Gating weirdness - hidden solution

[djp8djp]

Here's one for y'all. Let's see how long it takes you to figure out.

I'd finished a vero of a BluesBreaker II, with a few of my own mods. I'd tested and twiddled and finalized the design on breadboard before I went to vero. Anyway, I got the off-board wiring done and into the enclosure, and before finalizing, hooked up the board for testing and ... nothin'. Well, no surprise, I figured there'd be some debugging to do. I pulled down my magnifier - a round lens inside a circular fluorescent bulb, mounted on a hinged spring arm. It's really handy for the closeup stuff, as my eyes aren't what they used to be. Anyway, after testing for voltages I went to the audio probe, and found a very early-in-the-schematic bad solder joint. Easy fix. And then I had signal out of the first JFET, that serves as an input buffer. But it had this strange gating.

Now, I say "strange" gating, 'cuz it was really inconsistent. I had a little practice amp plugged in, a ratty ol' guitar in my lap, the circuits on my bench under my magnifier light, and audio probe in hand. I know, I should rig up a signal generator or MP3 for a testing signal, but I love the "Kraannggg" when a board first works. Anyway I huddle down and test, and get signal only on the first part of sounding a string. Partway through the volume drops off markedly, although not quite entirely. Many of you will have heard this behavior before. It's often due to some improper biasing. So I test voltages again. For as much good as that does with a JFET, it looked OK. I'd tested that JFET before, and had used the Fetzer Valve calculations for biasing it, and this is the exact one that I had working on breadboard. But also, on the input, where the bias resistor goes, I use a 1Meg trimpot. Lug 1 tied to input, 3 to ground, and the wiper goes to the gate. This lets me throttle the input signal strength, which allows me to twiddle into the perfect tube-y sweet spot. And it still keeps the bias to about 500k-1Meg - the wiper's position is generally not up very far, and I have a little extra resistance between the pot and ground, as insurance against overzealous adjustments.

So I peer through the magnifier, pin the probe in place, and adjust the trimpot while strumming. Still gating. Adjust the amp volume, repeat. Still gating, but at a bit different level. Twist the board around trying to look one more time for opens and shorts, strum and adjust again, and it's different again, still there. Weird.

OK, this is a simple enough little circuit, I just grab a small breadboard, plug in a few components and recreate it again. Plug in with the guitar and probe and KRAAnnngg. Yahoo. Inspect under the magnifier. Yup, looks good. Grab the screwdriver to try adjusting the input and suddenly the gating's back. Huh? Adjust the pot - still gating, all through the range of adjustment, with varying levels. "Zoom in" to check for shorts. Nothin'. I want to check voltages, but the board's kinda crowded around the JFET so I grab a couple of jumpers, to spring from the JFET et al, out to an open part of the board where my clumsy hands won't ruin something with the multimeter leads. OK, strum ... nuthin'. Dead. Huh? Zoom in. Nope, just some jumpers added, all good. Weird. Tear out the jumpers. Got sound again, but still gating. Nuts. So I push back, turn off my lamp, power supply and amp, and take a break.

When I come back, I reconnect power, warm up the amp, and just flick out a strum on the guitar while it's still standing there. Beautiful sound. Clear, and ringing slowly and smoothly allll the way down to silence. No gating. Whaaa...? Pick up the guitar, whang on it for a bit. Beautiful. Just wanna keep playing, but it's sounding a little hot, so I figure I'll twist down the input a little. Pull down the lamp and grab the screwdriver and strum and ... gating! Hey, I didn't even TOUCH the board yet. WTF. OK, let's try those jumpers again. Yup, silence. OK, yank 'em out. Gating. In disgust I push up the light and roll back my chair to think and absently strum and ... beautiful sound. And then it hit me. It should have been obvious to me, and I felt pretty thick for it having stumped me so long.

And now my problem's fixed. And so what did I change to get it to work? Nothing.

Can you guess what was going on?

[thermionix]

Something to do with the lamp?

[djp8djp]

Per-sactly! Turns out, the power unit in the head of the fixture is strong enough to have a decent little field. And as I was working on a FieldEffect transistor, it interfered. I was surprised at how much it interfered. Turn it off - no problems. Swing it up or away, no problems. Get it down by the board -weirdness.
Oh, and I figure that the jumpers I plugged in were just acting as antennae, strengthening the effect.
So - heads up 'bout this, to all with such lights. And to those considering getting one, maybe try one with LEDs?

[duck_arse]

the lamp/maggy I use is fitted with a 40ish W incandescent globe. the first few times I used it, I couldn't stand the smell (outgassing?) of the heated/ing plastic fittings, so the lamps stays unplugged now. I imagine a CFL would stink even worse, let alone the buzzings.

[PRR]

> maybe try one with LEDs?

Candle.

[Vitrolin]

i cant remember where but someone stated that never trust anything about a circuit  that isn't in its shielded box

[Jamdog]

So,  are you going to build an effects around lamp gating?

[deadastronaut]

ahhhh lighting , dont get me started....

i rigged up led colour strip lighting under my kitchen worktop cupboards...whilst listening
to DAB RADIO..

job done...

now turn lights on to test......ssshskhkrhhckshhhhsssshhcccrrshhh ..nasty radio noise

just annoying.

lights look good though...just cant have both it seems..

[djp8djp]

Haha. Yeah, not the sort of thing one thinks of off the top, huh? In hindsight it's pretty clear (isn't everything?). But when I was dealing with the problem it was a real head-scratcher. I mentioned the lamp a few times in my story, which probably made it a bit more obvious. But at the time, it was just part of the environment, and I was hunting for mysterious shorts and opens or intermittent contacts.

Isn't this fun !?


(jamdog - yeah, right. Probably not!)