Neovibe LFO won't oscillate ... help, R.G.!

[R.G.]

George, just to eliminate the possibility of pot problems, remove the speed pot wires from the board and replace them with fixed 47K or so resistors at the wiring pads on the board.

Can you tell me the version number of the board?

Do you have documentation from Frank that came with the board or are you using the docos from my web site that are current?

[george]

Redhouse:  I made sure the depth pot was turned half-way

Alderbody:  sorry muy bad too! didn' read you post! no I'm not using all metal-film resistors - some are carbon film possibly 5% tolerance.  Good suggestion, I'll try it out.  I 'll try something a bit higher-gain for Q12 too.

RG:  OK I'll give that a try, I did check the continuity from the emitter of Q12 back through the speed pot(s) and it seemed OK and the resistance was about that of the parallel resistance of 220K and 4.7K

The version I think says Rev A.  I know there are some bugs with cap orientation with the early versions, but the only one that was shown the wrong way around in the layout was one of the 10uF caps that connects to the depth pot.  That's now in the right way and replaced with a tant.

I'm using the layout that Frank sent me.  I did check your doco for speed pot wiring though, assuming that the assignment of C, D and E points hadn't changed in any revision.

I've even scanned the board and drawn in the components against the traces then compared to the schemo that Frank also sent me.   Everything is correct that I can see.

Can you think of any reason why the voltage at 3K3/2M2 is 12.5 not the "correct" value of 11.7?

Thanks again

[R.G.]

I wouldn't sweat the 12.5V versus 11.7V. It's probably from the slight difference in the main power voltage. This thing is pretty tolerant of that.

If you scanned the top and bottom, can you email me the scans?

The description you give is exactly what happens when the oscillator has too little gain or too light a load. It's resonant all right, as evidenced by the few cycles of pulsing, but the feedback is not enough to keep it running, so it dies away. In many real uni's this happens from a capacitor going bad, or one of the transistors going low gain. I don't know what is happening on yours.
Send the scans and I'll take a look at them, maybe I can see something. Worst comes to worst, you can mail it here and I'll figure it out. That may be...s...l...o...w... because of my other commitments, but I've never seen one of these be unsalvageable.

[george]

Thanks R.G. you are truly a legend.

I replaced the caps so it must be the trannies, although I did test them on my DMM before inserting them - they were showing the right sort of hFE for the types I was using.

I'll send you my scan of the underside, and a photo of the top of the board and the components