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Korg Monotron Euro Module DIY
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page Previous  1, 2, 3, 4, 5, 6  Next [all]
Author Korg Monotron Euro Module DIY

ringroad1

If anyone's looking for this, it's here:

http://translate.google.co.uk/translate?hl=en&sl=ja&tl=en&u=http%3A%2F %2Fhoushu.at.webry.info%2F201006%2Farticle_3.html


Scott Willingham

There's also this version, which seems less "fiddly" to me:

http://translate.google.com/translate?hl=en&sl=ja&u=http://beatnic.jp/ monotron/monotron%2B.html


Scott Willingham

In my first post to this thread, I said:
Quote:
Also, while the OP462 has a very low offset voltage, its input bias currents are much higher than the LM324. This, combined with the large feedback resistor, produces about 80-100 mV offset at the filter output. This slightly unbalances the diode clipping in high-res situations, but I don't think this is too important.


This statement turns out to be wrong, and the real situation is more interesting!

After playing with my modified Monotron a bit more, I became dissatisfied with the result. It seemed that the filter resonance (peak) was only mildly effective until it all-of-a-sudden went into self oscillation. I then looked at the output waveform and it confirmed this; very little ringing on the sawtooth edge. Finally, when the peak knob is cranked, the oscillation startes in the middle of the wave.



So I started to experiment with processing an external wave, rather than the VCO sawtooth. In this case, it looks "right" but notice the sawtooth is inverted from the VCO case.



And putting in a square wave really illustrates the problem. The negative-going steps ring but the positive-going steps are well damped.



This got me thinking about asymmetry in the circuit and I realized that the offet current from the OP462 was the cause of the problem. Originally, I assumed the diodes really don't affect much until self-oscillation where they hard-clip to limit the amplitude. In that case, 80 mV out of 500-600 mV didn't seem like a big deal.

But the problem is more subtle. When increasing the filter resonance, the filter ringing is very sensitive to the gain of the feedback loop. This gain is reduced whenever the "clipping" diodes current is significant relative to the current in 330 kohm resistor R60. At a filter output swing of less than 100mV, the current swing in R60 is less than 300 nA! The diodes do conduct significant currents of that order and, more obviously, an offset current from the OP462 of 250 nA is a serious asymmetry. This explains why the square wave rings on one edge (diode current near zero) and is damped on the other edge (diode cunduction lowers the feedback gain).

There are a couple of ways to fix this. One is to add a trimpot and resistor network at IC4C, pin 9 to balance the offset current (trim until the DC voltage between pins 8 and 9 equals zero). This should work ok, but requires trimming. I haven't tried it. Instead, I took the approach of AC-coupling the diode clipping circuit. I cut the diode connection to pin 9 of IC4C and inserted a series 2.2 uF capacitor. Then I added a 3.3 Mohm resistor in parallel with the diodes (biggest SMD resistor I had handy).

With this change, the filter now works properly on the VCO sawtooth and the peak knob gives its full range of effect. The square wave asymmetry is fixed.



A further comment about the filter noise:

At Steve Woodward's blog
(http://stevewoodward.posterous.com/korg-monotron-hacking he reports that in a direct comparison, the MS-20 filter is indeed quieter. One possible reason for this is op amp related. The uA741 used in the original circuit has an input referred noise of about 20 nV/root-Hz. This is 6 dB better than the low-voltage LM324 used in the Monotron. It is also possible that the MS-20 VCO amplitude is a bit hotter (the filter input and output gains are pretty similar). Actually, another possibility is that the MS-20 in question has the newer OTA-style filter. It is probable that excessive noise in the original filter is one of the reasons Korg developed the newer filter.

And headphone output levels:

The output levels of the Monotron are a bit weak for running into other equipment. I changed R79 in the output amplifier from 68k to 33k and this works well to add 6 dB gain, while still keeping the volume range reasonable for earbuds. Also, a further reduction could reduce bass response unless capacitors are also changed.

-Scott-


ringroad1

Scott Willingham wrote:
Actually, another possibility is that the MS-20 in question has the newer OTA-style filter.


Hello Scott, this is all fascinating stuff. My MS-20 has the screw in the faceplate, so it's more than likely it's the newer version of the filter, though as I understand it there's no way to tell absolutely without opening it up.

If I get a chance over the next few days I'll record an example of the same sequence and upload it for comparison.

Also, thanks for pointing out Beatnic's page on the LFO as EG mod, I'd missed that.


Tim Stinchcombe

Scott Willingham wrote:
But the problem is more subtle. When increasing the filter resonance, the filter ringing is very sensitive to the gain of the feedback loop. This gain is reduced whenever the "clipping" diodes current is significant relative to the current in 330 kohm resistor R60. At a filter output swing of less than 100mV, the current swing in R60 is less than 300 nA! The diodes do conduct significant currents of that order and, more obviously, an offset current from the OP462 of 250 nA is a serious asymmetry. This explains why the square wave rings on one edge (diode current near zero) and is damped on the other edge (diode cunduction lowers the feedback gain).
Very interesting stuff, and I like your neat solution! But I'm still not clear on your bias vs. offset distinction, mainly as I can't tie-up the numbers you are mentioning with the datasheet, which gives input bias current max at 600nA, and input offset current max at 25nA (or do you simply have a typo...?)

Quote:
And headphone output levels:

The output levels of the Monotron are a bit weak for running into other equipment. I changed R79 in the output amplifier from 68k to 33k and this works well to add 6 dB gain, while still keeping the volume range reasonable for earbuds. Also, a further reduction could reduce bass response unless capacitors are also changed.
I had an idle thought about simply swapping to use the loudspeaker channel of the output amp instead, which will give an extra 12dB (x4) improvement, but I haven't spent any time analysing what the effect of the two caps at the input would do - I was kind of thinking about shorting one or other out, but really need to look at some frequency response curves first.

What time I have spent on this over the last week or so has been on trying to understand how the pole(s) of the op amp interact with the poles and zeroes of the filter, and how R74/C22 'helps', but the algebra approaches the damn near intractable at times. Finally I think I have an approximation that'll give me what I seek, which is specifically the frequency-dependence of the resonance, and then what happens when using a higher-bandwidth op amp.

Oh, and having fished out a pair of headphones, I most definitely now hear the noise, and understand that as a problem!

Tim


Scott Willingham

Tim Stinchcombe wrote:
But I'm still not clear on your bias vs. offset distinction, mainly as I can't tie-up the numbers you are mentioning with the datasheet, which gives input bias current max at 600nA, and input offset current max at 25nA (or do you simply have a typo...?)

You're right Tim; my numbers and terms are a bit off. The bias current flows through the feedback resistor, creating an offset voltage in the output swing. In my Monotron, I measured the voltage across R60 as 85 mV and this, divided by 330 kohm, is about 260 nA. Looking back at the datasheet, the typical bias current is 360 nA. I think I must have misread that as 260 after my measurement and calculation. Now that I think a little more deeply about it, I'll bet that in my measurement of 85 mV across R60, that some of the bias current is also flowing in one of the diodes. That would make up the discrepancy in my rough calculation.

Quote:
I had an idle thought about simply swapping to use the loudspeaker channel of the output amp instead, which will give an extra 12dB (x4) improvement, but I haven't spent any time analysing what the effect of the two caps at the input would do - I was kind of thinking about shorting one or other out, but really need to look at some frequency response curves first.

Your caution is correct; the signal path to the speaker has a restricted bandwidth, about 230 Hz to 5.9 kHz. I assume the designers tailored this for the speaker's characteristics. It would be a waste of power to try to get too much bass response. The treble rolloff may counteract some other harshness.

The headphone path is much more wideband. The mod I suggested (reducing the value of R79) is also very minimal -- you can just tack a resistor in parallel to R79. Pretty easy to do and to undo.

Quote:
What time I have spent on this over the last week or so has been on trying to understand how the pole(s) of the op amp interact with the poles and zeroes of the filter, and how R74/C22 'helps', but the algebra approaches the damn near intractable at times. Finally I think I have an approximation that'll give me what I seek, which is specifically the frequency-dependence of the resonance, and then what happens when using a higher-bandwidth op amp.

Sounds like an interesting challenge. I look forward to reading more about it.

-Scott-


ethrandil

Hi all,

my current project involves the monotron and the Doepfer DIY synth. I want to make a semi modular synth using both to get:

two oscillators (doepfer + korg)
two filters (doepfer multimode + korg lp)
two lfos (...)
one envelope

I can handle all the signal-passing but am a bit confused about power supply and ground routing...
I will have a +/- 12V and a +/-5 V power supply for the doepfer module. I guess I can run the monotron on the +5V line. But the monotron is biased around 1.35V as you said, so how exactly can I route monotron signals to the doepfer module?

Or is this a very very difficult task?

Regards,
- Eth


krellmusician

Just wondering. Is there anything in the nature of the components used in the Monotron that LIMITS the use of voltages greater than 5V and the generation of a 1.35 V bias? In other words, if one applied, say, a 9V battery for power, would anything be fried or otherwise permanently disabled? (I realize that this, if otherwise physically possible, would probably require adjustments -- maybe even major adjustments -- to various portions of the circuit to enable them to operate properly.) Assuming a 9V supply and a 4.5V bias "ground", wouldn't this give the circuit a bit more headroom?

Sorry if this line of questioning seems foolish, and feel free to ignore it if not worth the response.

Thanks for your time.


Scott Willingham

krellmusician wrote:
Just wondering. Is there anything in the nature of the components used in the Monotron that LIMITS the use of voltages greater than 5V and the generation of a 1.35 V bias?


The SN74AHC14 Schmitt-trigger chips and the TPA6111A2 audio amplifier are limited to 5V operation. As you suspect, there are many adjustments to bias circuits that would be needed for the rest of the circuitry.

In addition, the main limitations for the audio path are the filter. Its principles of operation limit the core signal swings to only a few millivolts. This is the noise bottleneck in the audio chain.

On the other hand, it is very practical to use larger control voltage swings, and there is no need to reference them to vbias. Indeed, the internal CV at the wiper of the Cutoff potentiometer is 0 to 3V. Both the pitch and cutoff CV mods consist of adding a resistor to an op amp "virtual ground" summing node. Only the "outer" ends of those resistors are exposed to any high voltage, so you can't damage the Monotron with reasonable control voltages. The exception here is the gate input; it should stay within 5V, but it would be easy to design around that too.

The pitch and cutoff control voltages can be also relative to the Monotron ground. For a 1V/oct CV input, there is nothing special about "zero" volts. A grounded input for the pitch CV will just cause a VCO pitch below that with an input of 1.35 V.

I hope my response is reasonably clear. Don't hesitate to ask followup questions.

-Scott-


sandyb

folks - this is a cool thread thumbs up
lots of useful stuff.
i think it's definitely veered into the realm of the diy section now so i'm going to move it there. it'll be easier to find in the future should people be looking for the sort of information it contains.

sandy


plasticanimal

A vactrol was the answer to sending the LFO to the AMP BTW. With the EG mod you get a simple decay envelope. Sounds nice.


Scott Willingham

Scott Willingham wrote:
My next project is trying to understand and reduce the filter gating "pops".


Been working on this a while, and finally have some progress to report on.

A problem with the Monotron sound, noted by several, is its often loud click or pop on the trailing end of a sound. This is especially annoying when the filter cutoff is closed down manually or by the LFO. The main source of this click is the Monotron's clever use of the filter circuit itself to gate the amplitude of the sound. When the gate signal goes low, the filter's cutoff is forced so low as to effectively cut-off even DC signals.

While clever, this trick has some problems. First is the click/pop. After some simulation and analysis, I have concluded that this is inherent in the design of the filter, which passes the signal through saturated bipolar transistors. When saturated, bipolar transistors store a lot of charge in their base region. At the point of cutoff, this charge has to redistribute and it mainly goes onto the filter capacitors. In absolute terms, this is a couple of millivolts. But in the Korg filter, the main signal is itself only a few millivolts. The straightforward way to diminish the pop would be to enlarge the capacitors, but this requires wholesale changes to many other components and probably will change the overall filter sound.

Another problem with this scheme is that the gating is somewhat slow at the beginning of notes. With some settings of the filter, the attack is quite soft. I didn't like this, but I suppose it can also be considered a feature.

Finally, this gating scheme comes before the main noise source in the filter, so it does nothing to mute the noise between tones.

So my conclusion is to defeat the original filter gating and add a simple gate later in the signal chain. This proves to be fairly simple, eliminating the filter's pop/click, improving the attack rate, and effectively gating the filter noise between sounds.

Step 1) Remove R51. This defeats the original filter gating.

Step 2) Add a 100k pullup resistor from Q10's collector to VA (conveniently tapped at the non-populated pads of C15). Optionally add 10nF from Q10's collector to ground. This filters power-supply noise, but is probably not crucial. This revised circuit creates a 0.6 to 5V gate signal, which like the original, is forced high whenever a signal is plugged into the Line-In.

Step 3) Gate the audio signal with a MOSFET (type RHK005N03 from Mouser). Korg would probably use a bipolar to reduce cost, but the MOSFET circuit is simpler and cheap enough for my use. This circuit is inserted between the output of the filter and the input to the headphone/speaker amp. The signal level here is more than 100 mV; much less susceptible to small pops. Conveniently, the signal is also AC-coupled at ground potential, so the MOS switch works very well. And following the filter, this point gates the filter noise.

In detail, there is a trace on the back of the circuit board between the top of VR7A (the peak pot) and VR2A (the volume pot). Cut this trace. The MOSFET source goes to the top of VR7A and the drain to the top of VR2A. The MOSFET gate is wired to the collector of Q10. Finally, a 0.1uF capacitor is added from the top of VR2A to ground. This cap minimizes the voltage disturbance from the MOSFETs gate charge.

Here's a sketch:


I'm very pleased with the results -- sharper attacks, no filter pop, and nice noise gating (but I still recommend my previous noise mod). Sometimes there is a very small pop remaining, but this is from the VCO gating circuit. Even most of it is cut off by the new circuit.

Cheers,
Scott


Scott Willingham

For those adding external CV/gate inputs to the Monotron, there is a frustrating detail in that the "gate" test-point on Korg's PCB interacts with the VCO pitch. This can be fixed by removing R11, but this defeats triggering the synth from the Monotron's built-in ribbon.

Another fix was suggested by Japenese modder 'Beatnic' (and possibly Masa). This was to add a bipolar transistor to pull down pin 9 of IC2C. This nicely preserves the internal gate and is a recommended method.

This post is a quick sketch for a third method, which I came up with based on Masa/Beatnic's idea. The nice feature of my method is that it only requires adding one resistor to the Monotron.

Step 1) Locate pin 13 of IC3. Cut the trace between pin 13 and pin 14 (VA). Now we can use this spare Schmitt trigger inverter.

Step 2) Wire pin 13 to your gate input connector. Add a 50k to 100k resistor pulldown on this node to ground.

Step 3) Lift R13's connection to VCC. I.e. remove R13 and resolder just one end at an angle. Or use a separate 100kohm through-hole resistor.

Step 4) Wire IC3F, pin 12 (output of the Schmitt trigger inverter) to the former VCC connection of R13.

That's it. Here's a sketch:


-Scott-


th0mas

Thanks Scott!

Here is a picture of my modded monotron. I did this last week with some parts I had laying around from other projects.



Extra wires are there as I still haven't wired up audio in/out, power, or any other potential mods to come.

One problem I have is the cutoff and pitch both are effected as I bring my hand closer to the synth, quite a bit.. like more than a few semitones. I still have the speaker connected, but the ribbon is disconnected.. not sure what else could be causing this? It's on batteries too still, although as you can see from the image I plan on adding the power supply mentioned on the DIN sync blog!


Scott Willingham

th0mas wrote:
One problem I have is the cutoff and pitch both are effected as I bring my hand closer to the synth, quite a bit.. like more than a few semitones. I still have the speaker connected, but the ribbon is disconnected.. not sure what else could be causing this? It's on batteries too still, although as you can see from the image I plan on adding the power supply mentioned on the DIN sync blog!


My guess is that you are experiencing this pitch shift when no external CV is hooked-up. The popular mods leave the external input "hanging" when no input is applied. With long wires, you might be inducing 50/60 Hertz hum or other ambient RF signal into the CV input. This can be rectified into a DC offset by the op-amp's input circuit. If you short the CV jack tip to its sleeve, do you still get this sensitivity? If not, then there is likely no big problem and the sensitivity will also go away when an actual CV is applied.

A possible improvement to the standard CV/gate mods is to add 50 to 100 kohm resistors to ground at the input jacks. I'd also be inclined to add some RF decoupling caps, maybe 10 to 100 nF. But I'm guessing at this; I haven't tested it myself.

-Scott-


th0mas

Scott Willingham wrote:


My guess is that you are experiencing this pitch shift when no external CV is hooked-up. (trim)...

A possible improvement to the standard CV/gate mods is to add 50 to 100 kohm resistors to ground at the input jacks. I'd also be inclined to add some RF decoupling caps, maybe 10 to 100 nF. But I'm guessing at this; I haven't tested it myself.

-Scott-


Thanks for the reply. Unfortunately the pitch/cutoff shift is happening when I have either one of the pitch or filter CV connected to a MIDI->CV converter and the other one left floating. I'll try shorting the CV that is currently left floating.

Out of interest, why would adding a resistor to ground improve the CV/gate mods?

Also, everyone.. I found a forum link describing taking the pre-headphone amp output to improve the noisy output situation. I haven't tried this yet but will be soon, possibly tonight. It's described here: http://monotron.adornomedia.com/viewtopic.php?f=3&t=19


Scott Willingham

th0mas wrote:
Thanks for the reply. Unfortunately the pitch/cutoff shift is happening when I have either one of the pitch or filter CV connected to a MIDI->CV converter and the other one left floating. I'll try shorting the CV that is currently left floating.


Hmm. Well, I said it was a guess. Are you wiring a ground to all the jacks? Some of the mod descriptions are not explicit about this.

Quote:
Out of interest, why would adding a resistor to ground improve the CV/gate mods?


The resistor provides a defined voltage input (0V) when no signal is connected to the jack. (To clarify my vaguely-worded instructions: I mean a resistor between tip and sleeve -- not a resistor in series with ground!)

Quote:
Also, everyone.. I found a forum link describing taking the pre-headphone amp output to improve the noisy output situation.


The headphone amp has very low noise. IMO, bypassing it is a downgrade. This is very easy to test for yourself -- just turn the volume knob to zero. Now you are listening to the total noise of just the headphone amp. It is vastly quieter than the filter circuit.

-Scott-


th0mas

Thanks again Scott! I was going to wire the resistor in series to ground from the jack sleeves but I get what you're saying now. Right now I have all my jack sleeves connected directly to ground.

Too bad about the headphone output, although I'm not surprised with everyone else adding their own noise gate or other solutions that it wouldn't be this simple.


Oran

Hi all! Long time lurker, first time poster. I wish I had something to contribute for my first post, but I come to you with a problem instead.
(Disclaimer: Though I'm pretty good at soldering and such, I'm only learning how to read schematics.)
I'm currently adding CV inputs to a Monotron for a friend, and in the process I'm attempting the fix the gate/pitch issue. The route I've taken is to remove R11, then replace it with a switch and a resistor. From my research, this seems viable.
My main issue though, is that I've removed R11, but the ribbon is still functional! As I understand it, this should not be the case, but the Monotron seems to behave as normal! I'm extremely confused. :(
Due to this, it's difficult to troubleshoot my switch setup.
With the switch connected, it behaves the same in either position, due to the above issue.

Now, I could very well have done something wrong...
I did break the upper trace to R11, thus I soldered the new resistor attached to the switch to a point that seemed to be going to R11.
As far as I can tell, I haven't bridged anything that shouldn't be.
At the moment, I do not even have my switch connected, just a resistor hanging off of the solder point I used, and a bare wire hanging off of the bottom solder point from R11.

If anyone can help me figure out why the ribbon is still functional I'd greatly appreciate it. I suspect I've done something wrong, or misunderstood something, thereby having not fixed the gate/pitch issue.

Thanks!
- Oran

This is essentially what I'm trying to do, though as I said, I don't have the switch wired up right now:



Scott Willingham

Oran wrote:
My main issue though, is that I've removed R11, but the ribbon is still functional! As I understand it, this should not be the case, but the Monotron seems to behave as normal! I'm extremely confused. :(
Due to this, it's difficult to troubleshoot my switch setup.
With the switch connected, it behaves the same in either position, due to the above issue.


Hi Oran. My best guess is that R11 is removed and there is not yet any other connection to IC2 pin 10 (the gate input/test-point). This LM324 has a PNP input stage and base current could cause this floating node to rise, which turns on the Monotron gate signal. Then playing the ribbon would sort-of sound normal. Can you check if the ribbon is retriggering the LFO each time you press it?

I would suggest tying the new gate node low with a 100kohm resistor, so that it doesn't float when no inptu is connected. This might fix your problem.

-Scott-


Oran

Scott Willingham wrote:

Hi Oran. My best guess is that R11 is removed and there is not yet any other connection to IC2 pin 10 (the gate input/test-point). This LM324 has a PNP input stage and base current could cause this floating node to rise, which turns on the Monotron gate signal. Then playing the ribbon would sort-of sound normal. Can you check if the ribbon is retriggering the LFO each time you press it?

I would suggest tying the new gate node low with a 100kohm resistor, so that it doesn't float when no inptu is connected. This might fix your problem.

-Scott-


Thanks a lot for the reply Scott! Unfortunately, in addition to learning to read schematics, I'm also just learning electronic music terminology. haha But hopefully I'll make sense.

I realize now that in my panic earlier I may not have properly understood what the Monotron was doing. I'll explain what I mean by that...
When I turn it on, it is already making a sound, as opposed to having to be triggered by touching the ribbon. I don't quite understand if this is what it's supposed to be doing....always making a sound. If you could clarify that I'd be grateful.
Now, it does indeed not have an input on the gate point, as I don't have the setup for that. (I'm not a musician, just a modder. hehe) And as you said, the ribbon does only sound kind of normal...since the Monotron is "constantly triggered", the ribbon just seems to be competing with the existing sound when I press it. As far as I can tell, the ribbon is not retriggering the LFO...the oscillation is just continual, and touching the ribbon doesn't seem to affect it...it just adds in a slightly different note, depending on where I touch.

Now my biggest problem seems to be that when I wire up my replacement resistor, it does not return the Monotron to normal operation...it just keeps making noise. I can only assume that one of my connections is bad.

So, my direct questions now are:

1. Is it normal, once R11 is removed, for the Monotron to continually make a sound? (e.g. constantly triggered I suppose)

2. By my above image, was I correct in utilizing the solder point that I did (after damaging the top trace)? (My wiring seems solid, so now I'm perplexed as to why my replacement resistor isn't working, even sans switch)

3. Can you elaborate on your last suggestion? I'm only just getting familiar with using resistors in any way other than in series. Do you mean a 100k resistor coming off the gate point going to ground (in parallel I suppose)? This is a "pulldown", correct?

Thank you so much for your help! ^_^
- Oran


teatanic

hey guys, just had the time to work with the monotron. maybe this could be interesting for some of you.

external powersupply (3V) works fine (soldered a 3,5mm jack parallel to the batteryinputs) -> decide by putting in batteries or the power jack so its still a mobile toy. Vcc can also drive a little binary stepper (i took 4026 q1-4 , q5 to reset, do not take more outputs /w leds -> monotron only supplies 5V, or take a external powersupply for the 4026). stepper clock adjustable by the lfo-pot of the monotron (lfo output downside). The cv-output of the stepper triggers the gate (also downside)of the monotron. -> monotron with little stepsequencer works real nice.

its also no problem to drive the doepfer diy-synth vco cvs with the outputs of the stepper. its still on the experimentalboard, need money to build a case.
more will come the next days. i also made a little clip...if u are interested i can upload it, schematics (haha made a quick pencil draw) as well.



edit:
oran no 2 of your points semm coorect. the point should do it. if u are not 100% sure, just take a little wire solder it to your resistor an try it by holding the wire on the point. alternative: take a multimeter in beep-mode wink if u dont have one, take a led two 1,5V batteries and two wires xD


Scott Willingham

Oran wrote:
1. Is it normal, once R11 is removed, for the Monotron to continually make a sound? (e.g. constantly triggered I suppose)


Probably normal to be constantly triggered if IC2 pin 10 is left floating.

Quote:
2. By my above image, was I correct in utilizing the solder point that I did (after damaging the top trace)? (My wiring seems solid, so now I'm perplexed as to why my replacement resistor isn't working, even sans switch)


Your drawing looks correct to me. When the switch is closed, you should only trigger a sound when the ribbon is pressed. I would debug with an ohm-meter. There should be 1kohm from the ribbon "center-tap" trace to one of your switch terminals. There should be a short circuit from IC2 pin 10 to the other terminal of your switch. Also, check that there is not an accidental short circuit (bridge) from IC2 pin 10 to VCC. Pin 4 is VCC and one side of R11 is nearby, so a little stray solder could be pulling the pin high, even with the switch closed. With the device on, measure the voltage at pin 10.

Quote:
3. Can you elaborate on your last suggestion? I'm only just getting familiar with using resistors in any way other than in series. Do you mean a 100k resistor coming off the gate point going to ground (in parallel I suppose)? This is a "pulldown", correct?


Yes, a resistor from gate to ground will keep the voltage on that wire from floating up to a high voltage when the switch is open and nothing else is connected.

-Scott-


th0mas

Scott Willingham wrote:
Oran wrote:
1. Is it normal, once R11 is removed, for the Monotron to continually make a sound? (e.g. constantly triggered I suppose)


Probably normal to be constantly triggered if IC2 pin 10 is left floating.


I can confirm this is normal. An alternative to tying the pin to ground would be to just leave it - once they plug a CV/gate interface in the sound will stop except when gated.

Tom


Oran

Thank you teatanic, Scott & th0mas, for your help!
I really appreciate it!

I've got it all worked out now, and finished two Monotrons this morning!
It turns out one of my original connections was not solid enough in my resistor replacement circuit, so once I got that worked out the switch worked great.
And thank you, now I understand that pin 10 on IC2 is floating until there is a CV input plugged in. I do think I'll leave it that way, as I think my client will enjoy being able to put it into that mode (without CV input) and easily twiddle two knobs at once. haha Though I do also now understand how I could wire up a pulldown resistor to fix that issue.

On one Monotron I did not cleanly remove R11, thus both pads broke off, so I have it wired like this now (please forgive my crude drawings, haha):


The other one I was more careful with, so I was able to re-use the solder pads for R11.

Again, thank you all for your help! I've been keeping a sharp eye on this thread since just after it started, and while some of it is over my head, if I understood everything I probably wouldn't learn anything! I shall continue to visit frequently.
Soon I'll be getting another Mono to mod, and I think I might like to pursue Scott's third method of fixing the gate/pitch issue, as it seems like a good plan. I'm also of a mind to add CV inputs to a Monotron, sans external control box. I've got some very small components collected, and mapped out where most will go, so that should be a fun project. (So far I'm leaving these on battery power, but I may approach building a separate power supply soon.) Also very interested in many of the other mods that people have been doing...though some are beyond me, either in terms of electronic complexity, or musical usage (i.e. I don't know what certain mods would be good for, as I don't make music).

Thanks!
^_^ Oran

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