Blomman "BLM" 277
Signal Delay Unit

I built a BLM 277 for myself. This PCB and panel is available from The Beast UK.

 

The PCB is a vivid blue. I made reference diagrams from the PCB. The front has no silk screen so I noted the pinout of the LED and the taper of the potentiometers.

 

BLM 277 Front Reference

BLM 277 Rear Reference

 

Construction is straightforward. The default long delay time is when the switch is open so it is oriented with "on" to the left.

 

I chose to use a vintage red lens for my module instead of the snap-in black holder. These lenses fit loose on 1.5mm Buchla panels. In the past I have used hot glue to hold them snug but now slice a 0.25" ID grommet in half and fit it over the rear to holds them snug. The other advantage is the LED is no longer fastened to the panel in case I have to remove the PCB.

 

 

Operation

I took this oscilloscope measurement to see if the delay between stages was equal. I used a narrow pulse to be able to confirm the delay time. As also in the 277r, the delay between 1/4T and 1/2T is the same as between 3/4T and T, but the delay between 1/2T and 3/4T is longer. I suspect this is due to the A/D conversion time and is a fixed time. Also note the RC decay increases between 1/2T and 3/4T due to the additional capacitor coupling between Spin chips.

 

I chose to not take any other measurements as the beauty of this module is in its sound. Episode 13, as featured in the Muffwiggler Morton Subotnick Buchla Interview thread from the Source of Uncertainty podcast, features a review of the 277 which has many sound samples. Morton is worth a listen as well.

 

 

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