Neovibe LFO Problem

[Craig V]

Hello,

I'm having a problem with the LFO in the Neovibe.  Here is what happens:  when the speed pot is all the way up, and the intensity pot is too, it has a nice deep pulse.  Touch the speed pot and the oscillation stops.

The intensity pot seems to be working well, although it is kind of useless up till the last 1/3 of the sweep.  I have a feeling there is something related to the speed control problem.

I used the pot from Small Bear, which should be the correct pot.  I have it wired just like the .pdf file calls for, unless the picture is a mirror image or something. (I cross checked it with the schematic.)  

When I started my debug, I didn't have any modulation, and I measured 18.2V on base and collector of Q12, which implied a short.  But,  with the transistor out, I measured the sockets and got 10.2V on the base.  I installed a jumper from base to emitter, and that got the oscillation to happen.  I tried a 2n3904, 2n5088, and BC108B in Q11.  They all have the same results.

Is there something that I am overlooking?

Thanks in advance.

Craig

[R.G.]

The univibe, and by extention the neovibe has just enough gain in the LFO to start and maintain oscillation. It really needs the darlington connection of the two transistors in the LFO to work right.

Is it possible that you have one bum socket?

[aruse]

I am having the same trouble with my neovibe I finished last night. Is there a good higher gain substitute transistor to replace in the LFO circuit? I am currently using a pair of 2n5088's. I know very little about the options out there. Some guidance would be greatly appreciated. Parts avaliable through small bear or mouser would be best. Thanks in advance for your help.

[Craig V]

RG,

I replaced the transistor socket with single-in-line sockets, and don't have the 18V on the base of Q12.  

I was able to get it vibing with 2n5088 and 2n3904 in any combination.  I also used a MSP13 Darlington in by itself and working with another transistor, and still have the same results - no slow speed.

I tried another dual gang pot from Radio Shack and tried that still has the same results - no sweep either.  I have double and triple checked that the wires are going to the correct spots on the board.  

Have any ideas?


Aruse,
Did you order your speed pot from Small Bear?  If yes, did yours come without the nut and washer?

Thanks for replying.

[Gearbuilder]

Hi,
What value is your dual pot?
Bruno

[Craig V]

Both of the pots I tried were the 100K dual ganged.  The Radio Shack one is the same one that Dave Fox uses to retrofit a wah or rocker pedal to a 'vibe speed control.  (or he recommends it - I cross checked the model numbers a while ago.)  According to RG, this pot would act as a slowness control when its working, not being reverse log.  The first pot is the standard Small Bear Neovibe pot.

[Craig V]

Aruse,

Make any progress?

[aruse]

I replaced the tranistors with similar combinations as explained above and ended up with a pair that worked about half the time. Seems like voltage is border line. Put the unit in box and am back to square one. NO SLOW SPEED! Sorry I don't have more positive news. Another observation; the fast speed of the neovibe is not nearly as fast as that of the sweet sound mojovibe I have. I am going to open up the mojo and see what that is using for its transistor selection.

[R.G.]

Hmmm... something wrong with the pots, I think. The neovibe is just the same circuit as the univibe, after all.  I wonder if the diodes in the LFO are giving you problems. One shorted, maybe.

[aruse]

I'll try the pots and diodes tonight. thanks

[Craig V]

RG,


I took out the diodes (socketed) and it was working exactly the same as it was with them in - anything other than full speed on the pot yielded no oscillation.  

According to your Technology of the Univibe:

The two diodes across the center capacitor limit the size of the LFO output waveform. An oddity of this particular way of building a Univibe is that the LFO amplitude goes up with increasing speed, even with the diodes there.

So with no diodes, should whats happening be happening?  If yes, than I think something isn't connected in mine right - shorts are less likely because I have done real well in soldering and Dremel-ing out the traces.


Thank you for reading and/or replying.

PS.  Your Tube Amp debug page is VERY helpful.  I was having a problem with squealing, but I think you've narrowed it down to to a bad preamp tube.

[Craig V]

Aruse,

What kind of diodes do you have in there?  The Neovibe file calls for 1n4148.  Are these what you're using?  Are they really small and orangish, or bigger and blackish?

Any improvements?

[aruse]

went through the LFO today and managed to get it to go slow as long as none of the pots or jacks ground touched each other. I think I blew the driver transistor in process. Would that stop the LFO from strobing? Diodes are small orange switching type as spec's call for.

[Craig V]

Yeah, the driver transistor is pretty important to the strobe, or just getting the lamp lit in general.  This would be a good place to have a socketed transistor.

What kind of Lamp are you using in yours?  I have a Radioshack 12V/25ma in there right now.

[R.G.]

went through the LFO today and managed to get it to go slow as long as none of the pots or jacks ground touched each other. I think I blew the driver transistor in process.

OK there's a clue. I've been trying to think of what's going wrong here, and I think I may have picked it up from that comment.

Do one or both of you supply power to the 'vibe circuit through a wall wart and use a metal-housing jack to connect it into the box? You can't (successfully at least) get that to work, because the board makes its own DC out of the incoming AC or DC through a bridge rectifier. That necessarily means that the negative end of the incoming power is two diode drops *below* the signal ground, and connecting the incoming power to the chassis is shorting out the negative side of those diodes, with various ill results. 'twon't work that way.

Verify that neither of your incoming power wires has continuity to the signal ground on the board or the chassis/box.

The original sidestepped this by putting the power transformer inside the box; we'd like to not do that, for obvious reasons.

[Craig V]

RG,

I am using a plastic jack, and measured no voltage on the chassis, so I think the board is getting power correctly.  I have a DC adapter, so the bridge rectifier wasn't necessary (I think I got that advice from you months ago).  

I have the same results, no oscillation except when speed pot is full on, with or without the diodes in their sockets.  I measured fluxuating voltages in all of the sockets.  Is it possible to have too much voltage from the darlington section?

I don't even understand how I'm getting oscillation without the diodes.  From reading the "Technology of," I thought the diodes were the key element to create the necessary feedback.  Maybe the socket do it, LOL.

On a side note, I have been plugging this into my gear setup, and it sounds wonderful at the one speed.  The chorus mode is just awesome.

Thanks.

[R.G.]

Drat.

The diodes don't really affect the oscillation other than to keep its amplitude (relatively) consistent. It should happily oscillate without them, just not at a consistent amplitude.

Is one of the caps isolating the depth pot perhaps shorted or backwards, so that the depth pot is siphoning off DC and stopping oscillation?

I've never run into this particular bug before and have hit a wall in counter-designing for what defect might cause it, which is why I'm acting so useless here.

[Gearbuilder]

Hi


  Try to desolder one side of C22 to be sure your LFO is well functionning without a charge  .Control the AC voltage at C22(meter on DC).If your LFO is OK,you will see oscillation on your meter even you pot is at slow speed .If you don't, the problem is really in the LFO part of the circuit ,try to debug it ,sold C22 and FIRST check your resistors values,it's possible to put the wrong value and this change the tensions in the LFO circuit (47k to the  place of a 4.7K ).If LFO is OK ,check for values of  R47,48,49,50,the trimmer pot,check the voltage on the transistor too,this one wich sets the right amount of mA  in your bulb
Bruno

[Craig V]

Hello,


I desoldered C22, and took measurements - it was only oscillating when the pot was full up.  The problem lies in the LFO.  I have triple checked the resistor values, and all are correct, save for R48 (I used 10 ohm instead of 68ohm.).  

I measured DC voltage on the lugs of the speed pot - should this be happening?  I have always thought there should be no voltage on a pot.

What should the voltages of the LFO be around C22?  It seemed to only go to about 4.7 when it was oscillating.  

Thank you for your help.

[Gearbuilder]

Hi,
 I'm not actually at home for doing some measurements on my vibe ,but
i will take the voltage tomorrow morning.It's looks like a short .What pot did you use for the trimmer ?You never say if there's a small glow when the pot is full(speed pot)or slow.This could help a lot too.
Bruno