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4017 based sequencer: voltages wont hold

Here is the schematic of a sequencer I'm working on, which may be familiar to some here as I've received lots of help with the design.

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I had to add a "voltage shifter", to convert the voltage levels from swinging between 0 and 12V to swing from 6V to 12V (this is because the sequencer feeds into a VCO that uses a rail splitter on the 12V supply, so works from 6-0-6, and needs a control voltage between 0 and

6V).

The problem is that the voltage at Vout from the sequencer wont hold its value - at each step, the voltage set by the associated pot appears then rapidly falls to zero. I think that I've looked at this too long now, and can't see the wood for the trees. It appears that I am discharging Vout through a capacitor. Do I need to add a "sample and hold" circuit?

If someone could help, I'd be grateful.

Thanks! Julian

Reply to
Julian Bunn
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Seriously, is this a test for the rest of us?

with all that you have done, it's obvious that C12 is going to block the DC from the ramp level..

You need to remove that and tailor the R's in the input to balance the circuit.

But you most likely already knew that any ways!

Reply to
Jamie

Seriously, is this a test for the rest of us?

with all that you have done, it's obvious that C12 is going to block the DC from the ramp level..

You need to remove that and tailor the R's in the input to balance the circuit.

But you most likely already knew that any ways!

Reply to
Jamie

C12 looks as if it was the bad idea. If you want Vout to be held indefinitely, remove C12. and connect R16 directly to the emitters of T11 and T12 as well as R6.

I imagine that C12 was originally intended to be connected to the other end of R16 (the common point with R16, R17 and IC5/3, with the other end returned to ground to slow down the transitions at Vout.

I've not looked at the circuit for long enough to have much idea of what it is supposed to be doing, so there could be other strange connections.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

Thanks Bill - yes, in retrospect C12 was a bit stupid of me! I'll short it out and see if that fixes the problem.

Also it seems I have R17 and R18's values the wrong way around.

Thanks again. Julian

Reply to
Julian Bunn

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As I said, I couldn't see the wood for the trees ... no test - I had spent too long in Spice and my brain was fuddled :-)

Reply to
Julian Bunn

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