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Rampage DIY build problem - fall-time on channel A very long
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Rampage DIY build problem - fall-time on channel A very long
TL072
Hi all,

I have been struggling to troubleshoot a DIY rampage for the last couple of months. It has led to much learning about analogue circuits but I am still not able to fix the problem. I re-flowed a lot of the circuit, switched the op-amps and I think checked most of the connections (there are lot).

The problem: channel A has a extremely long fall time. No matter what I do, I cannot seem to get it to be lower. There is some variation adjusting the controls but on the shortest time setting it is still several seconds long.

From reading about slope generators (kassutronics discussion of slope generators), the issue seems to be that the "variable resistor" created using 4 transistors (matched) is getting the incorrect input voltages (if I understand correctly: way too low). When I look at the link above, I can understand the circuit but when I look at Befaco's schematic (so nice they provide these btw - link) it is slightly different and I really struggle to understand what is going on, even after re-drawing the circuit several times. It seems to be similar to the original serge style one but not the same. Can anyone break it down for me? Even just a strong understanding of this may assist me to debug what is going on. Here is the picture:



The nice thing about having one working channel is that I can measure the one more or less working circuit (there is something that I have not yet got to where the rise sticks a little... but that is for another time) and compare it with the non-working one. I have noticed that the output of the following op-amp is consistently -12v. The fall time is a combination of 3 signals. It would make sense if this -12v was pulling the rise time improperly long.



If I understand the schematic correctly, this means that the voltage at pin 13 is fairly positive which seems wrong. I do not know how to measure that point because it is tapped off a virtual ground on IC5D.

The difficulty with this circuit is the whole thing is one big feedback network so trying to figure out what is causing what to occur is far from easy. If anyone can provide any suggestions I will be very grateful and even if someone can just explain what is happening in that rats-nest of matched transistors, I might be able to more easily figure out where the problem is... Many thanks!
jakobprogsch
One way to understand the Befaco circuit is to think of it as two exponential converters:


On the left are the usual expo converters as found in VCOs either as a current sink (NPN) or current source (PNP). Conveniently you can combine them as seen on the right. Whenever V is positive only the NPN part is active and the PNP are reverse biased and therefore don't contribute to the current. When V is negative the opposite is the case. The current linearly depends on V and exponentially on the CV voltages. You can use the grounded transistor bases as additional CV inputs with reverse polarity. You could also use this as a very primitive exponential VCA but it will suffer from some serious crossover distortion.

As far as the CV summing networks are concerned that is hard to guess. I would check that all the resistors have proper contact and are the expected value. The opamp being glued to a rail might suggest that the feedback resistor R21 is not properly connected?

Edit: Better picture to correspond to the actual Befaco circuit
TL072
Many thanks jakob!

For some reason I did not get an email notice on your response so just seeing it now.

Your explanation is going to require some digesting for someone who is still learning but how you have broken the circuit apart in an image is immediately helpful and I think I more or less can see what is going on!

I have read about temperature compensated exponential converters in eurorack in the past (found the in depth article again if you are interested) but I guess what little I understood from that article did not lead me to see the underlying structure in the Befaco circuit. I also think that how you have broken apart the circuit may help me in the future with other tangles of components - so many thanks for that insight as well.

As for the R21, I will give it another check tonight. I have to believe I did re-flow / check it but that has been famous last words when troubleshooting other modules - so also thank you.

Edit: with your response to me you hit 100 posts. Congratulations!

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