One part that was difficult to locate was the 5Meg Release potentiometer. It would appear that stock rationalisation has in many cases limited the availability of high value potentiometers. The highest value obtainable was 2Meg, so I set about trying to do the same job with a dual 2Meg pot configured in series.

The Release pot configuration is that of a variable resistor, with the fixed resistor setting the lower value and the pot resistance the upper value. Adjusting this fixed resistor value slightly allows me to match the lower value in the CCW position. The maximum value is now 4Meg instead of 5Meg, which reduces the maximum release time by about 25%.

Here'e the original and alternative circuits.


Release pot replacement circuit



















And a spreadsheet plot of the end-to-end resistance curves of both linear and logarithmic types of pots for both the 5Meg and 2Meg dual configurations. Notice how the 274K resistor has been changed to 300K to achieve the CW resistance of 260K.

Release pot resistance curve















At the last moment, prior to assembly, two 5 Meg linear pots were located and ended up being used.

Interestingly, their end-to-end resistance was only about 4.2Meg, so the dual 2Meg configuration would have been an accurate substitute anyhow.