Forum Vibe LFO/driver problem

[LordoftheLivingRoom]

Hi all,

I'm building a Forum Vibe using Redhouses mods.  http://www.classicamplification.net/fv/files/fv_mod_01.htm

My problem is that the bulb isn't pulsing with sufficient amplitude.  With the Intensity control all the way up I get some bulb pulsing, but very little.  I've seen a couple of univibe style biasing vids on youtube and my bulb just isn't pulsing anything like the clear off to bright type pulsing.  Both the Bias Offset and Trim controls work properly as far as I can tell.

When I first turn the effect on, the first cycle or two of the bulb is very good, almost like I see in the youtube vids but the bulb brightness soon settles down to much less.  I'm using MPSA13's for the LFO and bulb driver circuit.  I even put in a darlington pair of MPSA13's in so the gain should be off the charts for these two circuits.  All of the components are correct.  All voltages look OK to me, but I can't be sure since no one appears to publish voltages for these circuits.

If anyone has some tips for this circuit, I'd greatly appreciate them.  Cheers.

[R.G.]

What voltages are you getting?

[LordoftheLivingRoom]

Hi R.G.!  Thanks for replying, I appreciate your help with this.

I spent some time this afternoon probing around with multimeter and cro.  Supply voltage is 15.9VDC.  Speed pot set at approx 66% and Intensity to 100%.  Voltage readings given as min. to max. VDC.

Junction of 3k3 R38, 4k7 R39, 2M2 R40 on The Redhouse schematic  (R39, R40, R41 on Neovibe schematic) = 7 to 8.5V

Base of Q11 and Q12 (MPSA13's) = 7 to 8.5V

Emitter of Q11 and Q12 = 6.8 to 9.1 V  (Noting that I'm using an LED in series with the 4k7 as a rate LED, no difference to performance when the LED is in or shorted out of circuit.  The rate LED barely varies when in circuit).

On each side of the oscillator diodes D1 and D2 = 6.8 to 8.6V

Wiper (lug 2) of Intensity pot = 3.8 to 6V (max Intensity.  The waveform amplitude varies nicely with Intensity pot rotation, from 0 to 2.2 Vpk-pk.)

Base of Q13 and Q14 (bulb driver pair, MPSA13's) = 5.8 to 7.6V

Emitter of Q13 and Q14 = 5.2 to 7V

Collector of Q13 and Q14 = 14 to 14.8V 


A few other points;

* When I advance the speed pot the amplitude rises slightly, as expected.  But past about 90% rotation the oscillation collapses and I have to back the pot off and wait for 30 seconds to a minute for oscillation to re-establish itself.

* After I had been investigating the speed pot phenomenon, and returned the controls to my original 66% Speed and 100% Intensity, oscillation would collapse whenever I tried to take a reading on the base of Q11 and Q12 with either my cro or multimeter.  This happened consistently regardless of any Speed setting.

[LordoftheLivingRoom]

No one has any tips or ideas?

[R.G.]

My first guess is that your bulb isn't an appropriate one or that there is some issue with the bulb. Your voltages don't seem grossly off at first blush.

Can you list voltages with amplitude turned fully down?

* When I advance the speed pot the amplitude rises slightly, as expected.  But past about 90% rotation the oscillation collapses and I have to back the pot off and wait for 30 seconds to a minute for oscillation to re-establish itself.

That's reasonably normal. It means that the speed pot is getting to too low a resistance for there to be sufficient loop gain to make oscillation happen. There are two cures - increase the resistances in series with the speed pot until it doesn't stop oscillating, or make the impedance at the input of the oscillator gain stage higher by using a JFET.

* After I had been investigating the speed pot phenomenon, and returned the controls to my original 66% Speed and 100% Intensity, oscillation would collapse whenever I tried to take a reading on the base of Q11 and Q12 with either my cro or multimeter.  This happened consistently regardless of any Speed setting.

Again, this is part of the phenomena about loop gain, but in terms of current, not voltage like most LFOs. Your meter is stealing enough current from the input to Q11/Q12 to prevent it from oscillating. Remove the meter, it starts again.

[LordoftheLivingRoom]

Thanks R.G.

The bulb is 5V 20mA.  I also tried a white LED in there.  Same problem. 

I had a PM from someone who's just made this circuit.  He suggested that the voltage at the junction of R38, R39 and R40 was stable in his build.  Mine oscillates.  I found a neovibe circuit with some voltages annotated and it's labeled as 11V IIRC.

Its good to know that my stalling oscillation is a normal problem.

I'm going to try some other devices in Q11 and Q13.  Maybe my Tayda MPSA13's are dodgy.

[LordoftheLivingRoom]

Solved!

Bad MPSA13's!!!

I tried a couple of the MPSA13 darlington transistors I got from Tayda...  All didn't work in circuit (yet test fine on my device tester).  So I tried the highest current handling small transistors I had on hand - metal can 2N2222A's.  They are set up in darlington pairs as per the vintage vibe.  And it works!  Now I need to change out Q10 which is a MPSA13 as per the mod vibe.  Reckon I'll just go with a 2N5089 like the rest of the audio side transistors.

These are the first bad devices I've ever got from Tayda.

So.  Thanks R.G for your help.