Pedal To The Meddle

Yes, a post that mixes Pink Floyd, Godsmack and guitar electronics. Sitting behind me is a big of guitar effects pedals in various states of disrepair; they were all broken to some extent when I picked them up on eBay and have since been scavenged for parts, semi-diagnosed or otherwise left for the proverbial rainy summer weekend day.

My pedal diagnostic plan follows three very simple steps:

1. Power Continuity, also known as the Jerry rule.

Back in the day when my father and I would hunch over a workbench building Heathkit projects, invariably something would go wrong and require test equipment more advanced than a screwdriver and a broom handle to push each other off of potential electrocution points. More than one half-assembled kit required us to visit Jerry, who very kindly offered to plug our printed circuit circuses into a maze of test leads that would have made Tesla jealous. Frustrated by the lack of a signal appearing at any test point, Jerry was about to throw in the towel one evening when my father noticed that the device under test was also being tested without power. Rule #1: Make sure it’s plugged in, and that even if it looks like it’s plugged in, make sure power is flowing from source to the obvious VCC test points.

2. Power Polarity, also known as the Matt rule.

Guitar pedals vary in terms of their input voltage and input voltage polarity – sometimes the positive lead is the tip, sometimes it’s the ring, some really old pedals have AC line voltage with on-board rectification and well capacitors. Bubba’s friend Matt once plugged in a hand-made pedal using the wrong polarity power adapter, and the resulting effects were more of the snap and fizzle rather than tone distortion type. Rule #2: Make sure the power is of the correct voltage and polarity.

3. Test top-down, also known as the Bartender’s rule.

When teaching new bartenders how to mix drinks, the usual advice is to pour the cheapest ingredients first, with the brand name booze coming last. Mistakes cost less that way. Pedal breakdowns tend to run along the opposite cost gradient: the most expensive, or most difficult to replace component will be the first to go (usually due to a violation of Rule #2), and if you have to replace one, you’ll end up having to find a matched mate or three for it. Given power of the right size and shape, I look for output coming from transistors, op amps or bucket bridge delay chips. White noise anywhere along those lines means unless you’re really good at dealing with germanium that’s a few microns thick, you need pull out the desoldering equipment.

There are always exceptions: Scorch marks on a circuit board indicate a bit too much input voltage resulted in a component being driven over its rating (with attendant snap and fizzle). Mechanical problems, usually from broken or intermittent switches, or input/output leads that have suffered metal fatigue, are common but sometimes masquerade as power problems when the input or output jacks act as battery switches as well. Big capacitors do in fact wear out (that old “electrons are particles” duality thing).

None of this explains how or why someone decided to rewire a vintage Electro-Harmonix Linear Power Boost without a battery and so that the output signal is shunted to ground, unless it’s an attempt to make me invent a fourth rule.

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