LFO 2 no triangle or sine waves

mfsp · Joined: Mar 24, 2017 Posts: 5 Location: Germany

Hello everyone,

I'm trying to build one LFO 2 but I'm experiencing some issues.

Since I had no triangle and sine waves, and after reading some of the topics related with the LFO's, I've decided to measure the resistors values and I've found that some of them are "burned" - the original values have decreased a lot.

R27 should be 22K now 19.38K
R12 should be 27K now 23.60K
R22 should be 49.9K now 30.70K

all of these should be 100K
but now they are
R11 58.6K
R9 58.5K
R20 66.2K
R19 66.4K
R21 65.9K

R24 R26 both should be 150K now 107K
R23 should be 200K now 125K

Any ideas of what might be causing this?

Thank you in advance,

Marco

Grumble · Posted: Mon Nov 27, 2017 12:31 am Post subject:

Did you measure these values while the resistors where in the circuit?
Because if you measure resistances or capacitors "in situ" their values will be influenced by the components connected to them.

mfsp · Joined: Mar 24, 2017 Posts: 5 Location: Germany

Hello,
Thank you so much for your answer.
Do you mean this much? Is it possible?

Grumble · Posted: Mon Nov 27, 2017 5:20 am Post subject:

I don't have the schematics of this LFO, but i'm sure it is not just possible that these values vary that much, but even very likely!
You might even discover that there will be a difference between the resistor values if you switch the test leads of your multi-meter caused by the internals of opamps or transistors.

Boerge · Joined: Sep 02, 2009 Posts: 82 Location: Germany

First of all: I assume, you built this one:

http://yusynth.net/Modular/EN/LFO/VC-LFO2.html

No tri, no sine means: you have rectangles and saw? That would proof, that your LFO generally works. So, your measuring could be wrong (never measure resistors in the circuit unless you are able to calculate roughly the influence of the circuit!).
I would check the tri/sine converter parts, IC3b/c/d, specially the orientation of the diodes (if you used the 1N4148 the ring is the cathode). Check the inputs and the outputs of IC3b-d with a scope. I'm quite sure, that your problem lies somewhere in that part of the circuit.

regards

Boerge

mfsp · Joined: Mar 24, 2017 Posts: 5 Location: Germany

Hello,

Thank you for your replies.

I've just checked and I can confirm that this values were indeed altered by the other components. Removed from the circuit, the values are as they should.

Back to square one Smile

Why are there no triangle and sine waves?

Correct that's the one I'm trying to build. Yes, I have saw and pulse. I've changed both TL074 and still no good. I have the 1N4148's as seen here
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Accepting the suggestion: testing, testing Smile

Thank you,

Marco

Boerge · Joined: Sep 02, 2009 Posts: 82 Location: Germany

Additional question: in what position is the trimmer T3 (Sine/Tri symmetry)? Always start with middle position of all trimmers! The trimmer should not have that big influence to prevent the circuit from working, but if R10 is to small, it could happen.

mfsp · Joined: Mar 24, 2017 Posts: 5 Location: Germany

I didn't know that Embarassed

That's something worth trying. Thank you.

mfsp · Joined: Mar 24, 2017 Posts: 5 Location: Germany

Hello,

Unfortunately I have to wait before proceeding with further tests. Yesterday I was using my oscilloscope and it started to smell like burned electronics Sad

I will be back with additional info regarding the lfo as soon as possible Smile

Thank you,

Marco

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Hi there, was there ever a solution to this problem? As I have a similar issue in that the triangle is a uniform saw which also resultis in a weird sine. I have a good saw and square, so that would leave me thinking that it’s somewhere within the triangle wave shaping that the problem lies. I have changed every ic every transistor, all caps several resistors. Also t2a and b have no effect on the circuit what so ever and I have also changed those. Oh and also I have changed the four diodes. Also the duty cycle of the pulse starts at 70%.

Thanks for any help, Ryan

gabbagabi · Posted: Mon Nov 18, 2019 9:42 am Post subject:

i can remember that ive forgot to solder in one of the brigdes, this was causing a strange sine

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Thanks for the reply. I am using a Soundtronics pcb that has two layers so there is no need to solder the bridges. I have tried everything I can think of. Funny thing is that the vco uses an almost identical saw to triangle wave shaper, and it seems to work fine. I’m at a loss.
Also the saw bias trimmers don’t seem to have any affect on the circuit whatsoever, I have tried changing them in case they were faulty but to no avail.

gabbagabi · Posted: Mon Nov 18, 2019 11:04 am Post subject:

well just fishing around:

measure the outta pins of the saw offset trimmers, one pin should have 0V and the other about 10V, if pots/trimmers have no effect often one side is not connected

if you have a good saw, where do u measure it? what is the amplitude?

note also that in yves schematic is an error, R17 (200k) exist two times, but one should be 100k (ic2a in the schemo)

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Here is a (cheap) scope image from pin 14 of ic 2, same reading as pin 7.
I’m only getting 20% of the duty cycle of the pulse too.

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RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Oh and I should add that I have been through the other posts on the forum and did spot your R17 suggestion, which fixed my initial problem of an irregular saw. So thanks for that one mate. I may have a fiddle with the PWM resistor value tomorrow to see if that can give me more range from the pulse width

gabbagabi · Posted: Tue Nov 19, 2019 8:33 am Post subject:

np,
are u shure the saw is now 10Vpp?
i cant say it from the scope.
you also do scoping on AC?

for more PW range you could try do decrease R37 and R38 to 10k

gl

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Thanks mate I was thinking Somthing along those lines, I’m quite new to all of this, especially using a scope, I will change the setting to dc when I’m home and take another reading. What would I be looking for to know the amplitude of the wave? Thanks Ryan

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Here is the scope set to DC. As I mentioned I’m very new to this so still not 100% sure how the read these things properly. Thanks for your help though mate

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gabbagabi · Posted: Wed Nov 20, 2019 4:14 am Post subject:

ur welcome, we all have started some-time-some-where

i can imagine there are tons of yt-videos about scopes,
but the most basic thing is measuring the amplitude of a signal:

count the divisions between the highest and the lowest point of your signal:

yours has roughly 7 Divisions, then you multiply the divisions with volt-per-division-value, which is in your case 1V/DIV

so your signal has an amplitude of about 7Vpp (peak to peak)

or on your scope u could calculate Vmax - Vmin=7,39V
but the waveshapers are expecting 10Vpp!

hm, that smells somehow like a 12V powersupply?

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gabbagabi · Posted: Wed Nov 20, 2019 5:05 am Post subject:

just saw that u mentioned a 12V power in the other tread.

so in theory the amplitude should be 12V*66%=7,92V
in reality you have measured 7,39V

u can correct that by changing the resistor combination R14/R17,
by default they should 100k/100k

theory: the factor between 10V and 7,92V is 1,262
so we could use:
100k / 126k
79k / 100k
68k / 85k
50k / 63k
39k / 50k

reality: the factor between 10V and 7,39V is 1,35
then it would be:
100k / 135k
74k / 100k
68k /92K
50k / 68k

or just use a trimer, do you have one laying around?

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Thanks for the detailed response. I have also plugged it in to a 15v supply with the same outcome. I have also replaced the bc557 to see if it helped as originally I used a 557c when it is specified 557b, same results. I may try another one, never know it may be faulty, although unlikely.
So would the trimmer replace the two resisters you mentioned? Ryan

gabbagabi · Posted: Wed Nov 20, 2019 6:10 am Post subject:

u have the same result with 15V? kinda strange.

if you are going to use a 100k trimmer, he would replace R14

if you are going to use a 50k trimmer, he would be used in series with R17

i cant imagine that swoping the bc557 will have any effect on the amplitude of the signal

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

Ahh ok, I thought it would be a long shot. I will try your suggestions this evening. I’ll let you know the outcome, hopefully it will be good news👍
Many thanks Ryan

RyBowk · Joined: Nov 25, 2018 Posts: 33 Location: England

No luck with the trimmer. I’m really at a loss now

gabbagabi · Posted: Thu Nov 21, 2019 5:23 am Post subject:

u give up?