I just finished a Systech overdrive clone. It not my first build by far, everything works fine except that, when i let a note ring, it ends quite suddenly, like a "gate" effect with a quite unpleasant distorsion (something like "crrrrshrrr"... ;-)). Not problem when i play non-stop without leting the notes ring too long, but when i let one ring, this effect occurs.
I verified everything many times,changed the AOP and the transistors several times and with several other models, verified the values of all my but i can't find anything :-/
Here's what i found during my tests :
- The "gating" effect occurs more when i turn the "disto" knob up. Under 60% it's quite not audible but the pedal is not as funny :-) Disto all the way up, this distorsion is quite as important as the guitar sound itself.
- I've got less of this unpleasant distorsion if i use a JRC4558D
- "disto" all the way down, i haven't got any more sound is it normal ?
- The pedal works quite the same with the 3904 soldered backwards. Normal ?
- Here are my mesurements :
5457 : D = 8,15V / S = 1,70V / G = almost 0V
3904 : C = 5,48V / B = 1,90V / E = 1,31 V
4558D : 1=4,0V / 2=4,0V / 3=4,0V / 4=0V / 5= 4,0V / 6= 4,0V / 7=4,0V / 8=8,1V
I'm thinking of an imporperly biased transistor or something like that, but my mesurements don't seem strange, no ?
If anybody would have any idea, i'd be very grateful :-)
In fact for the red, it's just a shade on the picture.
The black, indeed the solder joint doesn't look good.
I reflew them both carefully but i doesn't change anything.
In fact i more and more thinking about a bad biasing as, playing it again for a while, it sound pretty good. I just don't have this very long sustain there is at the end of the demo video, for exemple. On my pedal, in end quite fast and brutally.
I reversed the 3904 and indeed it does sound the same way, maybe just a bit less output.
You could also check the solder on the output wire it looks a bit off.
You wall voltage seems a bit low, 8V, could that be the reason ? And I have a little bit less than Vref on IC pin 5
You should not have any sustain issue, it doesn't sound gating at all. Keep looking and good luck
i resoldered the output wire just to be sure, but it was firmly connected, no pb.
Yes, the 8V on AOP pin8 suprises me too.
My wall voltage is exactly 9,00V then it goes through the 1N5817 diode when it's lowered to 8,76Vn then through the 100R resistor when it's again lowered to 8,1V, then in enters AOp pin 8.
- As far as i know, the voltages should be the same effect on or off (off, the signal simply goes directly from input to output). So the only reason for this voltage drop between on and off chould be the LED. I unsoldered th wire going to LED+. Indeed, the voltage doesn't drop anymore (8,34V effect on and off). There should be a little less hiss, but to be honnest, i don't hear any difference on the "gating" effect.
- I alo tried to shunt the 100R, just to get higher voltages. Inded, i get 8,78V on the AOP pin8, but no real difference on the "gating" neither...
i soldered directly th LED on the 9V chassis plug (through a 2,2K resistor) and there is no more voltage drop (8,34V on AOP pin 8)... but there is no difference on the sound and on the "gating" problem, neither.
hey! i had alike problem and replacement to another op amp was the clue. 1458 was too noisy on higher gain settings. 4558 was gated and not very pleasant sounding but tl072 solved my problem. Mayby you should try some another opamps too.
by the way i've tried 5532 too. Sound was so-so but if you got some - good idea to try it. For me it is strange that original chip was so noisy. It was from tayda and i don't think that it was fake but..