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 Forum index » DIY Hardware and Software » YuSynth
VC-LFO2 problem
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kynsi



Joined: Sep 10, 2008
Posts: 14
Location: cookietown, NL

PostPosted: Thu Sep 15, 2016 8:34 am    Post subject: VC-LFO2 problem
Subject description: strange outputs
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I've been given a VC-LFO2 module to check because "it won't go slow enough".
While trying to calibrate it I noticed the strange triangle and square waves.
Also if I try to modify the square wave there is no 10% - 90% scale. It goes to 'straight' line at +3 and -3.
See attached pictures.

Any idea? I have already replaced the BC557B, 7555 and U3. The original builder (not me) used TL084 and TL081 chips. Could that be the problem?
Also the resistors seem to have the right values.

Thanks!
KS


Square50.png
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Measured at pin 8 of U2
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Square50.png



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gdavis



Joined: Feb 27, 2013
Posts: 359
Location: San Diego
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PostPosted: Sun Sep 18, 2016 12:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Have you trimmed T1? Do the range switch and rate pot have any effect on the frequency?

I would focus on getting the saw waveform correct before worrying about the other outputs (the other outputs are all derived from the saw). The saw output should go between -5V and +5V.

The clipped triangle is odd though. Makes me wonder if pin 1 of the 7555 is connected to -15V instead of GND. Does it use a board etched from the Yusynth layout?

I don't think the 81 and 84 chips would be a problem.

The 7555 should create a ramp waveform at pin 5 of U2 from 5V to 10V. U2b buffers this ramp (pins 6 and 7 should match pin 5), then U2a inverts the waveform, doubles the amplitude and shifts the offset as adjusted by T2a and b. When set properly, the waveform on pin 1 of U2 should go from -5V to +5V.

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kynsi



Joined: Sep 10, 2008
Posts: 14
Location: cookietown, NL

PostPosted: Sun Sep 18, 2016 7:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for your reply.

I replaced both TL084's and that temporarily, kinda, fixed the problem, but it's still not working correctly. I notice the 7555 and bc557 are getting warm, where the bc557 is even getting too hot to touch for longer than a few seconds.

The problem still is that it 'stops' if I turn the frequency too low. The waveform just disappears.

All the controls are working correctly and pin 1 of 7555 is connected to and.
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gdavis



Joined: Feb 27, 2013
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PostPosted: Sun Sep 18, 2016 3:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Too much current. The 557 should be controlling the current so probably a problem around there somewhere. Check all the voltages around the 557, U4 and R6-7. Particularly the base of the 557 and how it varies with the rate knob and T1.
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kynsi



Joined: Sep 10, 2008
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Location: cookietown, NL

PostPosted: Mon Sep 19, 2016 8:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Here are the voltages, calibration settings, max rate:

7555
1: 0
2: 7.5
3: 13.6
4: 14.5
5: 9.4
6: 7.4
7: 7.4
8: 14.5

741
1: -14.9
2: 0
3: 0
4: -14.9
5: -14.9
6: -10.6
7: 14.4
8: 0

557:
B: 14
C: 7.44
E: 14.4

557 - min rate
B: 14.4


Voltages at pin 2 and 3 of the 741 look a bit strange?
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gdavis



Joined: Feb 27, 2013
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PostPosted: Mon Sep 19, 2016 3:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

kynsi wrote:

Voltages at pin 2 and 3 of the 741 look a bit strange?


No, 0V is correct for those pins. The base voltage looks low. Not grossly out of wack but too low. I think it just needs T1 adjusted to get into a reasonable range.

The frequency that is shown in the scope shots from your first post are way too high (if I'm reading it right, you're at 50uS/div which means about 5kHz, shouldn't be over about 100Hz).

See if you can adjust T1 to get the frequency in a more reasonable range (base voltage above 14.5V). I'm hoping that this will also bring the emitter voltage up close to 15V. I'm thinking that it's dropping due to the low base voltage.

Once it's trimmed in a good range, there may still be other issues to work out, but we need to get this sorted first.

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kynsi



Joined: Sep 10, 2008
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Location: cookietown, NL

PostPosted: Mon Oct 03, 2016 9:46 am    Post subject: Reply with quote  Mark this post and the followings unread

It took a while but finally an update.
I turned down the trimmer and wave shapes look ok and the transistor is not getting hot anymore.

However I am still left with one problem: I can't get the LFO to slow down. If I lower the rate the lfo drops out. I can go to about 10Hz and then it just stops.
This is in 1X mode, calibration step 18.
If I look on the scope I can see that it becomes unstable and 'flatlines' by itself.
Maybe a different capacitor? Or should I look somewhere else?
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gdavis



Joined: Feb 27, 2013
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PostPosted: Mon Oct 10, 2016 1:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry, just saw your post. Looks like you've made progress Smile

I'm having a difficult time thinking of anything in the circuit that would cause this behavior. Maybe a bad connection. Is the trim pot a multi turn pot? Did you replace Q1 and the 7555 after fixing the overheating?

It sounds to me like there might be a problem with scope setup. Have you tried connecting the LFO output to something like a VCO input to see if it's working?

Sounds like the scope might be AC coupled instead of DC, though the screen shots in your first post show it DC coupled so if you haven't changed it that should be OK. Is the trigger set to DC coupling? Is sweep set to auto?

Analog scopes are usually more problematic for measuring LFO's, I would think the digital scope should be pretty straight forward. I'm not very familiar with the Rigol scopes, but according to one of the manuals I looked up, it should be able to go up to 50Sec time base. That should give you a waveform, though it would take a little while to draw.

Look at all the things I mentioned above and if it still looks like a problem with the circuit we'll have to start probing things again. Set everything up for step 18, adjust the trimmer to where it just loses the waveform, measure voltages at U4 pin 6 and Q1 base (you can also measure these by probing each side of R6). You can also try probing the rate switch with the scope and see if you get a waveform there. See if it behaves the same as the output as you go down to 10Hz.

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kynsi



Joined: Sep 10, 2008
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Location: cookietown, NL

PostPosted: Sat Oct 15, 2016 12:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

gdavis, somehow missed your post...
I tried the lfo today with new bc557 and 7555. Alas, no improvement. The strange thing is that the led also 'stops' so it's not the scope. When I try to lower the frequency it just stops blinking, but not gradually. If I press the bc557 (which is not hot) the circuits retriggers and the led starts blinking again, but becomes slower and stops after a while.
You would expect a bad solder connection or broken pcb trace, but it all checks out ok. I'm actually thinking of making a new pcb, although it's very strange I can't get this to work..
I'll recheck all the connections and voltages tomorrow.
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gdavis



Joined: Feb 27, 2013
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PostPosted: Sat Oct 15, 2016 5:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

kynsi wrote:
gdavis, somehow missed your post...
I tried the lfo today with new bc557 and 7555. Alas, no improvement. The strange thing is that the led also 'stops' so it's not the scope.

Oh ya, I forgot about the LED.

Quote:
When I try to lower the frequency it just stops blinking, but not gradually. If I press the bc557 (which is not hot) the circuits retriggers and the led starts blinking again, but becomes slower and stops after a while.
You would expect a bad solder connection or broken pcb trace, but it all checks out ok. I'm actually thinking of making a new pcb, although it's very strange I can't get this to work..

Yes, that does sound like a bad connection. Could be a micro crack or bridge which can be really difficult to track down since it won't necessarily register reliably on a meter.

I've had boards pick up tiny metal shavings that behaved funny until I brushed them off. Also had a board from a fab that apparently had a hair during exposure, tiny little hairline copper bridge, that one was fun to find.

Quote:
I'll recheck all the connections and voltages tomorrow.

Good luck.

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prgdeltablues



Joined: Sep 25, 2006
Posts: 222
Location: UK
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PostPosted: Sun Oct 16, 2016 4:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Just a thought, but what temperature is your workplace? I work in a hut in the garden, and an lfo I built did just what you described. Then I found out that as the hut warmed up it started working properly. So I tried using a hairdryer on the board, and that made the lfo start up. Some chips are only specced down to about 5 degrees centigrade.
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kynsi



Joined: Sep 10, 2008
Posts: 14
Location: cookietown, NL

PostPosted: Mon Oct 17, 2016 3:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Rechecked everything and resoldered some connections. And I cleaned the board with isopropyl and a brush.
I did some more testing and I now noticed that the LFO is slowing down at each rate, not only the slow ones. Even at high speed (100Hz) it slows down, but it was less noticeable. On the scope I can see it's slowing down. This happens with both capacitors (1x and 0.1x).
So I don't think it has something to do with the transistor after all. Touching it just changed it slightly because of temperature change.
I changed the 741 just in case but this had no effect.

The temperature is around 18-20 degrees C so shouldn't be an issue..

Any ideas? This is driving me nuts.. I've fixed OBX's, ancient digital reverbs, guitar amps, klee seqs etc, and now a simple lfo getting the better of me? Shocked
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prgdeltablues



Joined: Sep 25, 2006
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PostPosted: Mon Oct 17, 2016 4:14 am    Post subject: Reply with quote  Mark this post and the followings unread

Weird. That suggests leakage somewhere.
I'd suggest replacing C1a and C1b - possible they may have been damaged by the excess current you previously had going through the BC557 (which would have been charging those capacitors). Maybe take them both out and try with just one initially. Leakage in the rate switch would be another thought? If you could take that out of the circuit, that might help too (or show that it wasn't the cause). Actually, if you disconnected the saw bias section, and R9, that would isolate the 555 from the downstream circuitry (the rate is obviously determined by the 555, and I can't see how anything downstream would affect that, but often helpful to eliminate anything that isn't part of the sub-circuit that isn't behaving).

Peter
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gdavis



Joined: Feb 27, 2013
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PostPosted: Mon Oct 17, 2016 1:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'd suggest also looking at Q2. You may just want to temporarily remove Q2 to rule it out. I'd also look at C2 and it's connections, though I think a problem there would affect the amplitude as well as the frequency.
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kynsi



Joined: Sep 10, 2008
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PostPosted: Tue Oct 18, 2016 4:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes!! I took out Q2 and now it's working. I'll check if I have any spares in my shop.
Thanks!!!
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gdavis



Joined: Feb 27, 2013
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PostPosted: Tue Oct 18, 2016 4:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Awesome, congrats! If the replacement still doesn't work, make sure the gate is being pulled low by R36.
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kynsi



Joined: Sep 10, 2008
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Location: cookietown, NL

PostPosted: Wed Oct 19, 2016 10:27 am    Post subject: Reply with quote  Mark this post and the followings unread

Replaced BS170 and now it's working perfectly!
Thanks!!
wave
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AlanP



Joined: Mar 11, 2014
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PostPosted: Thu Oct 20, 2016 12:04 am    Post subject: Reply with quote  Mark this post and the followings unread

BS170's are ESD sensitive, so I'm not surprised that that was the failed part... Sad
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