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OBX voice card (with non-custom CEM3310 equivalent circuits)
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page Previous  1, 2, 3 ... 67, 68, 69 [all]
Author OBX voice card (with non-custom CEM3310 equivalent circuits)
jvq
Some updates about how my crowbx is behaving.

VCO2 keeps going out of tune on two out of four voice cards. It takes several days/weeks for the tuning to go bust. I've just replaced the tune trimmers (maybe I turned them too much?) on one voice's VCO2, waiting to see if the tuning will stick now. So far so good.

Something else: sometimes the voice on/off and square/saw switches stop working for all voices. When this happens I can turn the on/off switches to 'off' and the voices all keep making sound. When I set VCO2 to 'off' it goes to 'half' instead. And I think it locks on 'saw' for both VCO's. Other switches keep working. Unfortunately it needs some warm-up time to reach this state (and turning off/on fixes it) so it will be hard to diagnose. I wonder if it's a problem with the 4011N quad nand IC on the host board.
oldcrow
Heya folks, a quick note. I am working on getting 8-voice stuff finished among my other projects. Since 3310 EGs are going to be made again I am going to try a 3310 edition of the crOwBX voice card. It will not replace my non-custom EG version, but it does simplify the build somewhat:



More later,

Crow
/**/
oldcrow
The 4011 is either defective or the +5V going to it has an issue. VCO tuning issues are usually due to a bad timing cap or bad trimmer. --Crow

jvq wrote:
Some updates about how my crowbx is behaving.

VCO2 keeps going out of tune on two out of four voice cards. It takes several days/weeks for the tuning to go bust. I've just replaced the tune trimmers (maybe I turned them too much?) on one voice's VCO2, waiting to see if the tuning will stick now. So far so good.

Something else: sometimes the voice on/off and square/saw switches stop working for all voices. When this happens I can turn the on/off switches to 'off' and the voices all keep making sound. When I set VCO2 to 'off' it goes to 'half' instead. And I think it locks on 'saw' for both VCO's. Other switches keep working. Unfortunately it needs some warm-up time to reach this state (and turning off/on fixes it) so it will be hard to diagnose. I wonder if it's a problem with the 4011N quad nand IC on the host board.
jvq
Thanks oldcrow!

I can rule out "bad trimmer" now. Replacing the trimmers fixed one of the bad VCO's, the other one is still misbehaving. It keeps drifting. Slow enough that I can sometimes get it in tune, but then within 5 minutes it is out of tune again.

I put thermal goop between all the LS318's and their tempco resistors but I wonder if the thermal bond got damaged on this VCO somehow.

I'll try replacing the 4011 on the host board and the timing cap on the remaining bad VCO2.
PWM
jvq, did you use styroflex timing caps? These are quite easily overheated.
jvq
PWM: I used WIMA FKS2 http://nl.mouser.com/search/ProductDetail.aspx?r=505-FKS2D011001AKC00

I don't know if they're quite the right type, I just decided to use the WIMA brand and picked parts by price and trying to match specs from the BOM.
PWM
I really can't tell you if these are adequate for this application. I tend to use 1% or 0.5% tolerance types. These Wima's are 5% and up.

I don't think this cap is the problem though, you would've encountered it in all VCOs otherwise.
Also, I don't think the tempco is the problem. If I understand correctly, the VCO drops in frequency and stays there? Does it still track, after dropping? If the tempco or it's mounting is faulty I would suspect fluctuations in the frequency, not a drop.

You could build a connector with wires. This way you can troubleshoot the Voice PCB on your workbench and measure values in a working voice.
jvq
PWM: so I had two bad VCO's. One of them was dropping but then staying where it was. I replaced the trimmers on that one and now it's steady.

The other bad VCO is drifting. I replaced its trimmers too but no change. When I play that voice it sounds like it has a different tuning problem from the first one. The fact that changing the trimmers didn't help backs that up.

It's almost as if the tuning of the bad VCO is "floating". It stays in one place for a while, and then it just drifts somewhere else. In the course of seconds / minutes.
PWM
Ah, ok. I understand now. Had the two VCOs mixed up. My apologies.
Drifting could very well be a tempco problem.
ultravox
jvq, I had a VCO stability/tuning issue on a different synth (TTSH) that turned out to be a grounding issue. Two of the three VCO's would drift and the 1V/Oct scaling would not align. I corrected the mainboard grounding and everything aligned and tracked perfectly.

This may not be your issue but something to keep in mind.
jvq
Thanks ultravox!

My best guess is that I did a bad job soldering the tempco underneath the matched transistor pair and that flexing the voice board repeatedly has damaged one of the tempco solder joints. Or that I didn't use enough heat transfer paste.

I'm not an experienced solderer, especially not with SMT. So I certainly won't rule out a bad solder joint.

I've been getting a little frustrated with this tuning problem so I'm going to take it a little slow and focus on another project for a moment.

I've also built and installed the Hexinverter MIDI-CV converter into my CrowBX. Sadly, it's a source of further frustration. It's linearity is poor compared to my clunky outboard MIDI-CV converter which makes it harder to tune all the voices. A so-so CV source is not the sort of obstacle I want when trying to fix a tuning problem. smile I think I will just build my own converter. It was encouraging to see Oberling's project at https://github.com/oberling/AVRMIDI-CV and I've been getting some MCU programming experience of my own recently.

All in due time. smile
jimfowler
I strongly recommend Peter's Midi2CV and MidiGal boards. Easy to build, easy to use, more flexible than Hexinverter. https://midisizer.com/
jimfowler
oldcrow wrote:
...but you can wire 100K pots that have CW lug to +15V, CCW lug to -15V and the wiper to the leftmost pin of each FT header to provide fine tune options.

--Crow
/**/


This nets you a 2 octave "fine tune" per VCO. So far I kinda like it as you can do some neat stuff in unison mode and still have the ability to slightly detune each VCO...but getting a 5-10 cent variation is kinda fiddly. Somebody who knows more about this than me can tell you where to put a resistor to relegate the adjustment to a proper fine tune range.
jvq
Thanks jimfowler, the midisizer.com boards look really nice!

They use the same DAC family as the Hexinverter MIDI-CV though: MCP4922 / MCP4822 12-bit. I'd like to build something that uses a 16-bit DAC such as the AD5676R.

Detuning is nice/awesome, but given the choice I'd rather have a more accurate MIDI-CV DAC that I can detune via software (e.g. a MIDI CC) when needed than a less accurate DAC that makes my job harder while tuning the CrowBX expo converters. smile

Plus, building this myself is fun.
livewire
FWIW, I am interested in obtaining four more of the bare "old style rev B" Crowbx boards. Have not seen any of these for sale in a long time. If I have to buy the "kit" with all the other support boards, so be it. I am restoring an old OB-X and still need boards.
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