"gating" on a Systech Overdrive clone

[wdm]

I think i need a little help.

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 :-)

[wdm]

And here are some pics :

[alltrax74]

So here are my voltages : wall 9.63V
5457 : D 9.2 / S 1.38 / G 0
3904 : C 5.68 / B 2.25 / E 1.8
1458 (1 to 8) : 4.6 / 4.6 / 4.6 / 0 / 4.15 / 4.65 / 9.2

On the solder side of your board I see 2 potential issues :
In red, there seem to be an unwanted trace cut between 2 solder joints
In black, you should reflow that solder

I hope this helps

[wdm]

Thanks !
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.

Thanks anyway for the help !

[alltrax74]

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

[wdm]

Thanks again.
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.

[alltrax74]

My wall voltage is 9.65V and I have 9.2V on pin 8, 0.45V drop, half the drop you have

[wdm]

I already replaced the 1N5817 just in case (i tested it ok) and mesured the 100R resistor and it's exactly... 100,0R.

But i noticed i've got 8,0V on AOP pin8 when the pedal is on, but 8,35 when it's off. Shouldn't it be the same voltages ?

[alltrax74]

Sorry I have no idea

[wdm]

Thanks anyway.

- 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...

[alltrax74]

There should not be any voltage drop due to the indicator LED
The LED is just gonna suck a bit more current from your power supply so there is something weird here

[wdm]

I agree.
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.

[wdm]

No other idea, anybody ?

[psalm69]

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.  

[wdm]

Hello,

Thanks for the clue.
I tried several other OP amps, including a TL072, and the 4558 was the less noisy, but i'll try again a TL072, thanks!

[Silver Blues]

While you're rolling chips why not try a 5532?

[wdm]

I haven't got some but i will order and try some !
Thanks.

[psalm69]

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..

[wdm]

Hello,

Thanks for the advice. Mine was from Tayda too...
So i think i'll try 5532 and 1458 from another provider...

[wdm]

Hello,

It took a while but it's done !
 I tryied 5532 from another provider. I have to say that the pedal sounds way better with it than with the 4558 but... i've got even more gatting effect.

I also built another one of this pedal just in case it would come from somewhere else but it sounds exactly the same...
I this this pedal is definitively not for me :-)