sep 13, 2009 8:48 am Post subject:
Boerge wrote:
The only schematic I found (earlier in this thread, and also in an service manual) is quite different from the circuit on Andys PCB. Can anybody point me to the right schematic?
This is the schematic for the Dual VCO (MKIII Odyssey), taken from the Service Manual, maybe you're looking at the original MKI Oddy schematics?
Joined: Mar 18, 2008 Posts: 454 Location: London, UK
Posted: Sun Sep 13, 2009 9:52 am Post subject:
I powered up my PCB last weekend and it mostly all works! I haven't got anything coming out of VCO2's pulse output but otherwise it sounds fantastic! Thanks Andy for a great PCB.
This is the schematic for the Dual VCO (MKIII Odyssey), taken from the Service Manual, maybe you're looking at the original MKI Oddy schematics
Andy
Thank you. I found out over the weekend that the schematic is identical to the ARP Avatar VCO. The only things I have changed are:
2N5910 => I used 2N3906
CA3086 => I used CA3046
I'll try BF245 for the FETs this evening.
regards
Boerge _________________ ...ich will doch nur löten...
Joined: Mar 18, 2008 Posts: 454 Location: London, UK
Posted: Tue Sep 29, 2009 8:48 am Post subject:
I've finally got some time to try and debug the 2nd VCO which isn't quite working as it should. I've checked the wiring which it all seems correct so I'm a little stumped!
VCO1 seems to work fine - I get a healthy saw and pulse output from the respective outs. VCO2's saw doesn't have the same level as VCO1's and I get nothing out of the pulse output. I've attached some oscilloscope pics.
Where should I start looking for problems?
Thanks a lot!
PS. Each square is 2 volts on the oscilloscope.
arp2photo.jpg
Description:
VCO1 Saw Output - looks ok!
Filesize:
125.06 KB
Viewed:
292 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.
arpphoto.jpg
Description:
VCO1 pulse output - looks ok!
Filesize:
124.7 KB
Viewed:
301 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.
arp3photo.jpg
Description:
VCO2 Saw level - looks low.
Filesize:
116.51 KB
Viewed:
300 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.
arp4photo.jpg
Description:
VCO2 Pulse output - non-existent!
Filesize:
117.84 KB
Viewed:
315 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.
Joined: Feb 16, 2007 Posts: 347 Location: Manchester UK
Audio files: 6
Posted: Tue Sep 29, 2009 2:00 pm Post subject:
LetterBeacon wrote:
Where should I start looking for problems?
I recently built an experimental single version of the VCO and had exactly the same problem - I did manage to fix it... but did I write any referral notes down - no of course I didn't
I'm desperately trying to remember what I did, but first of all check the sync input. Do you have the switch fitted from VCO 1, because VCO 2 will have problems if you leave the sync input floating - It either has to have a signal from VCO 1 or must be tied down via the 100k and 150k right next to the sync switch pads.
The next thing is to check the CD4011 - I definitely remember there was also something about loading on the output (one of the nand gates is used as a buffer for the square wave output).
I hope this helps - next time I'm problem solving, I'll takes notes!
Joined: Mar 18, 2008 Posts: 454 Location: London, UK
Posted: Tue Sep 29, 2009 2:09 pm Post subject:
AndyR1960 wrote:
I'm desperately trying to remember what I did, but first of all check the sync input. Do you have the switch fitted from VCO 1, because VCO 2 will have problems if you leave the sync input floating - It either has to have a signal from VCO 1 or must be tied down via the 100k and 150k right next to the sync switch pads.
I do have the switch in and I can hear it syncing when I turn the switch on, so I assume it's working. I'll check the wiring in the morning just in case.
Quote:
The next thing is to check the CD4011 - I definitely remember there was also something about loading on the output (one of the nand gates is used as a buffer for the square wave output).
When you say 'check the 4011' do you mean check it's getting the correct voltages? I don't suppose you have a list of the correct voltages at the pins at all? I have a couple of spare 4011s so I'll try swapping them in.
Joined: Feb 16, 2007 Posts: 347 Location: Manchester UK
Audio files: 6
Posted: Tue Sep 29, 2009 2:26 pm Post subject:
BTW, as an addition to this thread, I mentioned above a single VCO version I was working on. I was building a compact clone of the Odyssey - got as far as the two VCO's, S/H mixer, LFO, noise and VCF mixer, and then I got a MKIII Odyssey off ebay.
I guess I should finish this project... one day... maybe...
VCO1 Front.jpg
Description:
I've used 30mm sliders - the width of each section is the same as the Odyssey, but the depth is 4 inches shorter.
Filesize:
183.19 KB
Viewed:
286 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.
VCO1 Back.jpg
Description:
No 3086! I ran out of them, so just used matched pairs of 3904's... works fine! Plug in VCO's not much bigger than the 2600 module.
Filesize:
191.58 KB
Viewed:
287 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.
Joined: Mar 18, 2008 Posts: 454 Location: London, UK
Posted: Sat Oct 03, 2009 9:57 am Post subject:
forbin wrote:
what sort of travesty are you involved in? wearing a Moog tee shirt while fixing an ARP oscillator! No wonder your having problems!
Heh, I have tried it with my ARP t-shirt on but no luck either!
I tried some troubleshooting today and noticed something a little odd. When I turn up the Coarse pot on VCO 2 the saw wave increases in amplitude on the oscilloscope. I've tried swapping out the 4011 with a new one to no avail! The Sync switch appear to be wired correctly and I can hear it working too.
I've taken a video of the oscilloscope if that helps at all. It starts off with the Coarse pot completely CCW and then I turn it up. At about 0:28 I plug the oscilloscope into the pulse out.
Joined: Sep 10, 2008 Posts: 3 Location: Montreal Canada
Posted: Sat Nov 07, 2009 11:50 am Post subject:
Hi everybody,
This is not about building a new oscillator but I hope you guys could help me since you seems to know the ARP Odyssey oscillators very well.
I'm trying to fix a white ARP Odyssey 2800.
The problem is that the octaves on VCO-1 are not linear. For example, if I play a low C on the keyboard the next C will be an A.
I made the adjustments like described in the service manual but I not able to get the octaves in tune.
I replaced the capacitors in the VCO-1 circuit. It improved a little bit the linearity of the octaves but I'm still not able to tune it properly.
Does anybody have some suggestions to fix this problem?
Thank you!
Joined: Feb 16, 2007 Posts: 347 Location: Manchester UK
Audio files: 6
Posted: Sun Nov 08, 2009 3:14 am Post subject:
omnitek wrote:
I replaced the capacitors in the VCO-1 circuit. It improved a little bit the linearity of the octaves but I'm still not able to tune it properly.
Does anybody have some suggestions to fix this problem?
Thank you!
Nicolas
The first thing I would do is replace the exponential transistor pair - that's the TZ81/581 - you can change them with a pair of 2N3904/3906's.
Joined: Feb 16, 2007 Posts: 347 Location: Manchester UK
Audio files: 6
Posted: Tue Nov 10, 2009 2:02 am Post subject:
rjd2 wrote:
could the CA3086's be swapped with matched pairs of either 2n3904's or 06's? i had a friend match up a bunch of these a while ago. curious-thanks.
Yes, you can use 3904's, but they don't need to be matched - You don't even have to match the exponential pair, I've built several oscillators without matched pairs and they all work perfectly... great tracking and no high frequency adjustment
I'm currently working on a new single VCO PCB (no CA3086/46) - I'll get the details up as soon as I can, but there's a pic I uploaded a few posts ago.
Joined: Feb 16, 2007 Posts: 347 Location: Manchester UK
Audio files: 6
Posted: Tue Nov 10, 2009 2:08 am Post subject:
omnitek wrote:
I replaced them and the other transistors as well but I'm still not able to tune the octaves properly by half a tone.
Is it because I need to put a matched pair of transistors?
Nicolas
I've never had problems not using matched pairs before... maybe the problem is with the keyboard CV circuitry on board 1.
Sorry to be so vague, but I'm only used to working with later MkII's and the MkIII Oddy. _________________ Kronos 2-88, Kronos 61, Studiologic Sledge V2/SL, Broadwave ARP 2600EX, Broadwave 18U ARP based Eurorack Modular, Broadwave Minimoog Clone, GEM S2 Turbo.
Joined: Sep 10, 2008 Posts: 3 Location: Montreal Canada
Posted: Wed Nov 11, 2009 7:47 am Post subject:
I was comparing the voltages beetwen VCO1 and VCO2 and the pitch of VCO1 is shifting when I touch the grid of its JFET (Q6) with my voltmeter probe while there is no effect on VCO2 if I touch the grid of its JFET (Q13).
Is this a clue?
Thanks
Joined: Nov 22, 2009 Posts: 667 Location: Frederick, MD
Posted: Mon Dec 14, 2009 5:49 pm Post subject:
AndyR1960 wrote:
...I've lifted this from Thomas Henry's Build a Better Music Synthesizer - it's from his CEM VCO design. He used it on the Triangle output (which is also 0-5v PP) to convert it to 10v PP centered around zero.
I haven't tested it, but in theory it should be fine.
Did anyone test this?
Just wondering because I'm actually working on some CEM VCO's and need to bump at least the Triangle wave up to 10v PP centered in order to convert it to a Sine wave. I had come up with a theoretical design (a differential amp and some voltage step down) but this Thomas Henry design is much simpler...half the # of resistors and small enough I might be able to build it on the small experimenter section that's already on the PCB (old PAIA EKx boards).
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum You cannot attach files in this forum You can download files in this forum
Forum index » DIY Hardware and Software
FAQ
Calendar
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
Chat Room
