USE AN AUDIO PROBE!!

[Mark Hammer]

This past weekend, I had hoped to get closer to finishing my Farm one-board modular synth ( http://www.aleph.co.jp/~takeda/radio/OneBoardFarm.html ), that has been sitting ignored for too long.  But, rummaging around for stuff, I stumbled on a wired-up-but-unboxed Ross/Ropez phaser.  I vaguely remembered that one or two of those I had made for myself were not working, and wondered if this one was functional or non-functional.  Gave it a battery and plugged it in, but only got clean signal.  Hmmm.

Maybe there was somethng amiss with the LFO.  Obviously it would sound like only clean signal if there was no sweep.  So I checked everything around the LFO, and reflowed all the joints.  I even went so far as to replace the chip (LM13600).  Nope, nothing.

Okay, possible one of the chips for the phase-shift stages was fried?  Maybe.  Replaced one, then the other, but still no go.  Maybe it's the op-amp?  Again, a replacement without bearing fruit.

So, just for the hell of it, I thought I'd use an audio probe.  Plugged into my trusty battery-powered amp and ran a cable from the hot lug of the output jack to an alligator clip with a nice stiff piece of wire in its jaws, and poked around.  Hmmm, very modest vibrato at pin 8 of the first phase shift stage.  Okay, that checks out good.  Touch the probe to pin 9 on the second stage and that checked out fine, with a slightly more robust vibrato.  But nothing out of pins 8 or 9 on the other chip. 

Obviously the LFO was working fine, since I had modulation, and the op-amp was working fine since it ent the signal to the phase shift stages.  So why did the signal crap out on me between stages 2 and 3?  I knew that the chip was fine and there were no solder bridges or anythng like that.  So what was up?

Hmmm, let's look at that schematic and Tonepad layout again.  BAZINGA!!  The Tonepad layout has two pads marked X and Y.  These pads are used for inserting additional swept or fixed stages.  If you cut the trace between them, X is the output of stage 2 and Y is the input of stage 3.  Somewhere along the way, I had cut that trace i preparation for inserting more stages, but somehow hadn't done so or remembered doing it.  I inserted a wire link to join them and bingo-bango, instant luscious phasing.  I did a few mods to season it to taste (dropped the 10k resistor on the LFO output to 9k1, and ran a 47uf cap from the OTA side of that resistor to ground, as per 1st issue Small Stones), and it now awaits final boxing.  And I owe it all to use of an audio probe that showed me "It works up to here, and not further".

Incidentally, I found the vibrato available from the output of stage 2 to be far more pleasant than that achieved by simply cancelling the dry signal and using all 4 phase-shift stages, so I wired up a vibrato switch that selects between standard phaser, and the wet-only output of stage 2.  There's a volume issue I still have to suss out, but it's a really nice sound.  I imagine wet-only Phase 45 pedals sound similar.  I look forward to using it with a tremolo, for that "sincere but nervous" sound.

[Paul Marossy]

Viva audio probe! It's kept me out of the rubber room more than once. 

And while I'm at at, viva light bulb current limiter! I had to use one a few days ago on an amp that was blowing fuses as soon as I turned it on.

[alparent]

Yes, a probe is great for debuging.

But it's also a great learning tool. Probe arround a  build to discover what does what. Lots of fun!

Every nooby should build a probe. I know! I'm an Ex-Nooby, gratuated last week! 

[midwayfair]

Once I figured out how to use one, it dropped my debugging times from days to minutes. Second-best tool in the shed (after the multimeter).

[EATyourGuitar]

I made a video a while ago that shows how to do it

[Paul Marossy]

Once I figured out how to use one, it dropped my debugging times from days to minutes. Second-best tool in the shed (after the multimeter).

+1

[Govmnt_Lacky]

We should have a build competition....

Best audio probe 









*** JUST KIDDING***

[Paul Marossy]

We should have a build competition....

Best audio probe 

I still use this one all the time:




All I need is a scope probe and a 1/4" cable.