Nurse Quacky problem

lightningfingers · October 10, 2004, 03:29:28 PM

I have a problem with the NQ, I DID search the archives before posting and many builders have described the problem but I couldn't find a definitive answer; I get the guitar signal OK but there is no quack to speak of. One of the LEDs is lit all the time and the other follows the guitar signal. I think (from looking at the schem; i'm no expert) the transistor is acting as a variable resistor for the wah filter, but I pulled the transistor out and measured its hFE; its fine. Erm, :oops: a few other things, i skipped the Range and Attack pots (just shorted 'em out), I used 2n2 for the filter caps 'coz thats all I had and I used a 10uf to ground in the EF because I like the decay better that way. Also the LEDs are different kinds, both 3mm red ones matched within a few mV of each other, one has a clear lens and one has a diffused lens (I think this is highly unlikely to be the problem).

Any suggestions?

B Tremblay · October 10, 2004, 05:19:45 PM

JD Sleep sent to me a sample of PCBs he's making for the Nurse Quacky. I built one and came to the conclusion that the circuit still needs a 1458 or LM358 to function properly. I thought that including the Dr. Quack bias LEDs was enough to allow other dual op-amp types to be used, but apparently I was wrong.

So, first try using one of those op-amps. Also, shorting the Attack pot is OK, but doing the same for the Range may cause problems. If you replaced the pot with a 12k/12k voltage divider, it may be alright for testing - but you'll definitely want this as at least a trimmer, if not a panel pot.

Torchy · October 10, 2004, 06:43:52 PM

I posted a question on the quacky a while back because although it worked, the volume drop between clean and effected signal was pants.

I thought I screwed the first build, so I rebuilt it completely. Both builds worked and they quacked like a manic duck, but at really low volume. I followed the schem exactly and subbed various dual opamps, but all opamps I used had low output volume. BTW my builds were on veroboard.

On the last one I used the volume recovery schem (provided by RDV - thanks) and the output got back to about unity, and Im happy with it. Mine definitely werent fussy about which opamp used. :?

Samuel · October 10, 2004, 07:44:39 PM

my experience was definitely in line with Tremblay's - the thing only quacked with a 72 when I slammed the strings, popping a 4558 in there completely solved the problem...

lightningfingers · October 11, 2004, 04:41:21 AM

I have a 1458 in there, its still not quacking..........
I'll put in a 12K/12K divider, if that doesn't help i'll see if it wahs manually with a pot...not giving up...

thomas2 · October 11, 2004, 06:06:38 AM

i also omitted the range and attack pots with no problems and used 3mm red leds.. the circuit works fine, but needs at least humbucking pickups to work properly. so it's not as sensitive as i'd like it to be.. though if used with a booster it's fantastic! :lol:

lightningfingers · October 11, 2004, 06:47:21 AM

it wont wah manually with a pot either.....is the ROG layout known to work?

Thomas: what did you put in place of the range pot?

Mark Hammer · October 12, 2004, 11:09:21 AM

PUT THE RANGE TRIMPOT IN.

You will note that between the output pin of the envelope follower stage and the base of the transistor there is NO DC BLOCKING. As far as the transistor is concerned, any DC that comes through from the gain stage (with a gain of how many thousand?) is an "instruction" of where the filter should be right now, and if you've instructed it to set the filter bassband at 15khz, it will oblige you nicely. The trimpot (or chassis pot) setting of the range control trims back any stray DC, and also compensates for how the specific transistor used responds to the DC. From where I stand, the only way you could get a Nurse Quacky to behave well in the absence of that trimpot would be if you employed ridiculously stringent component selection criteria and also used an instrumentation amp with fine tuning of DC offset for the envelope follower.

Fine to short the attack control. Just note that any and all resistance between the second LED and trimpot reduces current. Removing that current will produce bigger sweeps that may well start too high...especially when the trimpot is removed.

It IS true that the original Dr Q was very fussy about op-amps, and that optimal performance came with a 1458. I know, I subbed a bunch of chips and only the 1458 delivered the goods. The LED compensation circuit Jack Orman designed, though, solved the problem. You should experience no difference in sweeping between op-amps, although you may well notice differences in things like noise or possibly clipping.

B Tremblay · October 12, 2004, 11:16:18 AM

Quote from: Mark HammerIt IS true that the original Dr Q was very fussy about op-amps, and that optimal performance came with a 1458. I know, I subbed a bunch of chips and only the 1458 delivered the goods. The LED compensation circuit Jack Orman designed, though, solved the problem. You should experience no difference in sweeping between op-amps, although you may well notice differences in things like noise or possibly clipping.

I thought the same until I built another Nurse last week. I had to -really- bear down on the strings to get any sweep with a TL072 or TL082. I needed to use a 1458 or LM358 to get the proper response. Maybe Dr. Quack's buffer (omitted in the Nurse version) does more than I thought.

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Nurse Quacky problem