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previous: Jim L. Jack Ormans "True-Bypass" -- 5/10/1999 2:33 PM View Thread

Ormans "True-Bypass" and Fixes for It

5/10/1999 3:24 PM
Ormans "True-Bypass" and Fixes for It
This is one of the ills of using a darlington for this kind of switching.  
From your description, you have a darlington with a fair amount of leakage. The initial switch turns the darlington off hard by sucking the base charge from the darlington into the output coupling cap of the effect. The darlington leakage then charges the output cap back up enough to cause collector-emitter leakage to light the LED.  
You can do a couple of things.  
(1)hand select darlingtons for low leakage  
(2)parallel the LED with a resistor to make it less sensitive to darlington leakage  
(3)Use a discrete MOSFET device with diode leakage pullup as in the Millenium 2 bypass.  
I recommend option 3. It's less sensitive to device characteristics. To make what you have into a Millenium 2, just get a to-92 mosfet like the BS170 or 2N7000, and replace the MPSA13 with it, drain for collector, gate for base, source for emitter. Then replace the 10M? 22M? pullup resistor with a 1N914 or 1N4148 signal diode, anode to +9, cathode to gate. This works just as the earlier one did, but without either the leakage or possible clicks.  
For long term protection, use a low leakage diode from ground to the gate of the MOSFET, something like the collector base of an NPN transistor or the gate/source junction of a JFET - reverse biased of course.

Jim L. RG's Y2K Bypass PROBS. -- 5/11/1999 1:42 AM