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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Unstable VCO
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elkayem



Joined: Jul 04, 2017
Posts: 22
Location: Santa Clara, CA

PostPosted: Mon Dec 11, 2017 10:17 pm    Post subject: Unstable VCO Reply with quote  Mark this post and the followings unread

After 5 months, I finally have my Sound Lab ULTIMATE and ULTIMATE EXPANDER. (See related post with pics.). Wow! What an amazing sound I get with all three VCOs at the same time. I've been having tons of fun with my keyboard. However...

One of my VCOs just won't stay in tune. The other two are rock solid, but I can watch this one on my tuner, and it goes up and down by +/-5 cents over the course of a few seconds. And it I check a few minutes later, it is off by 50 cents!

I need to debug this, but have next to zero experience with analog synthesizers. What would be the most common root cause? The SSM2210? The tempco resistor? One of the pots? I don't want to swap everything out, so need some place to start. Plus, I don't have spares on some of those exotic parts. Any tips?
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elkayem



Joined: Jul 04, 2017
Posts: 22
Location: Santa Clara, CA

PostPosted: Sun Dec 17, 2017 11:39 am    Post subject: Reply with quote  Mark this post and the followings unread

No replies to this query, but maybe for the best. Whatever problem I had last week has not recurred. I left my scope on all day yesterday and monitored the VCO frequencies, and all three stayed rock solid. 440 Hz stays within about 1 Hz, which is good enough for me. I've also been playing my synth a lot lately with my keyboard, and it stays in tune pretty well. Maybe it wasn't fully warmed up when I encountered that issue? Who knows. It's been fun seeing how well I can approximate the sounds of a Minimoog. It does a pretty good job. This is a fun little machine!
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elkayem



Joined: Jul 04, 2017
Posts: 22
Location: Santa Clara, CA

PostPosted: Sun Dec 17, 2017 9:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

I take it back. I'm watching VCO1 right now in real time and it is possessed! It is easily shifting up and down by 50 cents in a matter of seconds. It has been behaving itself so well all weekend. Guess I'll start swapping out components. Anyone who's seen this type of thing before, I could sure use a hint.
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g.gabba



Joined: Nov 29, 2008
Posts: 286
Location: berlin
Audio files: 18

PostPosted: Mon Dec 18, 2017 12:39 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi elkayem,

i guess the normal first question:
does it behave the same when the vco is the only module connected to the Power-Source? is not valid here, cos its all build one one Board?

putting extra stabil.caps to the corresponding opamp pos-ground and neg-ground?

can u measure someting strange on the cv inputs?

does the frequency change if you turn any other knob eg. LFO?

Since i had some bad experiences with china pots and non-coducting frontpanels i tend to ground the cases of the pots. You only need a crococlip to test it.

cheers, gabbagabi
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elkayem



Joined: Jul 04, 2017
Posts: 22
Location: Santa Clara, CA

PostPosted: Thu Dec 21, 2017 12:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Some good ideas there. I've got too much connected to the same power source (on the same board), so it's impossible to see of something else is responsible. I am driving all three VCOs with the same CV input, and the other two are solid, so I'm inclined to think it isn't the CV source.

It is possible that this is related to some other activity I am doing on the board. The frequency can stay steady for many hours, but then start moving around when I play with my synth. Coincidence? Maybe.

It is true that my front panel is non-conducting, but I tried to follow Ray's advice to ground the pots, and put a sheet of aluminum foil on the back. The pot cases are all grounded.

I am deeply suspicious of the pots, for absolutely no good reason. I did have one that was flaky that I needed to replace. I purchased a bunch of "TT Electronics" pots from Mouser. They feel quality, and are conductive plastic, but it seems to me that they could be the weak link.

I'll keep observing and see if it is correlated to anything else.

Another thing I just noticed yesterday. The Line Out volume will suddenly shift down to half volume for a few minutes at a time, and then pop back up to full volume. Probably not related, but just shows me that there are a few gremlins in this box that I need to find. Other than that, it is a terrific instrument, and I am grateful for Ray having designed it.

g.gabba wrote:
Hi elkayem,

i guess the normal first question:
does it behave the same when the vco is the only module connected to the Power-Source? is not valid here, cos its all build one one Board?

putting extra stabil.caps to the corresponding opamp pos-ground and neg-ground?

can u measure someting strange on the cv inputs?

does the frequency change if you turn any other knob eg. LFO?

Since i had some bad experiences with china pots and non-coducting frontpanels i tend to ground the cases of the pots. You only need a crococlip to test it.

cheers, gabbagabi
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RF



Joined: Mar 23, 2007
Posts: 1496
Location: Northern Minnesota, USA
Audio files: 28

PostPosted: Wed Jan 24, 2018 7:33 pm    Post subject: Re: Unstable VCO Reply with quote  Mark this post and the followings unread

elkayem wrote:

One of my VCOs just won't stay in tune. The other two are rock solid, but I can watch this one on my tuner, and it goes up and down by +/-5 cents over the course....


I have the same issue with my ultimate/expander - VCO #2.
- VCO 1&3 are perfect.

After troubleshooting all the components and changing several out of frustration - I eventually replaced the course tune pot and that seemed to fix it for a few months. Then the same problem returned. Sad to say I have not even looked at it for several months.

I'm a pretty good troubleshooter.....but...No solution here yet. I'll let you know if I come up with anything - Hope you will do the same.

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wackelpeter



Joined: May 05, 2013
Posts: 395
Location: germany
Audio files: 10

PostPosted: Mon Jan 29, 2018 8:01 am    Post subject: Reply with quote  Mark this post and the followings unread

this sounds like a faulty pot to me.... had this too once.... some pots also are very sensitive when you mount them on the panel or case and pull the fixing screw too much, then they can break somehow...


I was Troubleshooting all the way through the PCB and then i noticed that this also occured when i push a little harder on the case... Well could be a bad solder point then or a loose connection but, after re-soldering all Points i was pretty sure it wasn't that.

Edit it can also be a faulty input socket... also had one or two in the past years, were there was a short or temporary short between the gnd and signal tap of those... suspect then 1V/oct, linear FM or exponential FM Input

Also i would hook a DVM at the power rails and measure if the voltage remains stable... especially at the moments when this drift occurs. just a few mV drops will or at least can affect the pitch

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