Hey a buddy of mine gave me his Fulltone Dejavibe 2 (the one in the foot treadle enclosure) for repair.  I found out that the bulb was fried for the Photo section. I swapped with another Led and its pulsing and such.  Effect is totally different though, chopping and intense albeit restored to working.  Now for my Questions:

1. Does anybody know the specs or what type of bulb they put in stock?  Or any suggested sizes, current rating, color to use since it was bad.

2. The speed led worked when I got it then when I repaired it, it went on the fritz.  I thought maybe I blew the led moving the board around.  Turns out I measured the voltages that were present at the Led electrodes and found almost 15 volts of dc on both pos and negative.  What gives??  Help??



Thanks as always!

I dont know about that circuit,but if there is a trimmer you can adjust the intensity of the led to smooth it out.(on/off cycle).I was under the impression those were an exact copy of a univibe circuit..but if it has a led instead of a bulb it is not the same as a univibe..can you post some pics?

It turns out that it is a T1 style bulb.  Ive never seen any.  They are on order and Im hoping that it solves effected sound.  I still however am in the dark as to the specs of this bulb.  Especially current.  I know what voltage is present after testing with DMM.  So Trial and error it must be!!!

the deja vibe isn't using a LED under a light shield but an Incandescent bulb, 'they behave quite differently from a led,which will affect the sound a lot. it turns on and off slower than a LED. which explains why the effect now sounds more choppy. I dont know the specs of the one in the fulltone. but I've used a 12v 20mA with succes, you can get them in most online part stores,. I got mine here; http://www.banzaimusic.com/Incandescent-Bulbs/
R.G says in his technology of the univibe article on geofex;
The bulb and it's time response are important to getting the right sound. The original bulb in all except one original unit I've repaired is a nominal 28V, 40ma bulb. The cold resistance is just over 100 ohms. I have sucessfully used 12V/40ma and 12V/80ma bulbs here.

Thanks Liquitone!  I realized the T1 style bulb was the correct replacement.  However when I measured DC voltage at the nodes where the bulb is, it measured around 18Volts.  This effect employs voltage doubling.  I replaced with a 24volt bulb and it's closer but is still chopping a bit.  Definitely not like stock, and I have the internal trim pot for adjustment of brightness set to the lowest level too!!!   I guess I better try a few more bulbs until I get closer.  May have to swap trimpot for more control over the bulb.  Any suggestions???

I have a similar situation here,  no oscillation, or weak. 

I suspect that it may be the LED driver circuit of the Vactrols that substitute the dual 250K pot.

One vactrol measures differently than the other, and the forward voltage of the LED is higher than the other Vactrol's

What might have happened is that the LED are driven above their rated current, damaging them, and one behaves differently than the other.  The circuit only works if both give the same approximate value

I've asked for help at Fulltone.. will see if they reply.

Also will take the vactrol out and see if I can measure/repair them.

Quote from: maxdm on October 13, 2024, 11:08:46 AMI have a similar situation here,  no oscillation, or weak. 

No, or weak oscillation in 'vibe style circuits is often tied to lack of current gain in the darlington-connected transistors in the oscillator. Changing these, or subbing in a real single-package darlington like an MPSA18 often fixes the issue.

Quotesuspect that it may be the LED driver circuit of the Vactrols that substitute the dual 250K pot.

One vactrol measures differently than the other, and the forward voltage of the LED is higher than the other Vactrol's

Could be, I guess. I like to use the VTLC5-2 modules. These are center-tapped single LDRs. Maybe someone "fixed" at it before. Or maybe they were just tired on the manufacturing line that day and used whatever they had to make quota.

QuoteWhat might have happened is that the LED are driven above their rated current, damaging them, and one behaves differently than the other.  The circuit only works if both give the same approximate value

I've asked for help at Fulltone.. will see if they reply.

Also will take the vactrol out and see if I can measure/repair them.

Not a bad idea.

Quote from: R.G. on October 13, 2024, 02:41:11 PMin a real single-package darlington like an MPSA18

I was under the impression that the MPSA18 is just a single NPN transistor? That's what its datasheet says.

Fulltone did not help..

I fixed it. 

It was, as I suspected a blown LED in the Vactrols which substitute the dual 250K pot (on the schematic)

It would have been better, IMO, to use a dual-gang pot, but I imagine that it costs more.