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I recall there being a speed modification for this circuit that involves switching C21 to something like 0.47uF (putting a SPST switch in there to go between that and the stock 2.2uF). Has anyone done that? Or have thoughts on that? I believe the 0.47uF will give you much faster range for the speed.
As well, I have seen 2 ways of doing a rate LED. One way is tapping off pin 7 of the IC4 for the positive voltage with a 4K7 resistor to ground. Another was I have see is tapping off pin 1 of IC4 in the same fashion. Is there any difference here? I ask, because it would be very easy to tap off of pin 1 as it goes to Pot1 and you could easily solder a wire onto the pot leg coming through the board, whereas soldering to pin 7 (or any components connected there) might be a bit more challenging.
Thoughts?
I usually will do this by removing C21, wire the middle lugs of a DPDT switch to the C21 pads. Mount 2 different cap values to either side of the DPDT. Now you will have 2 different speed ranges.
I marked up the schematic for the mod tapping points. I prefer to use the junction of R23 & R25 (they’re also joined to pin #7 of IC4B) for the Rate LED indicator – only because I have an inkling there’s a bit more current on tap at that point to drive the LED properly. But that’s just me.
Do you have a set of preferred caps for the selectable speed ranges?
I ended up checking at pin 1. Seems to drive the LED just fine there. Don’t think there is any detriment to tapping at that location. Also, it was accessible after the board was mounted in the enclosure. Though, I can see that the junction of those 2 resistors would be an easy place on the back of the board to soldering in a wire for this mod.
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