Qs about the Dr. Q

[Samuel]

So I just finished wiring up a Dr. Quack. Schematic at www.generalguitargadgets.com.

After getting nothing the first time around I whipped together a quick-n-dirty audio probe, and found my (multiple) cold solder joints and broken traces. :oops:  :oops:  :oops: In any case I have tested every joint on the board, and everything beeps correctly.

So the thing is up and running, sort of. I get signal through it, but only get the envelope follower happening when I SLAM the strings, and only with certain settings on the pots.

Potential Issues:
1) Misbiased tranny? This is my prime suspect. Don't have time to go through with the voltmeter tonight, but I'll post those tests later

2) Bringing the "range" control out to the exterior. Can I just lift the three trimmer connects in order (as if the pot were plugging directly into the board) to a pot (b taper) of the same value? Or am I missing a concept here?

3) The LEDs. I had a pair of RatShack yellow LEDs. The schem specified red, but I didn't see why this would matter. The specs were roughly: 2.1 V, 50mA, 270mcd (ish I think?)

4) The opamp. Schem says "any dual opamp". I used a TL072. I assume this is OK? I have some 741s on hand (and a TL062 I think) if these would work better.

OK I think that is my list of potential deviances and issues. Anyone have a good idea where to start debugging this little guy?

PS Applause to Aron and RG for the excellent simple audio probe. Made it in about 2 minutes and it did its job.

[Mark Hammer]

You may be a victim of the ever popular "undersignalitis".  Although most envelope-controlled filters attempt to anticipate a broad range of input signal levels, they can't be everything to everyone, and often a given guitar may only elicit sweep when the sensitivity control is up full or near full.  Under those conditions, there are a few things you can do.  One is to beef up the signal going into the pedal (and by extension the envelope follower), perhaps by means of a preamp or other level-boosting device or by tweaking the degree of boost at the FET input stage.  The other is to increase the gain in the envelope follower by increasing the value of the feedback resistor.  The original Dr.Q used a 2.2M resistor, but I have found that 2.7M is probably a more suitable value for the sorts of signals that mortal single-coil pickups would feed it.

It is possible that the trimpot setting is such that it interferes with the amount of potential sweep, but if it is more than 1/4 of the way from either end-point of rotation then this is unlikely.  You CAN turn that trimpot into a chassis mount pot for filter tuning though I would take a peek at Tim Escobedo's "Phuncgnosis" design for a much better wayto accomplish that.  The stock DQ trimpot is really only useful for about 1/3 to 1/2 its rotation for filter-tuning purposes.

The original DQ was very fussy about op-amps, and I can vouch that it really only gave decent sweep with a 1458 (I tried a half dozen other types and they all provided either negligible or ridiculously high amounts of sweep).  jack's redesign really does let you use *anything*, so the op-amps are unlikely the issue.  A more likely issue is that the pinouts for the FET you are using *may* be different than you suspect.  It happens...all the time. :cry:

[Samuel]

Thanks for the response Mark. I very carefully checked through Mouser and NTEs manual pages for the proper pinout on NTEs 457 transistor, but it ended up being the inverse of what the parts layout showed. I was out of transistor sockets too unfortunately, but I'll see what reversing that gets me. Failing that, I'm assuming that any of the clean boost projects will help me out? I'd be totally fine with just building one into the box....

[Mark Hammer]

You'd be surprised at the sheer number of circuits that instantly spring to life when folks find out what the *actual* pinout of even a single component is.  I can't begin to tell you the number of times my ass was saved by sticking a bipolar trannie into a socket on my meter and moving it around until I got a plausible hfe reading.  I only wish there was an equally simple and painless way for identifying pins on a FET.  Yes, I know it is child's play to download datasheets for anything you want.  However, I don't know about you but half the time I find the pinout diagrams shown not especially clarifying ("Hmm, should that be looking *up* from the pin end or looking *down* from the top?").  Some are nice about it and show decently shaded 3-d renderings, but others are not quite as clear.

[Samuel]

This was my hugest aggravation getting started building pedals. I have it *mostly* figured out now, but "mostly" + a token gets me a ride on the subway as they say...

[Samuel]

Reversed the NTE457, and nothing doing. Got no signal at all that way.