FET Switching Help

[stoneattic]

I'm trying to true bypass my DOD FX10 and removed the transistors marked in red in the schematic below.  Since the FETs are "normally open" and when the stompbox is powered up it turns on I figured pulling them would keep it hot wired on.  I pulled the 2N5089 as well to isolate the LED.

I have the 3pdt wired up (correctly I pretty sure).  But when I switch it on there's no change between bypassed and on, or at least not that's obvious, and the tone and levels don't work other than I can hear some scratching (dirty pot?) when I tweak the tone knob when the effect is "on". 

Any ideas as to what I'm missing or where my logic has gone wrong?

[R.G.]

I'm trying to true bypass my DOD FX10 and removed the transistors marked in red in the schematic below.  Since the FETs are "normally open" and when the stompbox is powered up it turns on I figured pulling them would keep it hot wired on. 
...
Any ideas as to what I'm missing or where my logic has gone wrong?

I've only had one cup of coffee, so I'm not as sharp as I would otherwise be at this hour, but it seems to me that the JFETs are not really being "bypass" devices, but instead modifying the circuit's operation. That's not quite the same job as the JFETs do in Boss and Ibanez pedals. I'll get to digging through it when I'm more awake.

But as for logic, it seems to me that:
- this circuit never "bypasses" anyway, it only changes its buffered operation
- it would probably be simpler than pulling JFETs to do the following: (1) install a wire across the 0.047uF cap to ground the cathodes of the diodes which switch the JFETs and (2) short the LED with a wire.

The LED is turned on all the time. When the bipolar it on, it steals the LED's current.

[stoneattic]

Thanks for the response.   I'm very much a  beginner trying to learn and I'm sure you on one cup of coffee are better than me fully awake. 

If you're right about the FETs not really bypassing (and I very much trust that are) it seems to me that pulling the FETs would result in the circuit operating in the "on" mode and wouldn't the level and tone ppots operate in both modes?  Or, I guess my question is what am I not following?

[PRR]

These FETs are "open" when the effect is off, "short" when the effect is on.

When the FETs are "open", U1 and U2 work as simple unity-gain buffers. They are in the audio path, but do no damage to guitar signals.

When the FETs are "short", the signal-ground on the Level pot wiper does a 2-way thing, but basically forces U1 to a HIGH gain. The action of the U2 tone circuit is not clear (even on a fourth cuppa) but surely it gives a high gain, more at some frequency than another. I think it shelves the treble up/down; IAC it does a "something".

For "stuck-ON" operation, you could blob solder Source-Drain on each FET. There is a remote chance that the FET nonlinearity "matters" (i.e. adds nice distortion). And I'm not sure of the logic (and too lazy to look-up a '4007). Perhaps the best course is to set the foot-switch so that the effect IS on, measure the switch to see if it is on or off, and apply the appropriate jumper or no-connection at the switch leads. (Or like the safety switch on my tractor: tape it into the "OK" position, wrap the wires, and drop it inside.)
And as R.G. points out: Current is sent to the LED at all times, but diverted by the '5087 when in "off" mode. That's a little wasteful of battery but the whole affair is thrifty enough that it does not really matter.

[PRR]

Ah, I see what the '4007 does. The foot-switch is momentary. The '4007 is the memory.

I think that 1K from pin 6 to ground will jam the '4007 into "stuck-on" mode. But wait and see if anybody agrees.

[Paul Marossy]

Maybe not needed at this point, but this is a generalized DOD switching scheme with a fully detailed explanation - http://hammer.ampage.org/files/dodswitch.gif

[stoneattic]

I've got it working correctly.  The "problem" turned out to be my own mistake.  I was jumpering the gate and drain, d'oh! 

Thanks everyone for the help.