Vibracaster Troubleshooting

[dan83lz]

Hey all,

I'm relatively new to pedal-building, and I was attempting to build a vibracaster (schematic: http://www.aronnelson.com/gallery/main.php/v/frequencycentral/Vibracaster+Stock+12AU7+V2.JPG.html), but I haven't been able to get it to work correctly.  The valvecaster part of it works great, but I can't get it to oscillate.  I started out using all of the parts that the schematic calls for, but then I started to experiment with different cap and resistor values on the oscillator circuit.  Regardless of changes I made, the "speed" control has no effect except in some cases it would seem to lower the overall volume. 

My main suspicion is the fact that it won't oscillate because I'm using 9v instead of 12, but the original post for this pedal (http://www.diystompboxes.com/smfforum/index.php?topic=70160.0) suggested that it should work either way.  Has anyone else had trouble with this?  I really want to get this thing to work.  Thanks.

[GibsonGM]

Are the transistors and diode oriented the right way?   Often, parts in the wrong place/config are the cause of this type of problem.

What are your voltages on the transistors?    Can you lift the 100k pot end of the 4148 diode, and see if JUST the oscillator OR the audio section works?   Break it into 2 parts....

Read the "Debugging:" thread, and post voltages please...just sub in P, G, C for plate, grid and cathode )     I would think it would be OK at 9V, at least it would do SOMETHING, so let's walk thru.   I am most curious if you can get oscillation with that diode disconnected (or at least reasonable voltages).

[duck_arse]

what transistor are you using for the osc? if it is not oscillating, it may not be of high enuff gain. the 9V/12V should have very little effect on the osc.

the easiest way to see if it is oscillating is to stick a led in series with the 9V and the 10k resistor. and the most effective "osc works/osc doesn't work" resistor to fiddle is the 3M3. go as low as 1M, see what happens.

[dan83lz]

I used the BC547C, as the schematic recommended.  Unfortunately, I'm having issues with the leads on my DM at the moment, so I ordered new ones.  Won't be able to take measurements on the circuit until then, but in the meantime, I'll try to switch out the 3.3M resistor to see if that has an effect.  Thanks!

[dan83lz]

Ok, so I still don't see anything wrong with my original PCB, but I just breadboarded a new oscillator circuit and connected it to the Valvecaster PCB, and it works fine.  I reduced the 3.3M resistor at first, but ultimately decided that 3.3M sounded the best anyway.  Not sure why I couldn't get it to work before...  Thanks for the help!

[duck_arse]

it is odd the way this oscillator circuit causes lots of people trouble, but they get it working and are never sure what was wrong in the first place. so, how does it sound, are you happy with it?