NeoVibe depth too dependant on speed?

[zotch]

Hello, my NeoVibe (stock GGG board) loses depth at slower speed, and stops oscillating completely before the speed pot is fully CCW; voltages check out according to the drawing...when testing, the probe shorted Q11's base & collector and the bulb flashed brightly then oscillated a bit before going back to the 'weak depth' state. I changed Q11 just on principle, but there was no difference. Also, the depth is greater for any middle-range speed setting if the speed was first at 100%, then turned down--as opposed to coming UP in speed from the slowest (CCW) point. Measuring in-circuit, C19 reads 1uf, C20 reads like shorted leads do ("OL" as opposed to 0nf), and C21 varies with the position of the speed pot, reading as high as 11uf at midpoint. Is this the area to start removing components and testing values? Thanks in advance, for any replies.

Tom Oliveira

[Morocotopo]

Tom, that´s just how it works, because of thermal inertia in the light bulb or something like that. Someone explained it here better for sure, search, specially R.G.´s posts. I don´t know if it stopping at the lowest speed setting is normal, though.

Morocotopo

[R.G.]

Gain of the two LFO transistors is too low at the lowest frequencies to sustain oscillation, so it dies out. Common problem in older real univibes.

Use a 2N5088 or 2N5089 (or other NPN with gain >400 ) for the first LFO transistor.

Could also be a parts value problem, loading the LFO down too much, but if it works otherwise, try the transistor first.

[zotch]

Thanks for the replies; I had tried a mini-mag lamp (2-AA size) but that didn't even light up...strange--thought I had fried the new Q11 or worse. I've also heard U-Vibes on YouTube, and none really were fat @slow speeds...but I seem to remember mine sweeping like a wah when I first powered up, months ago (at med speed). Don't know what had changed; my ears maybe, heh.

I chose a 2N5089 in Q11 for it's slightly higher gain over the '88, and was rewarded with a bit more depth.  I still have space on the speed control where it's not flashing (0-40% or so), so I guess I'll try changing the 3904 in Q12 next and research the "loading" of the LFO.

Thanks again, sincerely; at this point, the unit's earned a spot on my expanded pedalboard (outboard Speed pedal planned too). Any 'next steps' suggestions are respectfully encouraged.

Tom O.