Still not got this debugging thing down

[stallik]

RG's hum free ABY. Worked well for 2 years then one chanel went quiet and distorted.  Replaced the TL072 on that Chanel, no joy. Replaced the Max1044 and it worked for a few mins then quiet again. Figuring something was taking out the 1044, replaced caps, resistors and anything connected. New chip lasted for a few mins then failed. Replaced all the chips. Failed again. Running out of chips, ordered some more.

After 4 hours, noticed the ground wire to one jack had come loose(poor soldering) re soldered. Pedal fine. Seems the wire had intermittent connection and stomping the foot switch was enough to connect/disconnect it. Empty the bin to retrieve what I thought were duff chips.

I really should remember to check the easy things first

[iainpunk]

i wish i didn't recognise the exact situation. i know how it feels when you find out that a cable has failed after you replaced all chips, caps and trannies on a PCB to find out where the problem lies. i have never had that happen to me since i have my audio probe to debug everything. i love that lil gadget

cheers, Iain

[bowanderror]

+1 on the Audio Probe. My debugging order is basically:

• Ocular Pat-down - Visually verify that the schematic, layout, and finished board all match - I find that I often forget traces on the solder side when doing perfboard
  
• Power - With a DMM, check that I'm getting power where I should be and ground where I should be
• Audio Probe - See here & here for better guides, but basically I use a looper to feed the pedal consistent input, then plug the audio probe into an amp, and clip the Ground lead of the probe to pedal ground. I usually start from the output and go backwards step-by-step through the signal path all the way back to input. It's a good way to see where volume drops & increases, where filtering is happening, and if the signal drops out completely you've either left the signal path or have found your problem!
• If all else fails... sleep on it. This seriously works wonders sometimes! I've solved many more problems in the shower trying NOT to think about some stupid pedal.

[ElectricDruid]

Intermittent faults like that are always a b!tch. The whole debugging process depends on a logical step-by-step approach, and that only works if the circuit behaves consistently from one test to the next. With an intermittent fault, of course, it *doesn't*, so the whole basis of the process goes out the window.

I had one on a synth once that didn't work, but as soon as I opened the synth up to work on it, everything was fine and testing here there and elsewhere I couldn't find anything. *Eventually* I worked out that it was to do with the front panel being lifted up, and it turned out there was a busted switch, and the broken pieces fell to the back when the panel was lifted and made the faulty connection. With the panel down, the bits fell forward and no dice. Nightmare.

About the only thing that causes me as much grief as intermittent faults is comms problems. I never seem to be able to find out whether the issue is the receiver not receiving correctly or the transmitter not sending properly.

[stallik]

Ah, audio probe. You'd think that would tell you everything. Unfortunately it only told me that everything was working perfectly. Which of course was true. Until I hit the stomp switch a few times.
Tom is right about intermittent faults but I think my main problem was that, for no particular reason, I decided what I thought was wrong before testing properly then blindly went down the wrong path.

My troubleshooting and the temporary replacement with a passive ABY prompts a question regarding phase.
I can reverse the phase of one amp either by switching the output signal af the isolated active ABY or by swapping the speaker leads on one amp. To my ears, there's no difference but technically, is there a preferred method?

[iainpunk]

no there's no prefered method some amps have the output phase option build in, its not like a speaker has any directionality in them.

sometimes with some amps, the tone will change slightly when you flip the phase, since a guitar signal is far from symmetrical, and asymmetric clipping can sound different one way around or the other. but its only a small subtle difference.

cheers, Iain

[stallik]

Thanks Iain, I'm still trying to get my head round the phase thing. I'd read that when 2 amps are out of phase, I should get some cancellation but instead of hearing that, I get a pretty impressive stereo spread. Switching phase on a foot switch might just be the simplest and most impressive effect I have.

[PRR]

> I'd read that when 2 amps are out of phase, I should get some cancellation

Only if they are very close, like 2 feet, and only in the deep bass, below 100Hz.

There's effects all up the audio band but much more subtle than 'cancellation'.

[iainpunk]

Thanks Iain, I’m still trying to get my head round the phase thing. I’d read that when 2 amps are out of phase, I should get some cancellation but instead of hearing that, I get a pretty impressive stereo spread. Switching phase on a foot switch might just be the simplest and most impressive effect I have.

i used to play 3 different cheap small guitar practice amps at the same times at a jam session i went to. i had them stacked up on a hand trolley, tied down, all 3 on a clean channel with all knobs on 10 so all 3 were overdriven/distorted through the clean channel, this sounded amazing, one session i found the time to place them around the sage a bit and it was super spatial and trippy, because the middle one was out of phase.

that effect can be done by a attenuverter with a bypass switch, simple and a nice utility if you use mixers a lot

cheers, Iain