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Almost no env. decay control, no scope; what to check?
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Almost no env. decay control, no scope; what to check?

by controlvoltage on Sat Nov 04, 2006 3:58 am

Hello,

I'm troubleshooting an x0x with a wonky env decay control... originally the decay pot had no effect until I discovered a backwards diode D29. Correcting this restored some functionality; then decay pot now provides a range of "extremely short" to "short" over its full range.

Is there any componentry likely to have been damaged by the backwards diode? I assume that if I get any variation in decay from the pot and if Accent_on decreases the decay time, then the 4066 is still OK. What actually determines the time constant of the envelope, and are there any typical voltages/ranges for min/max decay that I can check?
I can follow the schematic pretty well but I get lost in the dual pots sometimes... even a brief circuit description would be very helpful to reduce my bench time. Thanks to anyone who can help!

/\/cv
-=trimmer tweeker=-

controlvoltage
 
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Location: Between 0 and +5V

by guest on Sat Nov 04, 2006 3:38 pm

there are a number of things that could be wrong
but the most likely is that your pot is bad
there have been a number of complaints
about the dual pots lately

you can test the bad pot theory without removing it
if you remove r138 and replace it with a 1megohm
the decay should be much longer
you could clip in a few resistors and see if it varies
as you vary the resistance
if this is the case then there is something wrong with the pot

take a look at the pot and see if there is any solder
bridging the pins

the signal path for the envelope is as follows
check all of the parts listed to see that they are
of the right value and properly aligned and soldered

a short bit of charge passes through d37 and r152
and charges up c62 to 12v
this charge slowly flows out via r138 vr7 d28 d29 d26
r117 keeps the bottom of the decay at a .6v offset

as the voltage slowly drops across c62 creating the decay
this voltage is buffered by q40 and q41
with q39 acting as a constant current source
to keep the drop across q40 and q41 constant

ic12 shorts vr7 out of the loop
so only r138 determines the decay time


so its possible that ic12 isnt shutting off fully
or that q40 or q41 is leaky
but the bad pot is the most likely offender
guest
 
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by controlvoltage on Sun Nov 05, 2006 7:23 pm

Hi, thanks for the help! Seems to have been IC12 causing the trouble;
I replaced it and got much better decay range.
I also made a nice discovery; when making my tests I wired a 1meg pot in series with R138,
and it turns out this is a nice place to add a variable accent decay control
[as opposed to putting the pot between IC12-pin4 and D28(+)].
No need to cut any traces and it seems to have the desired effect.
I'm not sure if it would be preferable to put the pot in the other location or not.
Once you understand the interplay of the two knobs it adds a nice degree of flexibility to the groove.

Thanks again, that was very helpful!
I am trying to get this one finished this weekend,
and I was tearing my hair out over the decaylessness...

/\/cv
-=trimmer tweeker=-

controlvoltage
 
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by guest on Sun Nov 05, 2006 10:39 pm

you might want to check pin5 of ic12
just to make sure that it is going to 0v
when the accent is turned off
its possible its not getting fully shut off
if it goes down to .2v or less that should be fine
guest
 
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by bcbox on Mon Nov 06, 2006 4:29 pm

changing R138 will change the env decay time. Not just on accents but all notes. R138 is there to set the minimum decay time - the decay time when the decay pot is at minimum, since you don't want the envelope to go to nothing. On the stock 303 with R138 at 68k this corresponds to a minimum decay time of about 200ms - which is what the decay time is on accented notes since the decay pot is shorted out. The maximum env decay time on the stock 303 is about 2.4 seconds. If you make R138 10k you will get a minimum decay time of 30ms. Just changing R138 however will now limit the maximum decay time to 1.4 seconds. To bump back up the decay time so it'll reach the max time of the stock 303 (or longer) you would simply increase C62. This is the timing cap that creates the decay portion of the envelope based around Q39/Q40/Q41/C62. Changing C62 from 1uF to 1.5uF will increase the maximum decay time to 3.5 seconds.
In a nutshell I wouldn't set R138 on a pot. Leave it fixed at wherever the minimum decay is that you want. It's simply in series with VR6 so varying it isn't doing anything that the decay pot isn't doing already. Best case in my opinion is to set R138 to 10k and C62 to 1.5uF. This will give you an env decay range of 50ms to 3.5 seconds (versus the stock range of 200ms to 2.4 seconds). If you want to have a variable decay for accented notes (accent decay) you have to do what is shown in the Wiki and other places and put an accent decay pot in there between IC12pin4 and D28(+).
On a side note it is interesting to note that adding the accent decay pot does not truly give an independant decay time for accented notes. The new accent decay pot as proposed in the Wiki and in the Devilfish, banned TB3030 etc.. is always in parallel with the (normal) decay pot so the decay time on accented notes can never be greater than the setting of the (normal) decay pot; in fact accent decay can never be greater than 1.4 seconds (this corresponds to both the decay pot and accent decay pots at maximum). If you're willing to do a little thinking there are many ways to modify things to get independent accent decay.
bcbox
 
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Re: Almost no env. decay control, no scope; what to check?

by dc on Thu May 15, 2008 4:02 pm

controlvoltage wrote:Hello,

I'm troubleshooting an x0x with a wonky env decay control... originally the decay pot had no effect until I discovered a backwards diode D29. Correcting this restored some functionality; then decay pot now provides a range of "extremely short" to "short" over its full range.



I bought an x0xb0x on ebay recently which turned out to have this exact problem.

Based on this thread I determined that it was likely to be IC12.

I removed the chip completely and the decay pot worked fine, this allowed me to eliminate the pot itself as the source of the problem.

I was lucky enough to have a spare circuit board from a Juno 60 lying around and found a spare 4066 chip on it.

I substituted this for the chip that was in my x0xb0x and that solved the problem.

It sounds so much more like a 303 now that the decay is working.

I wonder if this is quite a common problem.

Currently there is a post on the Matrixsynth blog with video of an x0xb0x in which the guy can be clearly heard to ask in a confused tone of voice "what's happened to the decay?"

Is IC12 prone to failure?

Is there something in the circuit which leaves it open to getting fried?

When it comes to the insides of synths, I'm a meddler, I have no formal training in electronics but I seem to get most of what I need done most of the time ;-)

Just thought I'd post my experience with IC12 for the benefit of anyone experiencing similar problems.

dc.
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Re: Almost no env. decay control, no scope; what to check?

by MartinDidymus on Sun Sep 26, 2010 2:22 pm

Hi there,

I am finishing my x0xb0x and am getting a similar fault, there is no or negligible difference in the envelope decay when I turn VR6. I would prefer to narrow the possible list of culprits before I start removing components from the board, I am not very experienced in electronics and so would appreciate any assistance than may be offered.

I have visually checked the components listed by Guest in the reply to the original post. They are all of the correct type and value, they are correctly orientated and appear to be well soldered with no bridging.

I have checked the voltages at various points with the sequencer running they are as follows.

C62 the charge on the positive lead is 10v
R117 the voltage on one side is 5.3v and the other side is 0.6v
IC12 the voltages on the pins are:
Pin 1 - 3.4v dropping to 0v
Pin 2 - 0v
Pin 3 - 1.4v
Pin 4 - 0v
Pin 5 - 0v
Pin 6 - 11.7 dropping to 0v
Pin 7 - 0v
Pin 8 - 4.6v dropping to 2v
Pin 9 - 4.6v dropping to 2v
Pin 10 - 4.6v dropping to 2v
Pin 11 - 4.6v dropping to 2v
Pin 12 - 4.1v dropping to 0v
Pin 13 - 0v
Pin 14 - 11.9v
VR6 viewing from above with the pins facing you and turned fully counter-clockwise, the voltages on the pins are:
Pin 1 - 0v
Pin 2 - 0v
Pin 3 - 1.5v
Pin 4 - 1.5v
Pin 5 - 1.5v
Pin 6 - 0v
Turned fully clockwise, the voltages on the pins are:
Pin 1 - 0v
Pin 2 - 0v
Pin 3 - 9v
Pin 4 - 1.3v
Pin 5 - 1.3v
Pin 6 - 0v

As stated these measurements were taken with the sequencer running. Thanks in advance for any assistance.
MartinDidymus
 
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Re: Almost no env. decay control, no scope; what to check?

by guest on Sun Sep 26, 2010 8:24 pm

turn env mod up to full
and listen to the filter sweep
is there any sweep happening
does the sweep time change at all with the env mod

the pot voltages seem fine
guest
 
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Re: Almost no env. decay control, no scope; what to check?

by MartinDidymus on Mon Sep 27, 2010 11:10 am

Hi Guest,

Many thanks for taking the time to help me out. They say a picture is worth a thousand words so here are 3kโ€™s worth of images plus a recording for good measure.

Image

This shows Sawtooth with Cut Off 100%, Resonance 0%, Envelope Modulation 0%, Decay 0%.

Image

This shows Sawtooth with Cut Off 100%, Resonance 0%, Envelope Modulation 100%, Decay 0%.

Image

This shows Sawtooth with Cut Off 100%, Resonance 0%, Envelope Modulation 100%, Decay 0%.

x0xb0x test.mp3

This is the mp3 you will hear Sawtooth with Cut Off 100%, Resonance 0%, Envelope Modulation 0% and Decay 0%, then Envelope Modulation is increased to 100% and then Decay is increased to 100%.

As a side note TM3 is turned fully clockwise, Accent was set on 50% throughout but none was applied.
MartinDidymus
 
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Re: Almost no env. decay control, no scope; what to check?

by guest on Mon Sep 27, 2010 12:49 pm

i cant view the link
it says permission denied

can you run the test with cutoff at 50%
with the cutoff at 100%
the added signal from the envelope wont make much difference
guest
 
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Re: Almost no env. decay control, no scope; what to check?

by MartinDidymus on Mon Sep 27, 2010 1:51 pm

Access permissions sorted, sorry about that.

x0xb0x test2.mp3

As requested, with Cut Off frequency at 50 %, as before Envelope Modulation 0% to 100% and then Decay increased to 100% as well.
Last edited by MartinDidymus on Mon Sep 27, 2010 2:09 pm, edited 1 time in total.
MartinDidymus
 
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Re: Almost no env. decay control, no scope; what to check?

by guest on Mon Sep 27, 2010 2:04 pm

that sounds fine

the envelope is only supposed to effect the filter
and not the amplitude
guest
 
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Re: Almost no env. decay control, no scope; what to check?

by MartinDidymus on Mon Sep 27, 2010 2:11 pm

OK then, thanks for the help Guest and I guess I built the rascal fine then.

Off to the doctors to get my ears checked :oops:

Thanks again :D
MartinDidymus
 
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Re: Almost no env. decay control, no scope; what to check?

by guest on Mon Sep 27, 2010 3:00 pm

if it still doesnt sound right to you
feel free to elaborate
guest
 
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Re: Almost no env. decay control, no scope; what to check?

by MartinDidymus on Tue Sep 28, 2010 5:23 pm

guest wrote:if it still doesnt sound right to you
feel free to elaborate


I have had another listen and play around, and have rechecked the symptoms described in the various FAQ posts and have come to the conclusion my problem is probably Filter Warble. I will try some of the fixes suggested in that section starting with swapping IC12.

I shall post any further questions that may arise in that section.

Seriously Guest your help is very much appreciated.
MartinDidymus
 
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Please be positive and constructive with your questions and comments.