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gregcomposition

Joined: Oct 20, 2013 Posts: 15 Location: Poland
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Posted: Thu Nov 07, 2013 2:21 pm Post subject:
LFO troubleshooting |
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Hello everyone!
I've been watching this forum for a few months but now I decide to don't bother Yves by private mails and join the group.
Some years ago I bought on ebay a LFO's pcb as a project. This year finally I've started to make a 3VCO yusynth system and I wanted to put the old LFO in. When I finished, wired and trimmed the LFO module I realize unfortunately that it doesn't sound like an LFO. The sound is drifting down. I mean - the pitch of rate is slowing down constantly (no cycles at all). The waveforms on scope are not ideal but looks quite good.
As I have the basic electronic skills I don't know how to fix it. I've read all topics about LFO's issues but find no solution.
Anyone of you have similiar problem? Any advice?
Thanks for answer.
Greg
I've attached photos of pcb (as you can see it's in poor state in some areas)
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gregcomposition

Joined: Oct 20, 2013 Posts: 15 Location: Poland
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Posted: Sat Nov 09, 2013 12:05 pm Post subject:
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I replace BC557B, LM741 & TLC555. No results.
That's all I can do  |
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ranioItaly
Joined: Feb 18, 2013 Posts: 13 Location: Italy
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Posted: Wed Nov 27, 2013 8:45 am Post subject:
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hello,
could you attach some photos of oscilloscope wave shape?
so me and someone could try to help you.
bye |
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gregcomposition

Joined: Oct 20, 2013 Posts: 15 Location: Poland
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Posted: Thu Nov 28, 2013 10:25 am Post subject:
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Hi!
Thanks for interest.
First of all, I realized that I wasn't clearly enough in my first post.
Obviously my issue is connected with too much high frequency. For some reason I couldn't dive too low by the rate pot and T1 trimmer.
In the meantime, I resoldered all solder points and replace c10uF 25V for 35V (I don't know is it relevant?). Then I decide to make a test, the same as Sebo made for Yves:
Quote: | OK check the voltages at the output of the 741 (pin 6). First adjust T1 to have 0V at its wiper pin. Then when cranking the frequency pot from min to max you should measure a voltage varying from 0V to -7V. Next measure the voltage at the base o Q1, there you should get something a voltage from 14,55V to 14,35V, that is a variation of 0,2V. If you have values significantly diffrent from these then the problem comes either from R1,R3,R5,R6,R7 or U4. |
So I removed BS170 and measured 0V to -10.80V at pin 6 of the LM741 and 13,80V at base of Q1.
I replace R1,R3,R5,R6,R7 (by 1% in place of 5%).
That's all I did.
Now... Core signal is still unstable. When switch is on 1.0 position I have a frequency from 1000 to 8000 Hz and close to 8000Hz the signal is falling down about (50Hz/s), then about the middle scale of rate close to 3000Hz it is rising (10Hz/s)and when rate pot is close to minimum the signal is falling again.
In 0.1 switch position the frequency has the range from 20 do 1000 Hz - in the maximum the signal is rising up (10Hz/s), then close to 400Hz is drifting up and down and close to minimum - falling down again (5Hz/s).
So - the low frequency is appeared (that is a little progress) but when signal is starting to make a cycles (finally!) on a slow rate - it has maximum 1s long and soon it's falling down and became constant low (no cycles).
I tried to take a photos of the waves on my analog scope but with this unstable signal it is impossible - all photos are blurred.
Apropos: what about the waves? Now the waves are not good as before. 10V sawtooth looks like at added .jpg file.
The sine and triangle waves are far from their ideal, only the square/pulse looks good.
I hope this time I explain my issue better.
Thanks in advance for any help!
Greg
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ranioItaly
Joined: Feb 18, 2013 Posts: 13 Location: Italy
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Posted: Sat Nov 30, 2013 5:50 am Post subject:
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hello greg,
sorry if I do a stupid question, but Have you made a correct "Settings and trimming" as shown at the end of yusynth web page?
for me the frequencies that you say aren't right.
from web page:
" the frequency can be set with the RATE knob between 0.1Hz (10s repetition rate) and 100Hz (0.01s repetition rate) Switch on 1.0 position
or between 0.01Hz (100s rate) and 10 Hz switch on 0.1 position"
try to set properly T1 47K ohm trimmer for obtain the frequency as suggested by Yves.
bye |
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gregcomposition

Joined: Oct 20, 2013 Posts: 15 Location: Poland
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Posted: Sat Nov 30, 2013 11:33 am Post subject:
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Hi!
I'm a beginner in electronic world, but I think the most important thing in this case is an unstable rate impulse. In fact this is the main problem. Right now it's impossible to set the frequencies properly. The range of frequencies from my previous post was an example of rate changes. If we can set the stable rate we will set the right frequencies easily. I made LFO#1 before and everything was ok so the triming process was simple & successful.
Anyway thank you for trying help. I'm appreciated.
Greg |
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gregcomposition

Joined: Oct 20, 2013 Posts: 15 Location: Poland
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Posted: Sun Dec 22, 2013 10:32 am Post subject:
SOLVED! |
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Problem solved!
You never guess what was the reason of issue: partially-damaged wires.
I couldn't find the fault too long and I gave up - I made the new PCB, I bought new parts, I used the wires from the old PCB and... it wasn't working .
I checked several times all connections, solder points, wires and all seemed to be ok. Then I accidentally bent the wires and the unit back to life for a one second
So after replacing the wires, the LFO is in perfect working order.
I have to keep in memory : always change old wires to new one.
Best regards,
Greg |
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yusynth

Joined: Nov 24, 2005 Posts: 1311 Location: France
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Posted: Sun Dec 22, 2013 12:11 pm Post subject:
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Glad you found yourself. This typically the type of problem that cannot be remotely sorted out. One must have hands on it to figure out where the trouble comes from. _________________ Yves |
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gregcomposition

Joined: Oct 20, 2013 Posts: 15 Location: Poland
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Posted: Sun Dec 22, 2013 4:52 pm Post subject:
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Hi Yves!
Yes, you are completely right.
But I'd like to have just a some electronic knowledge to know which part is responsible for which reaction. Then I would think to myself: "Hmm, to fix the issue I have to check R6 & R7". But, I'm still learning. Who knows, maybe 50 years from now, I can understand it
Anyway thanks again for sharing your stuff. It's a great module, as the rest of your system.
If you don't mind, I'd like to ask about another thing. The last issue of the first cabinet I made is a "saw animator grounding problem". It's a very strange thing. I have to plug a jack PARTIALLY into a mod_in input to start the internal control of animator/knobs react. If I plug a jack FULLY - it "switched" animator to external control/knobs not react. With nothing plugged into a mod_in input there is no effect - you can tweak the knobs left or right and the leds are blinking slow or fast as they should, but it not animate.
What you think, what might be a reason?
All my best,
Greg
By the way, Merry Christmas! |
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yusynth

Joined: Nov 24, 2005 Posts: 1311 Location: France
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Posted: Tue Dec 24, 2013 12:32 pm Post subject:
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Check that the jack socket is connected correctly. In particular that you did not exchange the tip lug and the switch lug of the socket. _________________ Yves |
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gregcomposition

Joined: Oct 20, 2013 Posts: 15 Location: Poland
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Posted: Tue Dec 24, 2013 4:33 pm Post subject:
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I was afraid you say that
Yes, all seems like a simple incorrect connection, but I've checked it several times before and replaced twice with no results. I use a new one socket, but it doesn't help too. I guessed that it could be bad IC or something else. But if you said it should be a jack socket connection - I will try again to the bitter end. Thank you for your help.
On a side note, probably I could be wrong, but I think there is a typo in a saw animator's "The wiring diagram". Rate knob #2 has got misplaced connections according to the schematic.
Again Merry Christmas!
Greg |
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