ADSR V2 Sustain Always High

[Sebo]

Hi:
I recently build my second ADSR V2, the first worked OK, but this one have some trouble. Attack and Release works OK, but sustain is always high no matter the position of the pot.
I measured the gate after Q2 and is OK, but the signal after Q3 is not right, if I understood right it should be a pulse/trigger, but I'm meassuring a constant high signal with some sporadic very narrow pulses going to 0V.
I changed Q3, C4 and R10 with no luck, looked for shorts or breaks, but everything seems OK.
I tought to try a smaller cap (1nF), but I don't know why isn't working as it should.

[yusynth]

The behaviour you describe is normal, the signal at the collector of Q3 must be high with no gate and briefly go to 0V at the beginning of the GATE. The 555 is triggered by a falling edge.
Try and change the 555 that may be defective.

[Sebo]

I've already changed the 7555 (2 times) and didn't help.
I found another suspicious behaviour:
The output is very low about 3V (I'm not sure but at the begining of my test I think it was 10V, but I'm not sure).
The voltage between R12 y P4 is changing with the gate, also the Voltage at the other side of R12, that is conected to +15 is changing a little, I can't found any shorts or anything that justifies that.

[Sebo]

Hi Yves:
I found the problem, it was a short in the TL074 (in the LED circuit). It was har to find because was a short inside the socket.
Thanks a lot for the help.

[blue hell]

I love these detective stories

(Especially when they have a happy ending)

[yusynth]

Short in the socket, gee that was not an easy one to find Sebo !

[Sebo]

Yeah, and it was my fault, a little piece of copper from a multicore wire fall inside the socket, and when I inserted the chip gets trapped inside, I saw it by chance...

[Rolbista]

Hi, I see this thread is now 2 years old, but I seem to have similar problem - attack and release work fine but decay and sustain are always at max no matter the settings (actually I don't know about the decay part because I can't hear it with sustain always up. Anyway, ADSR seems to work ok when I touch pin 5 on the 555. Also, someone on the Muff Wiggler also encountered this type of fault, here are the findings: (i hope he doesn't mind)

"That's what I thought at first. I have it in a socket. But I pushed it and pulled it and scooched it around to get it to seat properly in the socket and the pressure and the movement didn't work. It only worked when I actually touched pin 5. Orrr touched both of the leads from the cap. But it didn't work if I just shorted pin 5 to ground. I reworked the joints for the socket and for the cap and I still got the same thing. I changed the value of the cap and it was still the same.

It was something about my resistance to ground was the only thing I could figger...

I clipped a resistor to the leads from the cap and Voila! It worked! I tried a few different values and soldered it on and it works great hmmm.....

I'm not sure why. Maybe has something to do with a long power lead and the cap discharging... That's just a guess hihi It's the only thing I can think of It's peanut butter jelly time! "

Any ideas how to fix it? I mean there is no additional resistor at this point in the circuit.

*EDIT* Well, I soldered a 470K resistor in parallel to the cap beetween pin 5 of the 555 and ground and now everything works like it should, although I have a feeling that this resistor affected the Decay's pot action a little bit

[smartattack]

I also bumped into this in one of mine - for me the cause was a faulty solder joint on one end of the jumper that connects pin 7 of the 555 to the sustain pot. Once I fixed that I had sustain and decay as expected.