Buchla 246 Sequential Voltage
Source Module

I was sent a 246 to modify similar to my 245 Sequential Voltage Source modifications.

 

I made reference designators from PCB images.  The components cover the silk screen legends once populated which makes it near impossible to find components.

PCB1 Rear Reference Designators

PCB2 Rear Reference Designators

 

Modifications

The original 246 used a 24V supply and the logic was a mix of DTL and early TTL that ran from +5V.  This redesign uses CMOS with a 15V supply and the 24V circuitry is simply driven from 15V with no compensating changes. In addition, a number of resistors were changed or added.

I modified a 245 and 246 and my customer's email back was "So the report is - they work flawlessly.  Null zone has been reduced to almost nil, adjacent stage pulse outputs now individually trigger the 281's - even three/four in a row.  I encountered no problems with the stage trapping - either open ended on either side or between two mid-points."

These modifications are the same as for the 245 so see my 245 Sequential Voltage Source page for detailed information. This page simply consists of the component reference numbers for the modifications specific to the 246.

 

Stage CV Input

PCB2:
With 16 stages there are no unused gates with floating inputs.
Add a 100 pF capacitor across R65.

 

Stage CV Output Level

PCB1:
Remove R27, R28 R29, and R31 (note PCB1).

PCB2:
Change R294, R296, R298, and R317 to 330K.
Remove C57, C58, C59, C61.
Remove R279, R280, R281, R282, R283, R284, R285, R286, R287, R302, R303, R304.
Remove Q75, Q76, Q77, Q79.
Connect the base to the collector with a link on Q75, Q76, Q77, Q79.
Replace R288, R289, R290, R307 with a link (corrected).
Replace IC31, IC32, IC33, IC34 with LT1637.
Swap pins 2 and 3 of IC31, IC32, IC33, IC34.

 

Stage Pulse Output Level

PCB2:
Change R55, R61, R72, R78, R86, R92, R100, R106, R114, R120, R128, R134, R142, R148, R156, R162, R170, R176, R184, R190, R198, R204, R212, R218, R226, R232, R240, R246, R254, R260, R268, R275 to 6K8.

 

Clock Waveshape

I adjusted the waveshape similar to the 245 but the baseline sits at 4V, not 2V (I believe) due to the number of stages. When I changed all the stage pulse output resistors to 6K8 the module would no longer advance.  I simply adjusted the clock waveshape trimmers until it functioned reliably.

PCB2:
Change R301 to a 10K trimmer.
Change R306 & R315 to a 20K trimmer (wiper at the node connecting R306 & R315).

 

Pulser Output

PCB2:
Change R30 to a link/wire.
Remove R42.

Reference  Output

PCB1:
Change R3 to a link/wire (note PCB1).
Remove R5.

 

This image shows the clock and CV output modifications on PCB2.

 

This image shows all the modifications to PCB2.

PCB2 modifications photo (larger PDF)

 

Here are the four CV outputs with a full 0 to 15V range.

 

 

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