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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
Troubleshooting Modulo Magic
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kkissinger



Joined: Mar 28, 2006
Posts: 1214
Location: Kansas City, Mo USA
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PostPosted: Tue Apr 13, 2010 11:59 am    Post subject: Troubleshooting Modulo Magic
Subject description: It is working now! Hurray!
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I finished wiring up a Modulo Magic circuit board and panel controls last Friday and have been troubleshooting since then.

The circuit is working fine and I've observed the modulo processing -- however at some combinations of input voltage, step size, and step initiation voltages, I get oscillation on the -.06v buss which, in turn, causes oscillation all over the board!

The symptoms and cause have been difficult to track -- however here is what I know:

. the oscillation frequency is determined by 10uf capacitor on the -0.6v supply -- a larger cap value slows down the oscillation.

. I have checked the board for solder bridges, particularly between the analog switch outputs and other traces. Everything looks fine.

. I replaced the nine decoupling capacitors from disc ceramic to film caps (little or no change to the results)

. I have observed noisy output from IC2's step initiation CV output (no consistent however noise would certainly impart jitter to the analog switches.

. When the oscillation occurs, the output swings to the negative rail and jumps up to around 0v (since this is fed from an inverting opamp, it is swinging positive internally).

. I have checked for a grounding issue -- sometimes when I move my hand to and from the circuit board, the oscillation will change in amplitude or may disappear entirely.

. I tried this without the 10uf capacitor -- everything appears to work ok except that an annoying high-frequency noise is ever-present (that is, it is oscillating at a high frequency).

. When I remove the 4066 chips, the board is quiet (no oscillation). If either of the 4066 chips in in place, then I get the oscillation.

. This problem is hard to localize because once the -.06v supply oscillates, the oscillation goes everywhere -- that is, some of it bleeds to the +/- 15v busses.

Here is a link to the schematic:

http://www.cgs.synth.net/modules/cgs40_modulo_magic.html

My next course of action is to remove the jumper that connects the output of the analog switches to the output stage (inverting summer). Once this is done, I'll check with a scope and see if the the oscillation goes away. (Can you tell that I'm getting desperate Confused )

Thanks in advance for any help or suggestions.

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Last edited by kkissinger on Sun Apr 18, 2010 7:59 pm; edited 1 time in total
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kkissinger



Joined: Mar 28, 2006
Posts: 1214
Location: Kansas City, Mo USA
Audio files: 29

PostPosted: Tue Apr 13, 2010 8:09 pm    Post subject: update Reply with quote  Mark this post and the followings unread

When I remove the jumper next to point M2 on the PCB, the oscillation stops and I can observe (on the scope) the correct operation of the analog switches.

I replaced the jumper with a 100K resistor (just for grins) and the module didn't oscillate -- and (as expected) the modulo function was very slight.

So, I've established that the oscillation is across the switches -- as if the switch output is backfeeding to the inverted inputs of the LM393s.

I am now scrutinizing the diode at the junction of the analog switch output bus and pin 6 (the inverting input) of IC1. If this diode is open (or if it has a bad connection) perhaps that's a problem... well, more work to do...

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kkissinger



Joined: Mar 28, 2006
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Location: Kansas City, Mo USA
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PostPosted: Tue Apr 13, 2010 10:42 pm    Post subject: Another update
Subject description: still troubleshooting
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Turns out the diode mentioned above may have have poor solder joints... I resoldered it and I'm not getting the feedback through the negative power buss... so that problem is solved.

However, the comparators are jittery -- there is still a lot of noise on the circuit and I haven't been able to isolate the source.

Anyway, it is late so I'll try again tomorrow evening.

Again, any advice will be appreciated. Normally I can figure these things out but this problem really has me scratching my head.

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kkissinger



Joined: Mar 28, 2006
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PostPosted: Wed Apr 14, 2010 7:53 am    Post subject: Slow progress -- still not fixed, though Reply with quote  Mark this post and the followings unread

Well, I finally have a consistant problem to solve --

I don't get noise when ALL of the analog switches are open or closed -- the problem is when only some of the switches are open or closed (that is, when I'm within the modulo processing range).

The next step will be to check for an open diode ... fortunately, they all connect to ground so I can simply use a spare diode as a probe -- with my "grounded diode probe" I can touch the pins 1, 11, 9, and 4 and 12, 13, 6, and 5 of the analog switches to check this. I think I can identify any bad diode connections with this method.

(Note: I suspect the diode connection at pin 11 of IC8 or pin 4 of IC9 -- I'll check those first)

Anyway, I may be on the path to fixing this.

One "advantage" of intense troubleshooting is that one really learns a circuit in-and-out. All the same, I look forward to getting this working -- it will be a nice addition to my synth.

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kkissinger



Joined: Mar 28, 2006
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PostPosted: Wed Apr 14, 2010 10:38 pm    Post subject: making slow progress
Subject description: finally have a problem definition
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Since I use sockets for all my chips, I've swapped chips in and out to assure that I don't have any bad chips and everything is ok.

I thought that I had eliminated the oscillation however I've determined the cause. Fixing the cause is proving a challenge.

The symptom is a "glitch" kind of output when any one of the analog switches changes state. When the switch changes state, the -0.6v buss pulls up to just above 0v. When this occurs, the analog switch gets jittery. The filter capacitor manages to charge back and brings the buss back to -0.6v -- and the cycle repeats -- that is why the filter cap impacts the oscillation frequency.

To use a higher value cap to decouple the analog switch ICs helps -- however with a low frequency or DC signal, glitching occurs when moving past a switch point for any one of the comparator steps.

The comparators are putting out clean signals. However, noisy transients appear at the control pins of the analog switches (namely, pins 5, 6, 12, and 13 of IC8 and IC9). This suggests that the source of the noise could be the diodes that are connected between the control pins and ground. I really don't think that the 100K resistors are contributing to transient noise here.

However, all the diodes look ok as far as their connections and operation.

Some of the other noise was the result of my connecting some of the flying resistors to the pot side of the cable rather than to the board. Fixed all of that -- I should have known better.

My focus is to figure out why the negative supply is pulled up into positive territory -- that's right, it rises ABOVE 0v -- but only when the analog switches are changing state.

Has anyone dealt with such problems before? I feel that my options are shrinking here...

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kkissinger



Joined: Mar 28, 2006
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PostPosted: Thu Apr 15, 2010 6:49 am    Post subject: OK, I think I figured out the problem Reply with quote  Mark this post and the followings unread

Well, I think I figured this one out...

The build list called for a "4066" and I just ordered a 4066 ... well... Embarassed

Not all 4066s have the same specs. I managed to order an 74HCT4066N which has a maximum power supply rating of 5v!

The CD4066BEE4, however, is designed to handle 15v supply voltages so I just ordered that chip instead.

I will admit, the 74HCT4066 must be pretty resilient.. it doesn't heat up or fail with the 15volt supply -- it just doesn't work well and, as noted above, shorts out the power supply when it operates.

Ah, this is one of the perils when one makes the jump from kit-building to sourcing one's own parts.

Moral of the story: check the ratings on the chips before ordering.

I'll post the results once I get the CD4066 chips installed -- hopefully my last post on this thread!

edit: chip identification numbers

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Last edited by kkissinger on Fri Apr 16, 2010 8:05 am; edited 1 time in total
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mark_olson



Joined: Oct 26, 2006
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PostPosted: Thu Apr 15, 2010 11:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Congratulations, Kevin!

I'd also like to say how much I (and I'm sure others) have appreciated the narration of your troubleshooting process on this. I have a kind of low tolerance for frustration, and sometimes I feel like I also have a low aptitude for linear thinking, so troubleshooting is hell for me. A story like yours with this module gives me hope.

Mark
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kkissinger



Joined: Mar 28, 2006
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PostPosted: Sun Apr 18, 2010 7:52 pm    Post subject: All working now! Reply with quote  Mark this post and the followings unread

I am happy to report that the Modulo Magic module is now working great.

In reality, every module that I've built has required some degree of troubleshooting -- I just consider it a "given" that I will have to do it. Usually the problem is something simple such as a cold solder joint. This one was a little trickier.

Thanks for reading. To write these notes helped me to narrow down the problem and, who knows, perhaps someone else will avoid the kind of problem that I created when I ordered the wrong part.

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otherunicorn



Joined: May 11, 2008
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PostPosted: Tue Jun 15, 2010 2:46 am    Post subject: Reply with quote  Mark this post and the followings unread

And I missed this topic until now. Yes, that 74xxx trap catches a few people, which is why I have it mentioned in my parts FAQ.
Certainly an interesting problem, and an interesting read.

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prgdeltablues



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PostPosted: Wed Jan 30, 2013 2:45 am    Post subject: Reply with quote  Mark this post and the followings unread

Ok, I'm stuck on troubleshooting my Modulo Magic build. IC2 was getting warm, and I measured between 1V and 3V on Pin 2 - which is meant to be a virtual ground. Double checked for solder bridges and misplaced components (this is a stripboard build). Took all the ICs out, and replaced them one by one, checking functionality - all OK. That is, with just IC2 in, it acts properly as an inverting amp for the REF input (up to about 9 volts - I'm on +/- 12 volt supply, and of course the TL072 won't swing rail-to-rail, so that makes sense). The comparators all work too, as do the analog switches. It's when I insert IC3 that things go wrong. Taking IC2 out - IC3 works as it should, as a two stage mixer. So it seems as if all the sub-sections of the circuit work as they should. But there's some interaction between IC2 and IC3.

With all ICs in place, if I apply less than about 4V to the Ref input (This is wired with a 100k pot as per Ken's Best of CGS layout on his website, with the input resistor increased to 470k as per) - everything appears OK (I haven't yet got as far as applying an input, so I can't say if the output is modulo, but all voltages appear OK, and nothing gets warm). When I increase the ref voltage, Pin2 of IC2 starts to rise above ground (and Pin 1 is no longer the inversion of the ref voltage). I've swapped out the opamp - no difference.

Any suggestions welcome!!!!

The only modification I think I have made is to add 'fine' controls for step size and ref, using 100k pots wired between V+ and Ground, through a 4M7 input resistor.

Oh, and with IC2 not in place, I get about 3V at where Pin2 would be, which seems odd.

Peter
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otherunicorn



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PostPosted: Wed Jan 30, 2013 2:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Check your power connections to the chip.
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prgdeltablues



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PostPosted: Fri Feb 01, 2013 3:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Update. Still not working. Checked the power supply to all chips -OK. Also went over the layout row by row checking for misplaced components (done that before with stripboard) - all looks fine.

Found that IC3 Pin1 (the output of the step/mix section) starts at about 5V and rises in a couple of seconds to the V+ rail - with no inputs. So something wrong there! The chip then starts to get warm. I assume once one chip is latched up, the whole circuit is unlikely to function. I can't see anything obviously wrong, but it's a pretty simple sub-circuit - just a two stage inverting mixer. It might be oscillating at high frequency, despite the 47p caps? Think I'll take that IC out, and its feedback resistor (so there's no DC path back to Ground or a power rail) - it looks to me as if the rest of the circuit should still function. If all is OK, I could then inject a small voltage direct into the top end of the analog switches (via the step/mix out socket), which should replicate the step size function.

Peter
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prgdeltablues



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PostPosted: Sat Feb 02, 2013 10:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Working!!

Finally found out that the Step out and Reference line were showing the same voltage, and tracked down a hidden solder bridge between them.

Peter
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