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Little LFO Problem |
redcon1
Learning to Wiggle
Joined: 26 Feb 2010
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Location: Fife
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 Posted: Sat Jun 30, 2012 2:44 pm Post subject: Little LFO Problem |
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One of my three boards doesn't oscillate. Swapped chips between the boards, swapped board (and connections) on the panel, problem stays with the board. Nothing obvious in terms of shorts/broken tracks/bent pins. Anyone got any thoughts?
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Synthbuilder
Super Deluxe Wiggler
Joined: 24 Mar 2009
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redcon1
Learning to Wiggle
Joined: 26 Feb 2010
Last Visit: 06 Jul 2012
Posts: 7
Location: Fife
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| Posted: Tue Jul 03, 2012 2:44 am Post subject: |
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| +1.88V and +7.34V respectively. Or so I thought. Leaving it on for several hours, it became apparent that while the pulse output wasn't changing, the triangle was, but with a period of several hours! (irrespective of pot settings). That looked like a huge time constant on the RC bit, but there are no shorts. Time to swap out the capacitor? |
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Synthbuilder
Super Deluxe Wiggler
Joined: 24 Mar 2009
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| Posted: Tue Jul 03, 2012 3:52 am Post subject: |
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| redcon1 wrote: | | Time to swap out the capacitor? |
Could be. I'm assuming you don't have the range switch fitted - if so change the range and if that oscillates it is probably the C1 capacitor or the tracking to it.
If you don't have a switch but do have C6 fitted then simply short out S1 and S2 with a little bit of wire (no need to solder it - just shove the wire in the holes). If that makes it oscillate, it's either C1 or the tracking to it again.
The other thing it could be is a broken pot. Set the rate pot to fast. Then check the voltage output of the wiper (middle pin) of the rate pot. It should be similar to the voltage on the pulse output. If not the rate pot is either busted or not connected somehow.
The shape pot is harder to test with just a DVM.
Tony
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redcon1
Learning to Wiggle
Joined: 26 Feb 2010
Last Visit: 06 Jul 2012
Posts: 7
Location: Fife
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| Posted: Wed Jul 04, 2012 3:58 pm Post subject: |
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Shorting S1 and S2 reduced the voltage as expected, but didn't start the board oscillating, unfortunately!
When the voltage on the wiper was 11.43V, the pulse output was 7.36V - different enough to suspect the pot? Otherwise, the pot behaves as expected with the resistance changing smoothly between 0 and 10K as it should, and the connectivity between the pot and pins 1 and 3 of U2 is fine.
I guess the next step is to swap the pots with those from one of the other two boards?  |
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Synthbuilder
Super Deluxe Wiggler
Joined: 24 Mar 2009
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| Posted: Thu Jul 05, 2012 2:36 am Post subject: |
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The rate pot sounds like it's OK.
| redcon1 wrote: | | I guess the next step is to swap the pots with those from one of the other two boards? |
Not yet - but it could be the shape pot.
With the power off, try checking the continuity between pin 1 of U2 and pin 2 of U1. Move the shape pot as you do this. One end (anti-clock) should produce a resistance of around 6K8. The other end should be much higher.
If you don't get around 6K8 in the anti-clock position you have a problem with the pot or the connections to the pot.
Tony
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redcon1
Learning to Wiggle
Joined: 26 Feb 2010
Last Visit: 06 Jul 2012
Posts: 7
Location: Fife
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| Posted: Thu Jul 05, 2012 11:31 am Post subject: |
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| Synthbuilder wrote: |
If you don't get around 6K8 in the anti-clock position you have a problem with the pot or the connections to the pot.
Tony |
Yup, it's a crap pot - teach me to check them before plugging them in.
Thanks a lot! |
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