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 Forum index » DIY Hardware and Software » YuSynth
LFO - floating frequency
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Moevot



Joined: Oct 16, 2019
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Location: Poland

PostPosted: Thu Nov 07, 2019 12:55 pm    Post subject: LFO - floating frequency Reply with quote  Mark this post and the followings unread

Fights continued ... At the LFO workshop. The first works very well. The second one is worse. I have a problem with frequency. At P1 the minimum frequency gradually decreases to zero, and at the maximum slowly increases to several kHz. In the middle position it also swims. I disassembled BS170 and then changed 555 and TL071 and TL074, checked all resistors, capacitors, switch, potentiometers, several BC557B. Symptoms indicate the leakage of current, i.e. capacitors, switch and elements of this circuit, unfortunately I checked everything and found nothing. It is possible that the fault is BC557 but I checked several. I noticed that the transistor temperature has a big impact on the frequency. The transistor gets very hot, which is probably the reason for the increase in frequency.When switched to the 0.1 range, the oscillator behaves similarly. I am inclined to do the whole again on a new pcb. Has anyone had such a case?
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Blue Hell
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PostPosted: Thu Nov 07, 2019 6:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

Did you check R12 to really be 27k?
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Jan
also .. could someone please turn down the thermostat a bit.
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Moevot



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PostPosted: Thu Nov 07, 2019 10:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes. I replaced a new one
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Blue Hell
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PostPosted: Fri Nov 08, 2019 10:34 am    Post subject: Reply with quote  Mark this post and the followings unread

Is Q2 shorting things out?

This a bit of an odd construction .. no current limiting ... anyways .. there can not be too many things causing Q1 to heat up ... so trying to search for that might help.

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Jan
also .. could someone please turn down the thermostat a bit.
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Moevot



Joined: Oct 16, 2019
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Location: Poland

PostPosted: Fri Nov 08, 2019 1:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Q2 I had soldered out. I increased R8 to 22k to limit the frequency. Oscillates to about 95 Hz, the transistor does not heat up. Heating increased the oscillation and the oscillation heated the transistor and a vicious circle formed. I bought a BC557 from another lot but it was still a problem. Increasing the resistor helped. The gradual decrease in frequency at P1 set to the minimum was caused by the transistor cooling after heating at high frequencies. One thing that worries me is the instability of the system. A change in temperature causes a change in frequency. I don't know if this is correct.
I also had a problem with synchronization. 4 x BS170 and Gate Out in the sequencer got off. Maybe some discards because I bought from a Chinese man Smile.
Sequencer repaired, LFO works, I'm worried about temperature instability.
I checked the second LFO I made, I did not pay attention to it before, but the frequency is not stable, but not to that extent.
Best regards.
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PHOBoS



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PostPosted: Fri Nov 08, 2019 1:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

yeah it's indeed a bit odd, if Q1 and Q2 would be 'on' at the same time it creates a dead short from +15V to GND.
Does everything (apart from the sync) work ok if you leave out Q2 ?

hmm, the 7555 could also cause a similar problem. At least an NE555 has an internal transistor that connects pin 7
to GND, but that might be different for a 7555.

edit: I guess I was a bit too slow typing my reply Laughing

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Blue Hell
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PostPosted: Fri Nov 08, 2019 3:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm wondering about that dot between Q2 and R5 .. isnt that a mistake? (as in that there should not be a dot there).
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also .. could someone please turn down the thermostat a bit.
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PHOBoS



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PostPosted: Fri Nov 08, 2019 4:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

You might be on to something, although I wouldn't expect any errors in his designs.
But without that connection the circuit does seem to make more sense.

Yves usually has a PCB design let me check that out,..
ok, it's also on there so I guess it's not a mistake.

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Moevot



Joined: Oct 16, 2019
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PostPosted: Fri Nov 08, 2019 4:45 pm    Post subject: Reply with quote  Mark this post and the followings unread

You interested me with this dot. If there was no dot, the discharge of the capacitor should occur through the internal transistor 7555 on pin 7 and not by shorting to ground. And that would also make sense.Out of curiosity, tomorrow I will check on a living organism what will happen without this dot.

The synchronization is working but there is one more problem described here.
http://electro-music.com/forum/viewtopic.php?highlight=lfo&t=33669
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