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Hi !

I'm building the "clean" variant with C11, C12 and C16 replaced with 220nF for use with a bass (suggestion by TH).

I wonder how to adjust the trim pots. Is there a specific bias voltage to look for ? Or I just tweak them until I like the sound ?

Thanks !

Ignore me :-)

I didn't wire up pins 5 and 7 and 10 and 12 together. It works now :-)

Cheers,
Ben.

Hi folks !

I hope somebody can be a assistance, otherwise I'll continue digging...

So I'm waiting for my Flunkee PCBs (a friend in the US ordered them for me and will get them to me, but it will take a while) so in the meantime I put it all together on a breadboard to play around with the kind of mods that can be done etc...

However, I have so far failed to get it going, and while I'm generally "ok" at debugging simpler circuits, I don't fully understand (yet) the 13700..

(Note: I've started from the dod schematics one can find around and switched to the Flunkee ones eventually, but I'm still using the two halves of the lm13700 the way the original dod does, though I don't think that matters).

There are my observations so far, any hint would be appreciated:

- First the bias voltage (VB) drops to a bit above 3V (ie, if I unplug the supply, the resistor bridge r23/r24 provides 4.5V but it looks like the various other resistors on that net will pull it further down). I assume it's normal but I'm asking nonetheless :-) (mostly I suspect via R8 -> R12 -> R18 which totals 45K extra path to ground)

- I've fed it with a sine wave from a generator (1KHz,  +/- 800mV). I can see the first part of the LM1458 (the buffer) output it just fine. The second LM1458 amplifies it. Past C5 (on the neg leg, or between the two diodes) I observe half sine waves with a roughly 4 to 6V amplitude depending on the SENSE pot, so far so good. C5 and R6 flatten and bias this, generating a VR envelope of about 3V to 4.5V (depending on SENSE) when a sound is played to a bit above 1V when not. So far it looks like somewhat reasonable but remember I don't fully understand the circuit :-)

- Where things get less clear is the other path out of the first 1458, via C6 and R7 to the + input of the first  half of the 13700 (in my case I use the first half like the original fx25, Flunkee use the second but my undertanding is that they are identical). I can see my signal the output of C6, however still biased positively by about 2.9V. I assume this is R7+R9 doing that. *However* if I probe past R7, ie, on the + input of the LM13700 (pin 3 in my case) the signal is a LOT flatter. IE, I see something that vaguely oscillate around the 2.9V bias voltage but it's only 200 to 300mV peak to peak. Pin 4 (the - input) gets between 2.7 and 2.8V (depends on envelope), and pin 5 is flat outputing about 7.8V. Nothing that look like a signal at that point. Pin 1 (coming from the envelope circuit) is at about 1.2..1.3V (depends on SENSE).

Now I tried 2 different LM13700, I've verified their V+ (9V) and V- (0V) on pins 11 and 6 respectively, etc... I basically get no signal past the first LM13700 stage.

Any idea what I might have got wrong here ? :-)

Cheers,
Ben.