Weirdest Thing Ever

[kurtlives]

So I did a super easy standard (or so I thought) mod to my EHX Small Clone. I changed the red LED indicator to a blue one and added a Bezel.

Now the LED works but now when the effect is on I don't get chorus. The bypass sound and engaged sound are identical. I did not touch and components and all wires are good.

Help please.....

[raulgrell]

Only thing I can think of:

Try changing it back to the original. If it starts working again, it might be an issue with current draw or voltage being put to ground.

No clue how to solve it though...

[kurtlives]

I no longer have the original red led, I will try another one though.

[hellwood]

maybe you could measure the resistance of the two LED's and add a resistor for the blue one to compensate for the difference?

[ayayay!]

maybe you could measure the resistance of the two LED's and add a resistor for the blue one to compensate for the difference?

That will probably fix it.  Blue (actually most all water-clear) LED's have twice the forward voltage as plain old red diffused ones.  Ever tried water-clear LEDs in clipping stages vs. standard red or green?  Usually WAY more output at the sacrifice of some distortion.  Anyway, chorus's are a different monster altogether to me, so all I can say is try it...

Hey kurtlives, did you see my note on your other thread about removing the shrillness from your DS-1? 

[kurtlives]

ya I used water clear high brightness.

No I did not.

[kurtlives]

Ok so ya it was the LED. I went back to an old boring read one and it worked. I would like a Blue LED though. So what kind should I get then? Couldn't I add a resistor to limit the current...wait I dont think that would work.

[Sock Puppet]

http://s273.photobucket.com/albums/jj219/designacryl/?action=view&current=led.jpg

This should work:

Replace R25 with a higher value, say 47k/100k.  Connect your LED with current limiting resistor added across this.
Short out the original LED so the footswitch common connects directly to Vcc.

Cheers,

S.

[ayayay!]

No, you're on the right track kurtlives.  Try it.  That's why it's called DIY.   

Try Sock Puppet's suggestion too. 

[kurtlives]

// AN - watch your language

Yesterday I put a red one in and it worked.

SO today I tried a blue one with a resistor, a green one and a red one. All did the same thing...the LED came on but no chorus.

Help me....I am goign to shoot someone soon...

[gez]

Could you post a link to the whole schematic?  Difficult to tell what's going on with the switching because only a fragment has been posted, so the following may not be correct (I'm just guessing). 

I'm assuming that the switch is 'no connection' when engaged as shown in the schematic linked to; and that throwing the switch pinches off the FET.  If so, then the LED is not an on/off indicator for the whole circuit, but a status indicator for chorus/whatever.  Yes?  If this is so, then perhaps the other LEDs have a lower threshold and aren't pulling the gate of the FET down to pinch-off level?  Varying R25 isn't going to do much, then, as the voltage is clampled by the threshold of the LED you're using.  A fix might be to incorporate a resistor between the anode of the LED and Vcc.

Don't start hacking up your circuit though, not until a schematic is posted (I'm a lazy ****er, don't like wasting time searching for them, and so the above may be total misinformation).

[gez]

Could you post a link to the whole schematic?  Difficult to tell what's going on with the switching because only a fragment has been posted, so the following may not be correct (I'm just guessing). 

I'm assuming that the switch is 'no connection' when engaged as shown in the schematic linked to; and that throwing the switch pinches off the FET.  If so, then the LED is not an on/off indicator for the whole circuit, but a status indicator for chorus/whatever.  Yes?  If this is so, then perhaps the other LEDs have a lower threshold and aren't pulling the gate of the FET down to pinch-off level?  Varying R25 isn't going to do much, then, as the voltage is clampled by the threshold of the LED you're using.  A fix might be to incorporate a resistor between the anode of the LED and Vcc.

Don't start hacking up your circuit though, not until a schematic is posted (I'm a lazy ****er, don't like wasting time searching for them, and so the above may be total misinformation).

Wasn't paying attention.  Was thinking about this whilst lying in bed and realised the above can't possibly be correct.

It's the other way round.  The LED pulls the gate up of the already pinched-off FET when the LED is connected.   As the threshold of the new LEDs is larger, it isn't being pulled up enough.

Fix?  Perhaps a different FET might work in there.  Alternatively, use a DPDT switch: one side switches on the LED, the other pulls up FET's gate.  Perhaps there's another fix, but I need some sleep now.  Good night...

[gez]

If you don't want to go down the route of using a DPDT, the following might work out OK (depends how much 'creative soldering' you have to do).  I think this is what Sock Puppet was mooting. 

Disconnect the 10K (R25) from the junction of the 4.7M (R24) and the switch, and wire in your LED between them: cathode to upper leg of the 10k and anode to the switch/R24 junction.  Keep the red LED soldered in as shown (for the moment), but use the blue/new one as your status indicator.

Ideally, you'd want to remove the red LED so that it can be used for another project, but pulling everything up to Vcc might introduce switching noise.  You could short the LED with croc-clips to see what happens.  If you get switch glitch, then making C16 slightly larger (or sticking a cap in parallel with it) might sort things out.  Alternatively, pull everything up via a resistor instead of the red LED.  5k1/10k??  Whatever option you take, you'll probably need to increase the value of R25 to adjust for brightness (and increase the value of any pullup resistor if used).

Easiest option, though, is to stick your new LED in, socket the FET and try different ones until it works (hopefully!)

PS, I didn't really pay much attention to what effect this was for, but if it's one of the new EH jobbies, don't bother posting the whole schemo (the fragment shown is enough, anyway).

[gez]

Easiest option, though, is to stick your new LED in, socket the FET and try different ones until it works (hopefully!)

Having thought about it some more, that's probably a poor option.  Due to the higher thresholds of blue LEDs the gate isn't going to be held much higher than half supply.  Although you might be able to find a FET that works 'fine', it will probably cause distortion of the signal.

[ayayay!]

Is that bezel plastic or metal?  Maybe one of the legs of the LED is touching the bezel?  Just a guess...

Also, maybe you could steal some voltage from somewhere else to light up your LED...

[kurtlives]

Thanks Gez

The bezel is metal.

[johngreene]

Typical values of voltage drop for a red LED are in the 2V range. Typical for Blue is around 4V. In this particular example, the gate of the FET is pulled to ground to turn it off. Typical values of Vgs(off) for a 2SK30A are in the range of .4V - 5V. The series diode on the gate of the FET prevents it from conducting by being pulled too high (but it is the diode leakage that is pulling the gate high so it probably needs to be quite high to work) so I'd put the blue LED in series with a 10K resistor and install it parallel to the original 10K (to provide a path to ground around the LED) and just short across the old LED with a small value resistor. Maybe 1K. Small enough to give you at least 7V at the switch.

--john

[ayayay!]

Wha?  You lost me John.  So put the Blue LED in series w/ a 10k r, install parallel to the original 10K, but why short the old LED?  Why can't he remove it? 

Did that full schem ever get posted?  Gez maybe you can help me out here...

[johngreene]

Wha?  You lost me John.  So put the Blue LED in series w/ a 10k r, install parallel to the original 10K, but why short the old LED?  Why can't he remove it? 

Did that full schem ever get posted?  Gez maybe you can help me out here...

The supply voltage comes from the old LED connection. You need to replace that with a short or a resistor. Remove it first, but put a short or resistor in its place. Basically the LED is now below the switch along with the current limiting resistor. The point the switch connects can now go to a higher voltage. This allows the gate of the FET to be pulled high enough to turn it on.

http://s273.photobucket.com/albums/jj219/designacryl/?action=view&current=aasmallclone.jpg

--john

[gez]

Did that full schem ever get posted?  Gez maybe you can help me out here...

The snippet in the link above shows a FET that switches out the wet signal, leaving only the dry signal ('bypass').  When the switch is in 'effect' mode, the LED pulls up the gate of of the FET, turning it on and allowing the wet signal to mix with the dry.  The diode to the gate will be reverse biased and prevents any serious current from flowing through the gate-source junction.  The 4.7M resistor and .047u cap ramp the gate up slowly, so as to avoid switching noise.  The 10k resistor connects the LED to ground, allowing it to turn on and limiting current at the same time.

Because red LEDs have a forward drop of around 2V or so, the gate is held at approx 7V, ensuring that the FET is fully turned on.  A blue LED, because of its larger forward drop, can't pull this particular FET up high enough to turn it on; or so it seems.  By sticking the LED and current limiting resistor where R25 is at present, the problem would be solved as the original LED will still pull everything up within a couple of volts of the + rail.  You could just short the original red LED, but the extra voltage - the FET's gate is now being pulled up to 9V instead of 7V - might cause a little extra "charge injection" into the gate, which could translate as switching noise.  Only way to tell is to suck it and see (short the LED).  No click, then fine: remove the LED and pull everything up to 9V.  If it causes a problem, however, it would be best to leave in the original LED or replace it with a resistor that will drop the same voltage as the forward drop of said LED.

Any clearer?