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 Forum index » DIY Hardware and Software » The layout factory
Cloning the CR78 with MIDI
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jbeuckm



Joined: Nov 30, 2008
Posts: 165
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PostPosted: Tue Jun 21, 2016 5:56 am    Post subject: Reply with quote  Mark this post and the followings unread

I agree about that 10k spoiling the squares' output. Why do you think they have R581 in the schemo?


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Grumble



Joined: Nov 23, 2015
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PostPosted: Tue Jun 21, 2016 10:38 am    Post subject: Reply with quote  Mark this post and the followings unread

I think I get it: In the original diagram the power supply is +15 volt.
When you use 15 volt the voltage at the base of Q510 is:
(+15volt - 0.7 volt) * ( 270k / (4M7 + 100k + 270k)) = 0.76 volt which is usually enough to set the transistor to ON (but not into saturation).
powering from +12 volt however:
(+12 volt - 0.7) * (270k / (4M7 +100k +270k)) = 0.6 volt and that might be just not enough.
And the three oscillators will add a pulse of:
5 volt * (10k / ((470k / 3) + 10k)) = 0.3 volt If they are all three high at the same time! And about 3/4 of this voltage (0.2volt) is added to the base voltage, so in case of 15 volt power supply the base voltage is around 1 volt, but in case of the use of 12 volt as power supply, the base voltage of Q510 is about 0.8 volt max.
In terms of base voltages that is a considerable difference.
The transistor in the original diagram is a 2SC900-F but I couldn't find a datasheet, I did however found a datasheet of the NTE199 which supposed to be a replacement for this transistor.
And looking at the specs (especially the VBE-on) it tells me that it should be 09 volt to be sure the transistor is ON. That means that powering this diagram from 12 volt just might not do the trick.
In that case the resistor R569 should be a little smaller (try 3M9)
When using 3M9:
(+12 volt - 0.7) * (270k / (3M9 +100k +270k)) = 0.71 volt


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jbeuckm



Joined: Nov 30, 2008
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PostPosted: Tue Jun 21, 2016 11:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Very nice work! I will try a smaller resistor tonight. Maybe I have to move to 15V as you have said before. I have 2n3904 installed which looks like this:


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wackelpeter



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PostPosted: Wed Jun 22, 2016 11:38 am    Post subject: Reply with quote  Mark this post and the followings unread

jbeuckm wrote:
@wackelpeter If you have a chance, could you tell me what the signal looks like at R566? I built the Metale Beat circuit but I get no output with the normal 12V signal pulsing to 0V for 2ms.


sorry for the late reply. Totally forgot about this...

Well on the side where the "trigger" appears it's sharp spikes going from 0V down to nearly -15V... Afterwards R566 its those spikes of roughly -2,5V or so... The signal there has some little positive offset, due to the other resistor at the Junction with the Transistor... Altough that's a 820K one...

hope that helps a bit...

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jbeuckm



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PostPosted: Wed Jun 22, 2016 1:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

What would be a good way to generate that -2v5 from my TTL positive gate? Maybe an opamp with -.5 gain? Any way to do it with a transitor?

Also, did you notice the period of the gate? I have tried 2-10ms on the other voices.
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Grumble



Joined: Nov 23, 2015
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PostPosted: Wed Jun 22, 2016 11:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

It is NOT possible that there are negative voltages in this part of the circuit, if there are, there must be something terribly wrong or wackelpeter is measuring wrong: there are only positive voltages! Also the trigger input should be positive!
When R667 is positive (5volt should be enough) then Q518 is ON, shorting the intersection of R566 and R567 to ground, so the Base of Q526 will be at ground potential causing Q526 to be OFF, thus C540 is charged thru D571 and R568. (it is no use placing a scope probe or multimeter here since the discharge resistor R569 is 4M7).
If R667 is at ground potential, Q518 will be OFF, and Q526 will be ON, there will be no more charge current and the capacitor C540 will discharge thru R569 etc.
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Grumble



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PostPosted: Thu Jun 23, 2016 12:11 am    Post subject: Reply with quote  Mark this post and the followings unread

Maybe a hint: Remove C541 (disabling the output of three oscillators) and remove L6, now check collector of Q510.
You should see a pulse with a leading edge of about 1 msec and a trailing edge up to about .5 sec
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wackelpeter



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PostPosted: Thu Jun 23, 2016 7:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Well that was the signal going to pin 22 of the voiceboard and comes from the logic board at pin17 (see at the right side it's a bit left between S103 and 104 of pg 17 of the SM)... Looking for that point in the schematic of the logic board i can find this pin 17 together with pin 16 and 18 located at the right side of pg. 22... but i have no idea if these where the same pins... seems rather like a switch...

I'm curious if i haven't measured it wrong... -15V spikes (okay maybe they where a bit less but i would say at least more than -12V) going through a 470k resistor, then the signal should be much smaller in amplitude... adding a 820K resistor going to +15V to this then you have the small negative spikes and the whole signal with some positive offset... makes sense to me or is it that i'm totally wrong there?

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wackelpeter



Joined: May 05, 2013
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PostPosted: Thu Jun 23, 2016 8:10 am    Post subject: Reply with quote  Mark this post and the followings unread

jbeuckm wrote:
What would be a good way to generate that -2v5 from my TTL positive gate? Maybe an opamp with -.5 gain? Any way to do it with a transitor?

Also, did you notice the period of the gate? I have tried 2-10ms on the other voices.


You can generate the -15 or -12V spikes and then you can go with the normal circuit i think...

The Roland TR77 is most voices except guiro triggered by -12 or -15V spikes... and there's also someone who midified his TR77... try a search at google and you should find the schematic he used to get the midi signals into that negative spikes... then you should be able to go on with these as in the normal circuit... maybe a few little changes in resistor values...

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heriks



Joined: Jan 14, 2018
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PostPosted: Fri Mar 23, 2018 9:29 am    Post subject: Reply with quote  Mark this post and the followings unread

I got something very similar to the posted Metallic Beat circuit to work, by lowering the value of R566. Someone in the thread mentioned that the trigger pulse was -15V, I think that's the problem; the base of Q526 never gets low enough for it to "shut off", and generate a puls on the collector.

I use a positive trigger on the input, with a circuit similar to the posted one, and I ended up using 1k for R566. Maybe no need to go that low, you can experiment yourself, but for me, lowering R566 was key in getting it to work.
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