Did it stall like that before you tweaked the trim pot? When it stalls out, if you move the trim pot back toward the factory setting, does it start working again?
Thought I would ask you a question in this thread relating to the Micro Vibe as you seem to know a lot about Univibes and clones. Please let me know if I should start a new thread... Can you tell me why the bias resistors for the lamp driver are reversed compared to the Univibe? Is it because the Micro Vibe runs on 9V? Reason I ask is because I want to do a ‘JC or Redhouse offset bias mod’ in my Micro ibe. And I’m trying to figure out how I would do it. I’m just starting to learn some basics but I don’t have any expertise in reading schematics and electronics in general.Screen cut off my name from the header, so I'm slow to respond.
Here's the schematic:
So, as near as I understand it, the trimmer sets the gain of Q2, which determines how much current it pulls through the bulb. Now, where LEDs can go bright-dark-bright near-instantaneously, filament-based bulbs have built-in lag. Indeed, it is that lag that provides the "feel" of bulb-based modulation devices. But since bulbs are essentially heat-based (i.e., the light results from heating up the filament inside them), if the bulb is pushed too hard (i.e., really bright), there is enough heat created that it doesn't lose heat fast enough to dim down. So the sweep will seem to disappear, as if it has "frozen" midsweep.
Given that the tone you like comes from having the LDRs shone on brightly, then the goal is to have the resistance provided by each LDR a little lower/smaller than they provide at default, such that workable bulb brightness is enough to do the job. I'm not promising this is the cure-all, but here is a suggestion. With the pedal turned completely off (no power) see if you can measure the dark resistance of one of the LDRs.
Keep in mind that vibes are essentially phase-shifters with staggered cap values. That first stage uses a 15nf cap to set the point/range where max phase-shift is created. If the LDR's default dark resistance was 1meg, then at the lowest/darkest point in the sweep, that stage would provide maximum phase shift (90 degrees) for content above 1/(2*pi*15nf*1meg) = around 10hz (or more properly, forthe entire audible audio spectrum). If the dark resistance was 500k, then max phase shift for that stage would commence around 21hz. Shine some light on the LDR to makeit's resistance drop to 50k and now the same stage is producing max phase shift starting around 212hz. And so on.
Maybe you can already guess where I'm going with this. If you can force the bulb to shine brightly enough to force the LDR resistance low enough, then make you can make the LDR resistance low enough to start with, that a modest bit of light will do the job. So again, if the dark resistance was 1meg, you could straddle each LDR with the same value parallel resistor to reduce its effective dark resistance. Strapping/tacking on 1meg would make the whole thing start an octave higher. If the dark resistance was 500k, tacking 1meg in parallel with each LDR would (in theory) make their max resistance 333k. And so on. That is, you would do with 4 extra resistors what you can't do using the trimmer.
With FET-based phasers and vibes, the trimmer one generally finds on the board, feeds a bias voltage to each FET to move it into a range where it can be swept up and down. Most such phasers let you dicker with the trimmer to get different feels coming from different ranges, from low and gurgly to high and swirly. But that adjusts the control element (FET) itself. What you were attempting to do was change the range of the thing (bulb) driving the control elements (LDRs). Sadly, the tonal range you sought was beyond the usable brightness range of the bulb. What I suggested here was a way of adjusting the control elements to compensate for what you couldn't do with the bulb.
Thx for replying! Thats the only schematic I have found as well. I did find a layout but I’m not aure if it is based on the same schematicGiven how many other circuits have B-C of 100k and B-gnd of 47k, I'd say the simplest answer is that the schematic has at least one error in it. That's what I get for posting the first Micro-Vibe schematic I found. You will note that it also doesn't have any way for the dry signal to get to the mixing stage. I salute the diligence of the person who took the time to draw it out, but they seemed to be a little hasty in posting.
Here is the thread where the schematic came fromGiven how many other circuits have B-C of 100k and B-gnd of 47k, I'd say the simplest answer is that the schematic has at least one error in it. That's what I get for posting the first Micro-Vibe schematic I found. You will note that it also doesn't have any way for the dry signal to get to the mixing stage. I salute the diligence of the person who took the time to draw it out, but they seemed to be a little hasty in posting.
No apologies needed. I never ‘expect’ a reply. Everyone is busy. Its nice when someone does and is always greatly appreciated!Sorry to take so long responding. I have a bit of a difference of philosophy with that site, so I don't go there.
That said, if you swap the values of the 47k and 100k resistors, to comply with other traditional posted schematics, you won't fry the transistor. And if it fosters intended performance, you're in business.
Q2 on the schematic. Q3 on the layout. I guess they are labelled differentlyAre we still talking about Q2?