Function f(x) - Gate Crasher

Seiche · Post by **Seiche** » 30 Apr 2020, 14:55

function f(x) has put a gutshot pic of their gate crasher on insta.

Not a lot going on, might be a reasonable "guesstimate trace" possible and maybe even CJ chimes in to help.

The lower part seems to be for the relay bypass switching and 10F320 PIC and not really relevant to the circuit itself.

There seems to be a CD4049UBE (the red llama chip), and a CD4070.

A quick google suggests the CD4070 has been used before on the fnord fuzz and xorcist, and in this circuit.

Seiche · Post by **Seiche** » 01 May 2020, 08:51

so after some research and listening to youtube clips of this pedal i found this post by parasit studios regarding a square wave fuzz using a CMOS hex inverter with a red llama like input section and two more stages configured as a schmitt trigger. Looking at the cap values this might be the ticket into a tim escobedo digital octaver fuzz type output section.

Seiche · Post by **Seiche** » 01 May 2020, 14:05

Guesstimate circuit with the part values of the gutshot.

I omitted power section (the big polarized caps at the top, 100R and diode, I'm guessing) and the relay circuit.

culturejam · Post by **culturejam** » 03 May 2020, 23:25

Not a bad guess!

You've got two square wave shapers in there, however. Only one is necessary. So either keep the 4049 setup as is and get rid of the first XOR gate, or keep both XOR gates as drawn and get rid of the second and third inverters. You'll get the same result (more or less) with either setup.

What's missing: Texture control (LPF at one of the inputs of the XOR) and gain makeup stage after the XOR using another inverter from the 4049.

Seiche · Post by **Seiche** » 04 May 2020, 05:57

Thanks for chiming in

So what's that 10K pot in the escobedo section that I labelled "TXT?" doing?

Nocentelli · Post by **Nocentelli** » 04 May 2020, 07:31

Seiche wrote:Thanks for chiming in

So what's that 10K pot in the escobedo section that I labelled "TXT?" doing?

Blend? Dials the in the amount of octave

Seiche · Post by **Seiche** » 04 May 2020, 10:34

so there is no "drive" control, but also no leftover 1M, if one of the is used as a pulldown (maybe not)

culturejam · Post by **culturejam** » 04 May 2020, 20:13

Nocentelli wrote:Blend? Dials the in the amount of octave

Correct.

Seiche wrote:so there is no "drive" control, but also no leftover 1M, if one of the is used as a pulldown (maybe not)

There is no control of the input gain directly, correct. It's fixed gain with the feedback resistor set to 330K. There is a 1M pulldown at the input of the first 4049 amplifier.

Nocentelli · Post by **Nocentelli** » 05 May 2020, 10:13

Working on schem...

Seiche · Post by **Seiche** » 05 May 2020, 10:46

I'm pretty sure the 100R connects to the gate control to stop oscillations on 0. You can see it on the pcb.

: gate control.JPG (15.75 KiB) Viewed 3317 times

So I'm thinking the input resistor is the 10k under the 4049 and the 330k goes in the feedback path so swapped from my first guess.

I also think they might be doing it with the schmitt-trigger and without the first XOR gate, as this gate control is also described in the Parasit Link. Not sure it works the same with the XOR square fuzz.

Nocentelli · Post by **Nocentelli** » 05 May 2020, 11:37

Seiche wrote:I also think they might be doing it with the schmitt-trigger and without the first XOR gate, as this gate control is also described in the Parasit Link

I thought there were not enough 1M resistors for a schmitt trigger made from two of the 4049 inverters?

culturejam wrote:There is a 1M pulldown at the input of the first 4049 amplifier.

Seiche · Post by **Seiche** » 05 May 2020, 11:52

I assumed they used one in the first feedback section instead of the drive control (= drive on full) but CJ clarified they used the 330k instead.

culturejam wrote:There is no control of the input gain directly, correct. It's fixed gain with the feedback resistor set to 330K.

So we have one pulldown 1M and have the other left over for the schmitt trigger feedback section.

Nocentelli · Post by **Nocentelli** » 05 May 2020, 12:01

I can only see one 1M in the upper section (assuming that all the components below the row of pots in the middle are used in the bypass switching).

Seiche · Post by **Seiche** » 05 May 2020, 12:04

Ah, I figured R1 could be the pulldown, because the signal has to come from the relay to go to the 4049. C1 might also go to gnd at the input.

These are just guesses, trying to match component values to a gutshot. The caps and resistors on the bottom left might also not be part of the switching, or maybe additional power filtering for the 7805.

culturejam · Post by **culturejam** » 05 May 2020, 17:12

Seiche wrote:Ah, I figured R1 could be the pulldown, because the signal has to come from the relay to go to the 4049. C1 might also go to gnd at the input.

Correct on both.

What you have drawn is almost exactly right. The few variances won't make a difference in functionality, but might change tone slightly.

Don't forget to tie off the unused logic gate inputs.