Shared Projects by midnightpharmacy

Replace a ua726 heated dual transistor with a surface-mount LM3046 - from quick test on my 702 VCO clone it seems to work ok.

*note that this board as is will only work on 726s with pin 2 connected to ground (eg. System 700) - for other VCOs with pin 1 connected to ground (eg. System 100, SH-2/1, 100M) it’ll need reworking

The rest is non-surface mount, cos that’s what I’ve got.

Square pin on the ua726 connection is pin 10.

• C1 - 10nF

• R1 - 12K

• R2 - 1K

• R3 - 39R

• R4 - 3K

• R5 - 100K

• R6 - 51K

• R7 - 4.7K

• RV1 - 500R

• U1 - LM3046

• U2 - TL081

• U3 - (ua726 connection)

• U4 - 78L10 10v regulator

Set RV1 at half-way by measuring the resistance with a multimeter before installing.

On powering up, measure the the voltage at TP (bottom end of the capacitor) and twiddle RV1 so that the 3046 reaches the desired temperature. I’ve set mine to 570mV, http://www.sinneb.net/?p=634 suggests 550mV.

If you’re replacing a ua726 in a circuit, you may need to fiddle with the CV input resistors to give the correct range to scale the oscillator.

For example, on the 702 I had to change R9 from 50k to 68k to give the 10k range trim pot at VR10 enough leeway to be tuned in.