208 pulser offset

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papz
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208 pulser offset

Post by papz » Fri Sep 27, 2019 3:57 pm

The 208's pulser ramp output at the yellow banana sockets oscillates between 1.5V and almost 15V and doesn't reach 0V.
The offset voltage depends on the period time, the wave moves with respect to 0V : when the period is in audio range, with the slider set to the max, it's about 0.5V only.

Although not a problem in most uses, a customer reported that he couldn't clock his Keen Association 220 keyboard with his 208p's pulser because of this.
It also gives a snappier sound when the pulser controls a LPG.

Adding an offset circuit on board 4 between IC1 pin 12 and IC4 pin 8 makes it possible to move the ramp bottom to 0V. Of course the former trace should be cut.
The circuit can be on veroboard and mounted with a short standoff to the back of the board.

When the trimpot is adjusted to cancel the 1.5V offset, the ramp bottom goes to -0.5V when the pulser runs in audio range.

This offset schematic by Jeroen Baars works fine.

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Last edited by papz on Sat Sep 28, 2019 11:13 am, edited 2 times in total.
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ArguZ
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Post by ArguZ » Sat Sep 28, 2019 1:48 am

That is very elaborate, thank you :)
Question, wouldn't a simple resistor change to the input of IC1 (pin14) or a value change to R41 work too ?
We need to lose around 1-1.5V only.

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Post by papz » Sat Sep 28, 2019 10:58 am

I first tried to change R41 value but it didn't help.
Finest EMS gear service and Music Easel/208 goodies

I'm not a synths dealer and I'm not aware of an inexpensive Synthi secret market, please don't ask me if I know one for sale.
I don't offer support of any kind to people attempting to build clones of EMS equipment.

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