Unlatch Mod?

[12StringStratMaker]

Hey guys,
I was wondering if it's possible to mod a Boss pedal (preferably the TR-2) to have the unlatch feature like on the old VB-2 Vibratos?  
I'll leave you geniouses to ponder.

[R.G.]

Trivial. Rewire the discrete FF to follow the switch instead of latch. Or just wire around the FF.

[12StringStratMaker]

Can you put that in stupid person terms?  I'd like to put in a switch to make it optional.  I'm glad it's possible though.

[12StringStratMaker]

bttt

[R.G.]

Look at this one:


You need a miniature DPDT toggle switch and one 56K resistor. You cut one PCB trace at the base of the flipflop transistor and patch the broken ends out to the switch. You undo the wire at P1 from the switch, then rewire P1 and the wire from the switch to the DPDT. Then add the 56K to +9.

Lemme know how it works.

[troubledtom]

that's one of the biggest slam down post i've see'n in a long time.
   it's nice to hear from R.G. ...........(&^%(&^$(     ............)
                       -tt

[12StringStratMaker]

So I tried it and it's backwards, it functions as it used to on one end of the switch and on the other end it's always ON unless I press the pedal???  What now?

Thanks for the great schematic though.

I'll recheck I did it right later tonight

[R.G.]

So I tried it and it's backwards, it functions as it used to on one end of the switch and on the other end it's always ON unless I press the pedal??? What now?

Therefore, we can see clearly that I got the wrong one of the two transistors in the flipflop - or you did.

Dang, I thought I had that correct. The transistor that has a low collector when the LED is on has to be the one that causes the thing to go into effect, not bypass, I reasoned.

However we got here, the solution is to do exactly that to the other transistor in the flipflop. That will reverse what the switch does.

So - are you sure that you got the transistor that does NOT have the check diode going to its collector?? That ought to be the correct one.

[12StringStratMaker]

Ok so no luck in getting it right but here's what I found,

1) the leads from the break by the transistor were backwards (lead from transistor going to outside lug of on-on dpdt, lead from other side of transistor break going to middle lug), this caused the pedal to be off only when you press it (functions normally with the switch normal way)

2) the leads in the correct original way you said give me it always on, pressing pedal has no effect (functions normally with the switch normal way)

3) going with the other transistor in the way you suggest puts it always off (functions normally with the switch normal way)

4) making those leads backwards like in 1) but on the other transistor, makes the check led function in the way we wanted, light only on when you press but no effect in on or off (functions normally with the switch normal way)

Hope this can help us find a solution.  Thanks for all your help so far.

[12StringStratMaker]

Oh wait, I think I thought of what might have gone wrong, I'm not at home right now so I'll let you know what I find.  Thanks all.

[12StringStratMaker]

The cases that I mentioned earlier stand, what I thought of didn't help...

[R.G.]

Hope this can help us find a solution. Thanks for all your help so far.

I'll do more navel contemplation and get back with a fix.

This just can't be that hard. The flipflop we're working with can be thought of as two cascaded inverter stages with feedback to the input, making a 100% positive feedback amplifier. This thing locks up with its output in one state or the other. The toggle switch introduces just enough of a kick to the input to make it flip over.

What I did was to break the feedback loop and feed the input from a fixed resistor to +9. That makes the output always high. The previously used "toggle" switch is a momentary to ground, and I hooked that to the input as well, so that when you press the toggle switch, the input of the two cascaded inverters is grounded, and the output is low.

The two inverters are identical, so you can call either one "input" and the other "output" depending on where you want to break the chain. I picked the one that I thought caused pressing the toggle switch to make the effect come on.

I can think of a couple of possibilities. One is that the resistor I used to pull the base up is too big, and will not turn on the transistor enough. Another is that your toggle switch may not pull the transistor fully off.

Can you do this: put a multimeter on the base of that transistor, and see what the voltage is there for the case of (a) footswitch not pressed and (b) footswitch pressed; then measure at the collector in both cases.

That will tell me what is going on.

[12StringStratMaker]

Can you do this: put a multimeter on the base of that transistor, and see what the voltage is there for the case of (a) footswitch not pressed and (b) footswitch pressed; then measure at the collector in both cases.

That will tell me what is going on.

Ok I measure:

Base:
 On = 0.024V
 Off = 0.023V

Collector:
 On = 0.028V
 Off = 0.613V

I did this with the pedal in stock state, I had to use the pedal for a gig last night so I had to take the switch out, but will try to mod it again when we figure something out.  Thanks

[R.G.]

We'll have to do debug in the altered state.

My simulations and tinkering here show it should work as drawn, so we need to see if we can find some wiring bug or something.

[12StringStratMaker]

Alright, I modded it back up as you said.  Since in the modded state it is always on, I could only test the transistor in that state.

Collector: 0.028V (on)
Base: 8.58V (on)

I then wired the leads from the transistor backwards (so that it's only turns off while you press the pedal), just to get some readings:

Collector: 0.025V (on), 0.595V(off)
Base: 7.06V (on), 0.00V (off)

[R.G.]

OK, we have a problem.

If the transistor has its emitter tied to ground as it should, the base simply can't get more than 0.7V or so positive. So there's some more detective work to do.

In the modded state, I need to know the base voltage when the footswitch is pressed and not pressed, even though it's always on. Could you do that for both transistors?

Also, is it possible that the trace cutting you did also opened up the emitter?

[12StringStratMaker]

OK, we have a problem.

If the transistor has its emitter tied to ground as it should, the base simply can't get more than 0.7V or so positive. So there's some more detective work to do.

In the modded state, I need to know the base voltage when the footswitch is pressed and not pressed, even though it's always on. Could you do that for both transistors?

Also, is it possible that the trace cutting you did also opened up the emitter?

Well I rechecked at the base with the switch in stock state and I still have what I said (0.024V both on and off with switch in stock mode) so unless I'm mistaken the emitter is opened up.  Wouldn't that make the stock mode not work right too? (but stock mode works fine)

As for pressed and not pressed in modded mode:

Q7:    
     Base:  0.00V (Pressed)      8.58V (Not Pressed)
     Collector: 0.028V for both cases

Q5 (check light transistor):    
     Base: 0.079V (Pressed)     0.79V (Not Pressed)
     Collector:    0.008V (Pressed)    0.658V (Not Pressed)

[R.G.]

I apologize for the following - I'm in the throes of a head cold and I don't have the sense to be complete in my questions.

I should have asked you to measure the voltages on all three transistor pins, including emittter, and I didn't. That would tell us whether the emitter is open. I think it has to be. The voltages don't make sense otherwise.

The simulation I've done tells me that the mod does work, so we just have to get the wiring details down to make the real one work.

So - could you measure collector, base and emitter for both transistors in both modded and original form, with footswitch pressed and not pressed. Sorry again - it's a pain, but it should give us what we need.

The base at 8.?? and 0V is an indication that the mod is doing the right thing to the base, I think there's just so nit that's wrong elsewhere.

[R.G.]

Oh, yeah - please measure the votlages on the transistor pins themselves, not on the pads to which they are theoretically soldered.

[12StringStratMaker]

Oh, yeah - please measure the votlages on the transistor pins themselves, not on the pads to which they are theoretically soldered.

I still put the other probe on the pedal ground right?