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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Echo Rockit VCF fault... troubleshooting
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muffin_man



Joined: Nov 08, 2018
Posts: 4
Location: Germany

PostPosted: Thu Nov 08, 2018 8:57 am    Post subject: Echo Rockit VCF fault... troubleshooting Reply with quote  Mark this post and the followings unread

Hi there,

I just built the Echo Rockit, and am having some issues that I can't seem to fix myself! I have a multimeter and a scope.

Input buffer, LFO and oscillator are working fine. However:

- first issue - something's dragging the virtual ground slightly towards Batt+ (running from an adaptor giving 11,1V, I get +4,8V/-6,5V)
Is that normal behaviour of this device? The LFO's output gets clipped at the top end Sad

- second: the signal does not pass even the first state of VCF! Signal amplitude at X24 is about 5V - thus after the voltage divider R2/R27, the LM13700 sees 50mV amplitude at the input. But - the Bias inputs (pins 1 and 16) are constantly dragging down close to Batt-... even though voltage at U1-7/R24/R26 is GND with the cutoff pot turn all the way up - after R24/R26 I get about -6V no matter what, and no output from the Opamp! I've re-checked numerous times for correct parts, placement, bad connections, tried 3 different ICs, even changed Q1 and Q2. Anyone has an idea what could be causing this behaviour?

- third issue is the unbearably noisy PT2399 which starts a loud crackle if delay's over 50%, and/or if LFO mod is over 2 o'clock... a common problem I guess, but I was wondering it really the IC itself is the problem. I ordered them from china. Are there huge differences in cheaply manufactured versions of this device?

Thank you all!
muffinman
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muffin_man



Joined: Nov 08, 2018
Posts: 4
Location: Germany

PostPosted: Thu Nov 08, 2018 1:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

I got a little further:

- if I remove R27, R34 and R22, the voltages get symmetrical, so the voltage drift is due to malfunction of VCF. Still, Im sure that everything between R1/R2 and C3/C4 is assembled as shown in the schematic. Again, double-checked all values, traces, joints etc.

- The noise only persists if the current into the VCO pin of PT2399 (pin 6) falls below about 2mA. That starts to happen if the voltage at U1-14 rises (!). If I remove U1 and put a resistor from Batt+ or GND to the base of Q3, the delay is working smoothly without any noise.
Odd thing, since I was expecting the current to increase as U1-14 rises. What's going on? Did I misunderstand the current sink circuit?
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muffin_man



Joined: Nov 08, 2018
Posts: 4
Location: Germany

PostPosted: Thu Nov 08, 2018 3:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Found the reason for the noise issue - the voltage was too high! I feel so stupid... didn't expect the circuit to be so picky. Now running off a 9V battery, delay/LFO mod works. Still, no sound coming thru VCF though.

For reference, in case someone really is reading this, here are the voltages:

Supply is now 3,4V/-4,3V, consumtion of powered circuit 50mA

LM13700 pins:

1: -3,1V
2: NC
3: 0,02V - 0,04V square wave from oscillator
4: 0,1V
5: 2,8V
6: -4,4V
7: 2,8V
8: 1,6V
9: 1,6V
10: 2,8V
11: 3,5V
12: 2,8V
13: 0V
14: 0V
15: NC
16: -3,1V

I'll assemble that portion on a breadboard to see if I'm really losing my mind or not...!
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muffin_man



Joined: Nov 08, 2018
Posts: 4
Location: Germany

PostPosted: Thu Nov 08, 2018 6:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Oops, sorry, of course amp bias pins 1 and 16 stay negative, since they're current biased!

I'm getting closer to the problem it seems: If I just leave out Q1, feed the oscillator and LFO to VCF, set cut off at 50% and resonance to 0, I can see the pulsing wave with a scope at the output U4-5! Only if the JFET is in place, the signal dies... tested with multiple MPF102 and a few 2N5486.

Any ideas?
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g.gabba



Joined: Nov 29, 2008
Posts: 445
Location: berlin
Audio files: 23

PostPosted: Thu Nov 08, 2018 10:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

hi muffin_man,
at least iam reading,
but have no idea how to help.
good luck!
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PHOBoS



Joined: Jan 14, 2010
Posts: 4088
Location: Moon Base
Audio files: 605

PostPosted: Tue Nov 13, 2018 4:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

mmmmm muffins,

If you are powering it from a 9V battery then the total voltage between pins 6 & 11 of the LM13700 seems a bit low as those are connected directly to the battery.
Could be the battery, could be something shorting out a bit pulling the supply voltage down, maybe one of the pins is connected wrong or there is a bad connection
somewhere.


also
welcome party!

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