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4001 is used in a "crowbar" formation, where it goes between +9V and ground, with the cathode pointed toward +9V. It shunts the voltage if you plug in the wrong polarity power supply -- which protects the components but will create a situation where your power supply can supply enough current to eventually (and by "eventually" I mean "very quickly in real life") destroy the diode, creating a dead short to ground. However, a battery can't supply enough current to kill a 4001, so it works to protect the components if you momentarily connect the battery the wrong way around. The worst that happens then is you drain the battery a little.

Using a 1n5817 in series with the power supply means that if you connect the wrong polarity of power supply, you will simply get no voltage passing, because the diode will block it completely. The price you pay is the loss of a trivial amount of voltage (0.2V in this case).

When people were powering their effects mostly with batteries, using the crowbar method with the 4001 was perfectly acceptable because it did exactly what was needed and sacrificed no voltage, which could matter when the battery became particularly drained. Now, though, almost no one uses batteries, especially in DIY pedals, so it's better to just use the way that won't kill a pedal.

Years ago, but thanks!

Get both trimmers to their mid-points... A/DAade the way these are adjusted kind of strange. The cRange trimmer adjusts how wide the difference is from min. frequency on the sweep to the highest frequency is on the sweep. The cMax trimmer adjusts what the max frequency is of the sweep.
So if you're getting 1.2mHz, check what the min frequency is of your sweep. If it's much below what the minimum frequency is supposed to be, then the cRange trimmer needs to be adjusted to bring the extreme ends of the sweep closer together.
It's kind of hard to explain... CRange adjusts how wide the frequency sweep will be. CMax adjusts how high the highest sweep frequency will be.
Hope this helps.

Thanks!
Yes, I watched some video(s) on calibrating the A/DA flanger and it seems to follow the same principle. One for moving the full interval and one for settint one end point.
Worked on the functional(?) Flanger and it takes many iterations to get close.

So for the Countdown Phaser it is the CLKRANGE and MAXFRE, but they amount to the same task.
Just to be clear. I got 1.278 Mega Hz.
Anfter some experimenting last night, the only pot that had any effect was RANGE which would make the TP frequency go from 1.225 MHz to 1.285 MHz. Which is too little, but at least some reaction.

I got a bit impatient so I cloned my questions to https://www.diystompboxes.com/smfforum/index.php?topic=129793.0

I also just finished the Flintlock Flanger, by Lectric-FX as well, and here the calibration works. So I can adjust the trimmers and the DMM is showing Hz as expected. Still a bit of tweaking to do, a few more steps and YT videos to check. But that project is moving along.
So I think the gear is OK to use for the Countdown as well.

On the Countdown, I played around with tweaking the knobs around, but it is pretty much stuck on 1.2something MHz as mentioned earlier.
I get bypass and some kind of effect signal, but it sounds more like a cocked wah than anything phase-ish.

(I might post this on DIY stompboxes forum as well)

Just finished with Countdown Phaser from Lectric-FX and managed to get hold of a DMM measuring Hz, I tried to calibrate it.
I have changed the Clkrange and Maxfre trimmers to "multi turn" ones.
I jumper T and R, I turn the big pots to full CCW, I switch on the board. It measures 9V and 15V where it should.
I connect black cable (COM) to ground and red (Hz etc) to TP and switch over to Hz. I get about 1.290 MHz. No matter how I turn, it is the same. (If I move around on the board I can get a couple of 50Hz which I guess is from the wall wart or what might remain from any AC). I can also get a few other readings on some of the legs, so I think the DMM is sound, battery and all.
Since this was a big project I decided to solder each component in, then check for continuity from its leg to the eye(s) on the PCB where it was connecting to. Also for each of the socket "holes". It was painstaking and made it all take much longer, but I am trying to avoid cumbersome troubleshooting with so many parts.
So I cannot swear I did not do any mistakes, but I feel I have at least minimized that part a bit.

Any ideas on what could be the problem, why there is no change when dialing the trimmers?