Tubescreamer 808 - Drive knob doesnt work..

[danelectro]

Hello everyone, this is my first post on this forum. it looks great!

Well i just built a TS808 using the www.generalguitargadgets.com's schematics, layouts, and circuit boards.

The problem is that my drive knob does nothing. im not using a switch right now so its hard wired on. I get a very slight distorted gain. the level control works, the tone works, just the drive does not.

My voltages at the IC and transistor pins are correct. Ive tested the diodes and the 47pf cap in the first opamp feedback loop. ive even tried it w/o the diodes adn cap, i get no level at all. i also tested the pot itself and it is working fine..

i build tube amps on the side, so i figured this pedal should be a breeze. i must be missing something, but i absolutely cant figure out what..

any ideas on how to fix it or debug it? thanks

[brett]

Hi.  You've got a mystery happening here.  Time for some detective work.  You said

ive even tried it w/o the diodes adn cap, i get no level at all.

I think that's because without an ACpath to ground in the feedback loop you get no gain.  With the cap *in* and the pot replaced by a 1M resistor, you should get heaps of gain?  (ie lots of distortion) If not, there's an unwanted source of feedback.  If so, there's something suspect about the pot (double check it's a 500k or 1M pot).

Keep looking...it's a simple circuit and there can't be too much wrong...

good luck

[danelectro]

ok, thanks for the quick reply.

let me rephrase what you quoted,

i got identical results from:

no diodes, no cap, 2 small diodes, 2 bigger diodes, 3 bigger diodes, and now w/ a 1M resistor in place of the pot..

i orderd some Ge and the recommended diode types.. they werent labelled so i have no idea which is which.

i wonder if i fried the first half of my opamp chip.. the second part must work because teh level works.. im not using a socket, but i solder fast. 40watt iron. hrm

[brett]

The thing that first struck me in this post was that you got the same results with "no cap".  That should have changed thing big time.  My next guesses are that you either have a feedback path from output to -ve input of the first stage, or you've burnt that stage.  Tho with a burnt stage, I'd usually expect no output...hmmm...

I've you've got a high-impedance DMM (>10 Mohms), check that both the +ve input, -ve input and output are at 4.5V.  Cheap meters (like mine), will give lower numbers, but all the number should be equal.  

Next step might be to build an "audio probe".  You'll find a schematic of it with a search of previous threads in this forum.

[petemoore]

Right there's you're prime suspect.
 I solder 'fast too, but fried some 082's the other day with just kwik touches of heat, trying to piggyback them...course I fried another pair trying to PB but that's cause I got North and S wrong lol...smoki the OA.
 With all the added connections though at OA central, use a socket. Pull the Oa you have in there and toss it, clean up the board and install the new empty socket ... I fried 'em jsut doing like touch up work, it doesn't take much at all to fry them, and the fear of partial fry/doupled with the ability to try different OA's in the circuit make sockets a godsend.
 I think this is the problem with your circuit...you'l like being able to swap em out too !

[1wahfreak]

I had the same exact problem with mine using 1n34A diodes. I switched to 1n914's and now it works perfectly.

[petemoore]

you can put about any config of diodes in and it won't kill the circuit.
 Going from 1n34's to 1n914's...will have an effect on the tone and output to some extent, but it will sound' either way.

[danelectro]

do you think the fact that when i took out the cap and then teh diodes and it had no effect that its not a diode issue?

i got the same amount of break up with no diodes at all... i guess the opamp was clipping..? but that would mean its not burned up...

hrmmmmmm

[Ge_Whiz]

Mis-wired drive pot?

[smoguzbenjamin]

Yeah that was he first thing that came to my mind. Check your wiring mate!

[danelectro]

im sure the pots fine. infact right now its off and i replaced it w/ a 1Mohm resistor. when i finally hear some gain ill put the pot back on

i think my opamp is dead.. ill have to order more from small bear and that took like 2 weeks last time. garrr

thanks all. anymore ideas would be appreciated

[smoguzbenjamin]

2 weeks? That's short. I am expecting a package from the States and its been 5 weeks now. I think the post people  lost it :x grrrr

[danelectro]

well, i got my shipment in. i soldered in an IC socket and used a new JRC opamp chip.. i also wired in the switch.

still not working.. on or off it sounds the same.. my knobs dont even effect the volume or tone anymore... i cant imagine what is wrong, ive checked everything a hundred times. does this PCB board and knobs, etc have to be mounted in teh chassis for it to work? im just testing it all sprawled out ona plastic bag right now.. can the transistors be shot? if so what are the symptoms?

i'm about to scrap it and start totally over..

thanks again

PS
the circuit is totally stock except im im using 3 germanium diodes.. i originally had the smaller glass diodes in there and i heard no difference. im goingi to try now w/o diodes at all.. and then w/ the smaller ones again

[petemoore]

Just me but I always get the circuit de-bugged before putting switches [bypass] and jacks on...just a few more things that could be problematic IMO.
 So you checked the OA pin voltages and they still look good?
 Something funny in there...you already know that...I've written off circuits before and then gotten them going as 'comebacks'...just goa bakc a forth over the whole thing, concentrating your focus 'around' the drive pot.. Like you haven;t already..

[danelectro]

well i just tried w/ no diodes, just the jumper, 2 diodes and jumper, jsut 2 diodes.. all the same. i dont even hear a clean volume boost..

and yeh, untill today's post i didnt use the switch. i thought maybe itd help, but no change.. the tone and level pots seem to be working again now. its the drive does nothing.

i suspect tho w/o any diodes i should still get a clean boost.. the opamp is just a noninverting opamp w/ a feedback loop eh?... the gain should be (500k+51k)/4.7k  .. so thats like a gain of 117x.. hrmmmmm

[Johan]

check the cap in the 4,5volt spliter. if it is not there or if the solderjoint is dry, you will get that same result. if it is not there, the 1/2volt will swing with the signal and in effect turn your tubescreamer into nothing more than a fancy buffer...

Johan

[danelectro]

damn i thought you had it!  i had a tantalum cap for C7 and it was backwards! but that didnt change a thing.. hrmmm cuold i have killed it?

edit: replaced it w/ an electrolytic to test.. no change.

[danelectro]

also, im using an electrolytic for C8.. its labelled with a "BP" in teh schematic, what does that mean?  i tried both orientations with no difference at all. the + side should be the side nearest the opamp tho right?

edit: well i bypassed the cap all together, no change in output...

[danelectro]

anyone have any ideas for piece by piece troubleshooting? .. i guess if not im gonna rip it all apart and start over.

[Impaler]

Ok, I built Tonepad's layout so I'm not too familiar with the GGG one. You may want to try that layout for PCB. Mine worked first time. Looking at the GGG layout as I write tho, This comes to mind...

1. There are 4 jumpers on this layout, make sure you have 4 jumpers on yours...

2. The 1µF caps at C2 and C8 are non polarized, so make sure yours are to... If using electrolytics, you may have them reversed as there is no polarity designated


If all else fails, check your circuit for shorts with the DMM's beeper and go thru the board looking for traces that are connected that shouldn't be...