MFOS 16 Step Quantized Sequencer- Last few problems

[hububalli]

Hello all,

After months of fiddling I have my sequencer almost completely working. I have a couple of problems left that I cannot solve though.

The quantizing works perfectly accept for on step 6. I can only get a few step durations, not the whole range like the other steps. After reading through the MFOS page and here, I am thinking this might be my answer "observe U1-A pin 2 , must be approx 54Khz""

I have a multimeter with a Frequency function but I am not sure how to use it in this situation. What state should the sequencer be in and what pins am I measuring across to get the reading? I have tried what I thought were the obvious choices but I am not getting any reading. I don't want to short anything by randomly sticking the probes in places they don't belong

Am I right in thinking U1-A pin 2, means IC U1 (CD40106), -A means the 1st inverter in the CD40106, and pin-2 of that inverter which is pin 2 on the IC (2nd down on the left)

Sorry for the rather noob question, this is a whole new world to me.

Any advice as to the exact locations to test would be appreciated.

Update:

Just tried using my better multimeter and I am getting a reading now. Not sure what's wrong with my little meter!? I am still unsure about where to probe. If I probe pin 2 and ground on the IC while the sequencer is running I get 84.8Khz.

[sixbyseven]

I noticed there are some issues (albeit older posts) that ask about the 54KHz +/- 1Khz for the clock speed on U1-2,

I just built this circuit and I too get 88.893 KHz. The LED timing is not accurate and the trim does little to solve it.

So I built just that section onto a breadboard and measured my resistor at 20.02 K and my capacitor at 681pF and sure enough I get 88.373 KHz.

I was using a multi layer ceramic (C0G) type, so I threw in a cheap ceramic disk type marked 681, but measured out at 1.2nF

I then got 49Khz.. I remembered a calculator online..

http://www.talkingelectronics.com/pay/BEC-2/Page49.html

It turns out that if you use a 20K resistor and a 680pF capacitor, the theoretical calculated frequency will be 88.235 Khz

So are the values on the schematic accurate?

There is a thread here on this site: http://electro-music.com/forum/topic-54288-50.html where he added a trimpot in place of R26. The theoretical ratio is so tight between the capacitor and the resistor that hitting the bounds of the component's tolerances will send it way over the +/-1KHz Ray's instruction ask.

I will try this this week and let folks know if I use a 50K trim pot in place of R26, to 'dial in' the 54Khz frequency required.

Does this sound reasonable or.... is there a better way?

[sixbyseven]

I pulled up one side of R26 and added a multi-turn 20K trimpot. Sure enough, I was able to tune it to exactly 54.21 Khz and the counter worked great, except I did not have a 2 count (just one led on) or a 4 count (the 2 and the 1). It was weird! It would go to no LEDs, 2 then 4, then 4&1..... perfect up to 4&3&2&1. But no 1 or 2&1

I tested the frequency drop and voltages on U2-14, U4-5 and U3-3 and noticed I could not get the appropriate voltage or frequency for a 2 or 3 count. Every other step had perfect voltage and frequency. As a side note, with each step the voltage at pin 3 of U3 should increase about 0.176 volts per step, and decrease about 3.318KHz per step (U2-14).

But at step 2 and 4, both frequency and voltage would just skip. Spent hours checking solder joints, component values and it had me perplexed. So I thought if the TL074 (U3) was flakey.

I replaced it and everything now works dead on. If I was doing this again, I would just replace R26 with a 35-50K trimpot.

[hububalli]

Glad to hear you got it sorted. The solution to my problem actually turned out to be a dodgy IC as well. Sorry I didn't post my solution in the end, might have saved you some time!

Was an interesting read, I used cheap ceramics as I didn't have anything else for a couple of values to hand. Seems might have saved myself a few problems.

[solaneu]

As someone who had the same problem with some step durations being skipped I wanted to post my experience here so it might be useful for someone in the future.

First of all, with 680p cap and 20k resistor it did not work at all for me, all the leds were on all the time. So finding this thread and using the calculator posted in this thread, I came to values 1nF for the cap and 22k for the resistor, which gave 54.54 kHz. With these values it kinda worked, but skipped some steps, like sixbyseven described. Trying out a few TL074s I came to the conclusion the problem was not the IC. So I replaced 22k with a 50k trimmer and got it to work. Measured the trimmer afterwards, it was around 24k. Luckily I had a few 24k resistors laying around replaced the trimpot with one and it finally worked. Calculator gives out 50 kHz for this combination but important thing is it works.

I used a 1nF C0G ceramic capacitor with 10% tolerance.