Forumvibe fv5 bulb issue

[pef]

Hello to all,
I post my first message on this forum because I have an issue with the forum vibe bulb. Indedd, I've finished to solder and hook-up everything, but the bulb don't light at all even when adjusting the trim pot. So there is some descriptions :
- I triple checked the orientation of components and the hook-up shemes : all is fine...I guess :-)
- I got the guitar signal when bypass on.
- I got the guitar signal when bypass off. If I hide the ambiant light to the light-resistors, I can hear the modulation of the effect.
- LFO is working (I can see it with the lfo's led pulsations)
- The bulb is not burned (it works with a 9v battery)
- I got the right 18v power supply (woodoo lab with current adaptor to 18v)
I havn't found any topics on this case, so if someone could help me to figure out the problem, i would be grateful :-)
Cheers.

[R.G.]

From just the set of symptoms you mention, I suspect a solder short between the bulb leads, or a copper short on the PCB. But this is a wild guess, based on essentially no information.

[GibsonGM]

Welcome Pef...can you get a voltage reading from the bulb holder?   Black meter probe on ground, red on the bulb holder (check both sides if you are unsure..).

Know how to read a meter?  This might tell you if you're getting ANYTHING to the bulb.

Another test is with no power on, see if you get continuity between the 2 bulb holder tabs or whatever you're using.  Shouldn't...

[pef]

Hi, thanks for your interest !
I'll check the voltages when i'll be back home tomorrow.
Cheers.

[pef]

Hello !
So, here's some infos
- There is 17.22v between ground and the bulb holder,
- I got continuity between the 2 bulb holder tabs,
- After inspection there is no solder short between the bulb leads nor on the PCB as R.G. suggested.

[GibsonGM]

Hello !
So, here's some infos
- There is 17.22v between ground and the bulb holder,
- I got continuity between the 2 bulb holder tabs,
- After inspection there is no solder short between the bulb leads nor on the PCB as R.G. suggested.

Hi Pef....well, this shows us something.  If you can ground one of your meter probes, and read voltage at BOTH bulb socket tabs, something is shorted.  With NO bulb inserted.

What I'm saying is that you should not have continuity between the tabs...they would not be connected together, as the bulb filament sits between those tabs...did you REMOVE the bulb first?  I may not have been clear about that!  That would explain continuity     Sorry.

I don't have the schematic for the vibe in front of me, so don't know if the 17V is ok. 

[pef]

Hi GibsonGM,

Thanks for your precisions :-) (it was logical i could have guess...) I remove the bulb and i have :
- Without power, I don't have continuity between bulb tabs.
- With power on, I can read 17.22v at on tab and 0 to the other.

According to the shematic, I think this voltage would be around 12v : http://basicaudio.net/univibe-ForumVibe-final.pdf But I may be wrong...

Thanks again for your support,
Cheers.

[GibsonGM]

Hi Pef,

Did you mark your pots as suggested in the build document?  Did you try to set it up as they suggest in the same document? 

If you have 17V at the bulb socket, shouldn't the bulb be lighting??    I would find which tab is the hot in the socket, and take the bulb directly to ground from there, see what happens.   If it lights, the trouble is on the "Trim" side of the circuit...do you know what I mean?   I would suspect Q11 thru Q14 as potentially having a problem.   


This is one place where access to an oscilloscope might really help! 

[pef]

Hi thanks for your quick answer :-)

Yes I did follow the instructions. As you suggested, I found the hot bulb tab and linked the buld to the ground : the bulb light !
How can i find the problem linked to the trim part ? I think I can borrow an oscilloscope. What would be the process ?
Cheers.

[GibsonGM]

I am not familiar enough with the way the vibe works to tell you exactly what the transistors should do, Pef!!   I understand the principle, but not the exact voltages that you should see.   So my help is not based on this circuit specifically.  But I can help you see if there are voltages present, which appear to be appropriate...

What I would do first is to take voltages from Q11, Q12, Q13, Q14.   I can at least tell you if they are conducting.   List them "Q11: C xx B XX Exx"
While you are there, make SURE that each transistor is placed in the circuit the correct way - often, they are accidentally reversed from the way their pinout should be!   Make sure they are the correct part number while you are there
 
You must also examine the solder side of the PCB here, to be sure there are NO solder bridges between transistors and components...a short would make the bulb stay on, perhaps, and not allow the LFO to oscillate.  Look for bad joints, missing solder...

Are the 1N914 diodes oriented in the right direction as shown in the schematic?  The polarity is important.   Are the electrolytic caps oriented in the right direction, with the "-" band facing the most negative direction?

At the bottom of the 4.7k resistor under Q12, is the circuit grounded??  (maybe the first thing I should have told you to look at!)  And only grounded at that node?   

If you connect the bulb as you just did, one side to ground, one side to "LFO B+", can you change the LFO speed?  I would like to see if LFO is working...adjusting Intensity may also help you to see the bulb go on and off.    A scope will also tell you this, if it is easy to get one.

Good luck!  The problem is most likely something very simple that we are overlooking! 

[GibsonGM]

I am not familiar enough with the way the vibe works to tell you exactly what the transistors should do, Pef!!   I understand the principle, but not the exact voltages that you should see.   So my help is not based on this circuit specifically.  But I can help you see if there are voltages present, which appear to be appropriate...

What I would do first is to take voltages from Q11, Q12, Q13, Q14.   I can at least tell you if they are conducting.   List them "Q11: C xx B XX Exx"
While you are there, make SURE that each transistor is placed in the circuit the correct way - often, they are accidentally reversed from the way their pinout should be!   Make sure they are the correct part number while you are there
 
You must also examine the solder side of the PCB here, to be sure there are NO solder bridges between transistors and components...a short would make the bulb stay on, perhaps, and not allow the LFO to oscillate.  Look for bad joints, missing solder...

Are the 1N914 diodes oriented in the right direction as shown in the schematic?  The polarity is important.   Are the electrolytic caps oriented in the right direction, with the "-" band facing the most negative direction?

At the bottom of the 4.7k resistor under Q12, is the circuit grounded??  (maybe the first thing I should have told you to look at!)  And only grounded at that node?   

If you connect the bulb as you just did, one side to ground, one side to "LFO B+", can you change the LFO speed?  I would like to see if LFO is working...adjusting Intensity may also help you to see the bulb go on and off.    A scope will also tell you this, if it is easy to get one.

Good luck!  The problem is most likely something very simple that we are overlooking!