LFO waveform shape of moogerfooger 102

[peter ripa]

according to the specs its supposed to be a square and a sinus but on my scope its definetely square/triangle.  
curious if someone else here has an mf102 and can confirm this as i dont suspect mine is broken, just some faults in the documentation

[Paul Perry (Frostwave)]

While yolu have the scope out Peter, can you check the peak to peak voltage?

[peter ripa]

its goes from +- 1.1 volt

[Paul Perry (Frostwave)]

Peter, according to the manual, it should be a sine, and it should be +- 1.5 volts. (unless you are inadvertently looking at the carrier output, which is +- 1.25V)
If the sine is generated by a triangle overdriving an OTA (as it is in my commercial Blue Ringer), then maybe the triangle for some reason is too low & not being flattened out to a sine by the OTA. Just a guess, I havn't seen inside one of the MoogerFoogers.

[peter ripa]

im looking at the LFO and its a triangle, just over +-1.1 V.

[mlabbee]

Peter - while you've got it open, can you see what the input drive section uses?  A while ago someone was showing me the Murf and how you could crank the input to get this amzing overdrive - very rich, creamy sound (the rest of the effect was basically turned off).  I'm curious to know what kind of input buffer/boost they use . . . .

[peter ripa]

yes the overdrive on the moogerfoogers is beatiful. i often use them in bypass mode, ie in the insert jack prefilter on my voyager.
i wouldnt know the circuit myself but i opened it and took some pictures. i´ll post the links when they´re up

[Paul Perry (Frostwave)]

I'm sure you could get that 'overdrive' by just wiring half a LM13700 (or NE5517) as an amplifier, and...overdriving it. (details in the data sheets).
I'm pretty sure that the LM13700 would be doing the work in there.
(appreciate a confirmation though, if anyone has one open).

In fact, a LM13700 amp, with a control to vary the current to the gain input (NOT more than 2ma, though!) would be a simple & useful project.
Remember the LM13700 is an OTA, not a normal op amp.
And note that Moog used to have a patent on using OTAs as distortion elements :wink:

[peter ripa]

there are four LM13700N on the board

if your particularly interested in the number of an IC and cant make it out of this pictures, let me know.

[Paul Perry (Frostwave)]

Thanks Peter, that's interesting! Those FPQ3906 are quad transistor packages, I believe.
I don't think I can say anything else, though.
All I can think, is try to trace the triangle backwards using the scope.
Be careful not to short pins 1 & 16 of the LM13700s to anything, though!
I've lost a few that way... :cry:

[puretube]

free information sucks...

[peter ripa]

it seems hard doing a sinewave. i have lots of analog gear but this is the only one claiming to have a sinus and apparently it doesnt.

puretube: which one do you mean? i got some closer up photos

[Paul Perry (Frostwave)]

It isn't hard to make a sine wave!
As I said before, you just make a triangle, and amplify it wiht an OTA (eg, half of one of those LM13700s). IF thelevel of the triangle is just the right height, you get a suprisingly good sine out (accuracy about 2%, which is surely 'close enough for rock and roll'). It's a standard thing to do. As close as most diode sine shaping networks.

[puretube]

U3 (?)

[peter ripa]

i dont know but you see the numbers clearer here