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External triggers fo analogue drum machine
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tonewill



Joined: Aug 21, 2009
Posts: 128
Location: England

PostPosted: Tue May 28, 2013 7:11 am    Post subject: External triggers fo analogue drum machine Reply with quote  Mark this post and the followings unread

Hello all,
I have an old Hammond auto-vari from the 1980s, not the original with separate boards for patterns/sounds etc, but with everything on one board.

Using my scope I've found out where the trigger for each sound emanates from the big 450 chip. The triggers are negative going.

I just need a little help on how to connect an external trigger here as I'm far from an electronics expert. I'm sure you can't just connect the external trigger directly to the trigger out from the IC, I think I'll need a resistor or maybe a diode to stop the external signal going to the IC and maybe the IC signal going to the external trigger. Any help or hint appreciated here.

Many thanks.
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johncronan



Joined: Feb 24, 2013
Posts: 42
Location: Chicago

PostPosted: Wed May 29, 2013 9:11 am    Post subject: Reply with quote  Mark this post and the followings unread

It's hard to know for sure without a schematic, but the only surefire way would be to cut the trace and take over that signal line entirely. Using both trigger sources at the same time likely involves cutting the trace anyway, although I guess it's possible you could rig up a diode OR gate without doing so. -John
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tonewill



Joined: Aug 21, 2009
Posts: 128
Location: England

PostPosted: Wed May 29, 2013 11:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks John, I have a schematic coming so I'm hoping that will help. I don't want to remove the internal triggers though.

I found a site where a guy fitted MIDI to a Roland CR-68 using a PIC. He seems to have just used 47K resistors between his trigger outs and the trigger ins on the drum machine.

Another guy on youtube added MIDI to a CR-78, he replaced some resistors with pairs of resistors making up the original value and put his triggers in between them.
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johncronan



Joined: Feb 24, 2013
Posts: 42
Location: Chicago

PostPosted: Wed May 29, 2013 12:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

tonewill wrote:

Another guy on youtube added MIDI to a CR-78, he replaced some resistors with pairs of resistors making up the original value and put his triggers in between them.

That sounds workable as a hack, although I think if it was me I'd try to work out how to get a diode OR gate to work with the negative voltage. Since it is more "proper" and still doesn't really cost anything. My approach would be to first produce a trigger signal that is able to take over for the original circuit, then once that is working figure out how to mix them. My two cents! Wink
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tonewill



Joined: Aug 21, 2009
Posts: 128
Location: England

PostPosted: Fri May 31, 2013 2:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
...try to work out how to get a diode OR gate to work with the negative voltage

Is there an obvious or standard way to do this? Shame the pulses aren't positive as using a couple of diodes on the input signals seems the most logical to me too, but I know very little other than what I've been reading on the subject recently.

Thanks for any help.
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johncronan



Joined: Feb 24, 2013
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Location: Chicago

PostPosted: Fri May 31, 2013 8:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, let's see... the standard diode OR gate has two diodes connected to a resistor to ground. We want the diodes to be forward biased when the signal goes negative, so you would just reverse the direction of the diodes. Each of the two inputs is connected to a cathode and the anodes connect to the resistor. Maybe start with 1k for the resistor (although if that is too low you will see the trigger signal attenuated from the load; make it higher).
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elmegil



Joined: Mar 20, 2012
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PostPosted: Fri May 31, 2013 11:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Does this help any?

http://www.musicfromouterspace.com/analogsynth_new/ELECTRONICS/mickeymouselogic.html
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tonewill



Joined: Aug 21, 2009
Posts: 128
Location: England

PostPosted: Sun Jun 02, 2013 11:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks chaps, I appreciate that. I watched some youtube videos by 00retrobrad00 that explained diodes for beginners like me, and I see now that reversing them makes sense to let the negative going pulse through.

Interesting link too, thanks a lot, going to read it right after posting this.

I found another guy who connected his trigger pulses directly to the sound board of a rhythm ace drum machine without adding diodes to the original trigger inputs, but maybe there are already diodes in the circuit of that machine so he didn't need to add his own. When the schematics arrive for mine I'll see what's what.

Thanks again.
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tonewill



Joined: Aug 21, 2009
Posts: 128
Location: England

PostPosted: Mon Jun 03, 2013 6:22 am    Post subject: Reply with quote  Mark this post and the followings unread

I've got the circuit diagram and service notes now.

There are two ICs that generate the patterns and send out the pulses to the sounds. One is for the main patterns and the other is for the intro/fill patterns. The ICs are connected directly for each sound pulse, or as the manual says:
Quote:
The voice output of the 460 (IC1) and 462 (IC2) are internally gated so that their outputs can be tied directly together...

Logically, doesn't this mean that if I create the same short pulses I can connect directly to one of the ICs without the need for cutting tracks and adding diodes?

Many thanks.
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johncronan



Joined: Feb 24, 2013
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Location: Chicago

PostPosted: Mon Jun 03, 2013 7:38 am    Post subject: Reply with quote  Mark this post and the followings unread

tonewill wrote:

Quote:
The voice output of the 460 (IC1) and 462 (IC2) are internally gated so that their outputs can be tied directly together...


It sounds something like open collector. Anything on that connection can pull the output low to indicate a trigger.
Quote:
Logically, doesn't this mean that if I create the same short pulses I can connect directly to one of the ICs without the need for cutting tracks and adding diodes?

I think so, but it also means that the circuitry you add needs to work like an s-trigger, not a v-trig. Connect it to your switch (transistor)... the other end of the switch goes to V- through 10k or something like that. Maybe you can see on the schematic. If this point is grounded through, say, 100k then you would want no more than around 10% of that or 10k. Just as an example. Or it's grounded with 10k and so you choose 1k. You momentary connect it to the negative rail at an impedance low enough that it overcomes the impedance the line is grounded with.

Hope that helps!
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tonewill



Joined: Aug 21, 2009
Posts: 128
Location: England

PostPosted: Mon Jun 03, 2013 8:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Hope that helps!

Thanks, yes, it helps me realise that I don't know enough about it. Essentially I have a MIDI to trigger kit that converts note-on to +5V and note off to 0V. I was hoping to convert the range to that required perhaps by modifying Ken Stone's gate-to-trigger circuit or something similar. I'll have to go and read some more I think.

Many thanks.
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tonewill



Joined: Aug 21, 2009
Posts: 128
Location: England

PostPosted: Mon Jun 03, 2013 11:13 am    Post subject: Reply with quote  Mark this post and the followings unread

I've attached a scan of the relevant part of the schematic. It shows where the trigger pulses come out of IC1 and IC2 and where some pulses go through an inverter and some through NAND gates.

The sounds going through the NAND gates can also be triggered externally by the bass pedals and lower manual in the case where this device is built-in to an organ.

Would it be a better idea to send inverted triggers and connect them just after the inverters and gates and before the following diode?

Many thanks again.

IC-11 and IC-12 are 14011
IC-13 is 4069


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diablojoy



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PostPosted: Tue Jun 04, 2013 3:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
Would it be a better idea to send inverted triggers and connect them just after the inverters and gates and before the following diode?


yes I would try breadboarding that
If you tap off each of the invertor and NAND gate outputs and put each of them through another invertor section say a CD40106 this will give you positive triggers that you can use , I think this is what you want ? i would then buffer each of those new outputs with an opamp

So 2 x CD40106 and 3 x TL074 plus some associated resistors and output jacks. not much more needed i think.

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tonewill



Joined: Aug 21, 2009
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PostPosted: Tue Jun 04, 2013 4:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks diablojoy,
Actually, it was getting a bit beyond my knowledge and as the drum machine wasn't expensive and doesn't sound remarkable anyway, I decided to just get a ribbon cable and carefully solder directly to the pads under the IC that creates the internal triggers (removing the IC first). As mentioned, there are two ICs that are used to create pulses that are directly connected without each other's pulses causing a problem to the other IC so I don't see why my pulses should cause a problem either if they are essentially the same.

I'm now able to create manual triggers that work perfectly using a microswitch, a 100n capacitor and a 100k pull-up resistor to create a short pulse at 0V. I haven't been able to successfully replace the microswitch with a MIDI controlled "5V when on" signal, but I'll still look into it just out of interest. I posted another thread about that side of things.

Thanks again.


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diablojoy



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PostPosted: Thu Jun 06, 2013 2:08 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
I haven't been able to successfully replace the microswitch with a MIDI controlled "5V when on" signal, but I'll still look into it just out of interest.

Ahh my mistake did not realise you were going the other way
perhaps try breadboarding this attachment i think it should work
R2 -R3 adjust voltage level into the 40106 invertor and may not be strictly neccesary, but those values will keep it above the cmos trigger level and below the rail voltage.
you may need to play with R1 value , it determines how hard the transistor turns on.
The Led and its associated resistor is optional and will depend greatly on the trigger length as to whether it will visibly light up or not.
the diode is neccesary i think, cmos outputs will sink current if connected directly together
so you should maybe connect after each of the existing diodes on your board to avoid any fuss and also avoid having 2 diode voltage drops in line.


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tonewill



Joined: Aug 21, 2009
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PostPosted: Thu Jun 06, 2013 1:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks a lot diablojoy, I appreciate that very much.

There isn't a +15V supply in this machine, only -14V so that won't work for me unfortunately. I don't want to create a +15V supply in there as it exceeds the threshold of being worth doing, if you see what I mean!

I also don't want to connect the trigger points to any other place than they are now either for the same reason. It works fine manually as described before, so it's just a case of whether there is a relatively simple solution to be able to do what the switch does with a 5V trigger instead, with the voltage available. Sorry, hope you didn't spend too much time on the diagram Embarassed.
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diablojoy



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PostPosted: Thu Jun 06, 2013 3:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

no problem at all Smile I think i spent all of 10 minutes on it.
I actually needed to do that scheme for myself anyway for a highly liquid MD24. So I had some thoughts on it already, will be trying it on a breadboard soon .
With only a negative supply rail available it may well be in the too hard basket to convert from +5v triggers
beyond my limited ability anyway.

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