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 Forum index » DIY Hardware and Software
Threeler Debugging Help?
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jhoweaa



Joined: Jan 09, 2016
Posts: 6
Location: Ann Arbor

PostPosted: Sat Jan 09, 2016 11:42 am    Post subject: Threeler Debugging Help?
Subject description: I need some help debugging my Threeler build
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I'm new to Synth DIY as well as new to this forum, so please excuse me if breach any forum etiquette with this posting.

I recently completed a build of the Threeler VCF which I purchased as a kit from Bridechamber. I purchased the DotCom version. I think I put everything together correctly, but it obviously isn't behaving properly. I'm hoping that I might be able to get some help diagnosing the problem from people on this forum. I'm fairly new to electronics so I'm not an expert in circuit knowledge. I do know how to use a multimeter and mostly know how to use an oscilloscope. I can mostly interpret a schematic and figure out how it maps to the PCB. With that being said, let me describe what I'm seeing.

The first clue I had that something wasn't right was the fact that I could barely get any self oscillation out of the filter when using no input and increasing the resonance. The only mode where I get any self oscillation is in mode 4 (3HP) and there is is very tiny and I have to crank to pot way up to see anything. I also was not getting any output at all on Output 3 in most of the modes. I used my multimeter to check to make sure I was getting connections in various place, making sure ground was connected everywhere, etc. I checked the input voltage as it came on the board. From my DotCom, going through an adapter which came with the kit, I see 14.74 on the positive rail and -14.34 on the negative. Not sure why they aren't the same.

I wondered if there might be a problem with the wiring of my mode switch. But I checked the voltages on the DG409 and I think I'm certain U3/U4 are getting the proper values from the switch. I've checked the other ICs and they all seem to be getting the same +V/-V values that come in on the input.

I've hooked the input and the outputs to my scope (I've attached a picture of the screen below). When set into 3LP mode, Output 1 looks reasonable to me. If I input a square wave and adjust the cutoff, the wave will change from square to sine. Output 2 behaves in a similar manner. However, Output 3 gives me no usable output at all, and is in fact generating a tiny wave form at about -13v. Playing with the resonance seems to have no effect, at least in 3LP mode. It has a *VERY* minor effect in 3HP. If I input no signal at all, I can only get a tiny bit of self oscillation when used in 3HP mode. The wave forms were generated with the input, cutoff, fine, and res pots set at about half way (IIRC).

I've looked over the board carefully to see if I have any bad solder joints, connections, etc. I'm not seeing anything (but I could be overlooking something). I'll post some links to high resolution images at the bottom of this post if you would like to look at them in more detail to see if I've messed up any connection.

At this point I've pretty much exhausted my thoughts on how to debug this thing. In many places I'm not sure exactly what voltages I should be seeing. Any suggestions on specific things to look at, measure, etc. would be greatly appreciated!

Wave Forms on the scope:

On the scope, Channel 1 is the input signal, Channel 2 is Output 1, Channel 3 is Output 2 and Channel 4 is Output 3. Also notice the difference in scale for Channel 3 (Output 2) and Channel 4 (Output 3). Finally, note that Channel 3 is very negative from the 0 volt line for that channel.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Some images of the build:

Back side of PCB
Front side of PCB
PCB panel connections

Pictures from my Osciliscope:

Output 1 Wave Form
Output 2 Wave Form
Output 3 Wave Form #1
Output 3 Wave Form #2

Thanks again for any help!

Jim
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elmegil



Joined: Mar 20, 2012
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Location: Chicago
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PostPosted: Mon Jan 11, 2016 12:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm assuming you've seen the plots Dave Brown has at his site for this circuit:

http://modularsynthesis.com/bridechamber/threeler/threeler.htm

Do the graphs on yours for 1 & 2 out change as expected with the mode switch as well?

And I'd assume then that 3 out is the same low voltage mess Smile

Have you tried to swap U4? I know that the DG409 is a relatively spendy chip; have you socketed them? It is possible for one or more pins to be 'fried' on a chip without disabling the whole thing, depending on the type of chip. If it's socketed, it's easy enough to trade one for another and see if behavior changes (even if you only have the two, swap them on the board, but it would be better if you have an extra to use that).
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elmegil



Joined: Mar 20, 2012
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PostPosted: Mon Jan 11, 2016 12:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

If you are using sockets make sure you haven't folded one of the legs somewhere up underneath the chip. Can be hard to see from "overhead" view....
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jhoweaa



Joined: Jan 09, 2016
Posts: 6
Location: Ann Arbor

PostPosted: Mon Jan 11, 2016 4:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
I'm assuming you've seen the plots Dave Brown has at his site for this circuit:

http://modularsynthesis.com/bridechamber/threeler/threeler.htm

Do the graphs on yours for 1 & 2 out change as expected with the mode switch as well?

And I'd assume then that 3 out is the same low voltage mess Smile

Have you tried to swap U4? I know that the DG409 is a relatively spendy chip; have you socketed them? It is possible for one or more pins to be 'fried' on a chip without disabling the whole thing, depending on the type of chip. If it's socketed, it's easy enough to trade one for another and see if behavior changes (even if you only have the two, swap them on the board, but it would be better if you have an extra to use that).


Yes, I've looked at Dave Brown's pictures. Parts of mine match in wave form, but the range is definitely off. However, anything involving Output 3 and Resonance is completely wrong.

I was going to try your suggestion with the DG409's, but I managed to bend (and then break) one of the pins when I tried to take out U3. I was going to try to swap U3 and U4 to see if anything changed. Now that I've fried the chip, I'll have to wait until I get my replacements. Oh well, part of the process I guess.

When I'm able to do more testing I'll let you know what I find out.

Thanks!

Jim
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elmegil



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PostPosted: Mon Jan 11, 2016 4:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

If you have any resistor leg cut offs you could improvise a replacement pin... at least for testing purposes Smile
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jhoweaa



Joined: Jan 09, 2016
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Location: Ann Arbor

PostPosted: Tue Jan 12, 2016 6:28 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
If you have any resistor leg cut offs you could improvise a replacement pin... at least for testing purposes Smile


That's an idea. However, since the DG409 is just a switch used to connect pins 8/9 to some other pins on the chip, couldn't I just bridge pin 8 to pin 4 and pin 9 to pin 13 to simulate mode 0, and similarly connect to other pins to simulate the other modes? Or is there some reason that would be a bad idea (assuming I'm careful not to short anything out)

Thanks.

Jim
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elmegil



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PostPosted: Tue Jan 12, 2016 8:12 am    Post subject: Reply with quote  Mark this post and the followings unread

That should also work Smile
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jhoweaa



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PostPosted: Sat Jan 16, 2016 7:49 am    Post subject: Reply with quote  Mark this post and the followings unread

After some getting some terrific help from Ian Fritz, we did finally determine that the problem was with one of the CA3280 chips. It seems like at least the 'B' side of the chip doesn't work properly. I've got an order for some CA3280's from China that I hope to get in a week or so and then I'll see what happens.

Jim
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airfrankenstein



Joined: Jan 10, 2010
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PostPosted: Mon Apr 25, 2016 1:18 pm    Post subject:
Subject description: threeler debugging
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I'm following this thread because of a similar problem. No self oscillation on the lower frequencies and on the bandpass/high frequency modes I only get self oscillation if I turn the resonance knob all the way up before switching to one of those modes. The resonance pot's action seems to be in the first tenth of its sweep. Tried swapping in a new CA3280 and then switching the CA3280's around. Tried changing the pot, reflowing solder joints. I also swapped one of the 220pF caps : initially I thought the problem might be a cap. Still searching.

Have you resolved your issue?
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jhoweaa



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PostPosted: Mon Apr 25, 2016 3:42 pm    Post subject:
Subject description: threeler debugging
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airfrankenstein wrote:
I'm following this thread because of a similar problem. No self oscillation on the lower frequencies and on the bandpass/high frequency modes I only get self oscillation if I turn the resonance knob all the way up before switching to one of those modes. The resonance pot's action seems to be in the first tenth of its sweep. Tried swapping in a new CA3280 and then switching the CA3280's around. Tried changing the pot, reflowing solder joints. I also swapped one of the 220pF caps : initially I thought the problem might be a cap. Still searching.

Have you resolved your issue?


Yes, my problem has been resolved. My solution was to replace the CA3280's since they seemed to be the problem (actually only one was a problem, but I replaced them both). I don't have any other insight into things to try. There is a thread on Muffwiggler where I worked with Ian to figure out the problem - https://www.muffwiggler.com/forum/topic-153227.html

Jim
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airfrankenstein



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PostPosted: Mon Apr 25, 2016 11:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks. So the Chinese CA3280's were ok?
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jhoweaa



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PostPosted: Tue Apr 26, 2016 2:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

airfrankenstein wrote:
Thanks. So the Chinese CA3280's were ok?


Ian Fritz sent me a couple of replacement CA3280's that were 'original'. I also have some Chinese ones, but I didn't try those since I got it to work with the ones that Ian sent me.
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