Shaka Tube debuggin help please

Started by stenns, May 05, 2006, 03:52:43 PM

Previous topic - Next topic

stenns

     My shaka tube has been in the debuggin stage for a while now.  When I first completed it, the TL072 opamp was installed backwards, and when I realized this and flipped it around, it still had problems.  Thinking the opamp was fried, I searched for another.  Diystompboxes member Ildar was kind enough to send me one, but this was unfortunately, not the solution to my problem. 
     Bias pot adjustment allows for some functionality:  an extremely buzzy guitar sound is made with terrible sustain.  It cuts out almost immediately after the string has been plucked and can only be heard if the strings are heavily wailed on.  Volume knob adjusts volume, tone adjusts tone, but the gain knob is useless.  Jumpering the tip of the input to different parts of the circuit revealed that a cleaner sound could be achieved at pin 2 and 7 of the 12ax7 (grids), so this suggests the problem is in the SS preamp stage. 
     Wires to the gain pot were replaced without results.  2.2uf capacitor was replaced without any either, except when the input tip was jumpered to pin 2 of the 12ax7 (grid 1) a mid frequency ossilation was heard that gradually increased in pitch. 

Voltages:

TL072

1     14.14
2     .1200
3     -11.6
4     -11.6
5     0
6     -10.16
7     -10.16
8     14.92

V-   ~ -11.00
v+   14.96
B     ~ -11.00


Schematic: http://www.diystompboxes.com/pedals/schems/shakatube.gif
Layout:       http://www.diyguitarist.com/PDF_Files/ST1590BB-PCB_7-14-05.pdf


Any ideas??????
Thanks!!!

aron

This is time for an audio probe. See Debugging page at top. Listen to the output of the TL072 and see if your problem is at the chip or after.

Aron

Paul Marossy

What's the specs on yyour power transformer? I used a 13VAC 800mA wall wart to power mine.

stenns

i think its a 12.6v (or perhaps just 12) 800ma radioshack ac-ac power converter for answering machines.  Thanks for the posts I'll be getting around to that probe any day now!

Paul Marossy

OK then, I guess it's audio probe time!

rody82

Well, if you used that Paul Marossy Layout (7/14/05, the TL072 version)
then i know the solution. That layout has a mistake, cause that 250pf capacitor near
the TL072 opamp should connect to the output of the opamp, but it doesnt... Check
the traces, its not difficult to connect them because they are very close.
I had the same issue, and this solved it very well...
I just cant believe, that no one noticed this so far.

The distortion is a bit grainy yet, and it only works with the bias fully turned down
(BIAS = V-), so it needs a bit of experimenting yet...

RaceDriver205

Just a word for future builds, stenn : - a damaged part is a very very low probability occurance. Ive even found this for static-sensitive parts. Its nearly always a faulty connection (i.e. no connection or a connection where there shouldn't be one), or a part the wrong way round.
Have you made sure your electrolytic caps are the right way round?

Speaking of debugging, I wonder if that XORUS of mine will ever work  ???. ARGH!

Paul Marossy

#7
QuoteWell, if you used that Paul Marossy Layout (7/14/05, the TL072 version)
then i know the solution. That layout has a mistake, cause that 250pf capacitor near
the TL072 opamp should connect to the output of the opamp, but it doesnt...

Doh! I'll have to check that out.

EDIT: OK, I compared my layout with the schematic. It looks like I missed a connection between the pot/250pF cap and Pin 1 of the opamp. I'll have to fix that.

aron

Check out the output of the op amp and then work down the line with the audio probe and you will hear where it starts gating (cutting out)