LFO troubleshooting

gregcomposition · Joined: Oct 20, 2013 Posts: 10 Location: Poland

Hello everyone!
I've been watching this forum for a few months but now I decide to don't bother Yves by private mails Wink

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)

LFO_top.jpg

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1.18 MB

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LFO_bottom.jpg

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1.26 MB

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gregcomposition · Joined: Oct 20, 2013 Posts: 10 Location: Poland

I replace BC557B, LM741 & TLC555. No results.
That's all I can do Crying or Very sad

ranioItaly · Joined: Feb 18, 2013 Posts: 13 Location: Italy

hello,
could you attach some photos of oscilloscope wave shape?
so me and someone could try to help you.
bye

gregcomposition · Joined: Oct 20, 2013 Posts: 10 Location: Poland

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:

ranioItaly · Joined: Feb 18, 2013 Posts: 13 Location: Italy

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

gregcomposition · Joined: Oct 20, 2013 Posts: 10 Location: Poland

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

gregcomposition · Joined: Oct 20, 2013 Posts: 10 Location: Poland

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 Mad

.
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 Very Happy

So after replacing the wires, the LFO is in perfect working order.
I have to keep in memory Idea

: always change old wires to new one.
Best regards,
Greg

yusynth · Joined: Nov 24, 2005 Posts: 1243 Location: France

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.

gregcomposition · Joined: Oct 20, 2013 Posts: 10 Location: Poland

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 Laughing

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, santa

Merry Christmas!

yusynth · Joined: Nov 24, 2005 Posts: 1243 Location: France

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.

gregcomposition · Joined: Oct 20, 2013 Posts: 10 Location: Poland

I was afraid you say that Smile

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