Testing AC Power Supply - Sanity Check please?

[ashcat_lt]

It's kinda like a crazy check, only...

I've been asked to help troubleshoot a Line6 MM4 that won't light up on adapter power.

The batteries in it are dead, it still doesn't work when they're removed.  I haven't tried fresh batteries alone yet.

Measuring AC voltage between the center and the sleeve of the adapter (plugged into wall power, not connected to the pedal) I get a millivolt or two.  I can't get any meaningful voltages - AC or DC anywhere on the board when I plug it in.  When I tried to get DC resistance through the transformer, the wall side read as open while the pedal side is just a bit more than an Ohm.  So it seems like the adapter has failed somehow.

But it measures about 12V from either of the pedal side conductors to my finger, and that's why I'm coming here.  Is it possible that I have somehow misused my multimeter?  Am I doing something wrong in my other measurements?

Or should I apply for that crazy check and retire?

[R.G.]

1. Test your meter. Use it to measure a 9V battery on a DC range, then use it on an AC range to measure - well, I hate to tell you to measure an AC wall socket, because that does require a modicum of care, so don't do that. Instead measure another wall wart with an AC output. If the meter measures both of these moderately well, chances are the meter is working. Check to see that you have the meter leads plugged into the correct sockets, as some meters have alternate meter positions.
2. If the meter is working, look carefully at the wall wart you're trying to measure, and read the expected output from its ratings area. It should tell you whether it's AC or DC, or DC with pulsating AC together. If it has a DC output and you measure it on an AC range, you won't get much. Likewise, if it's output is AC and you measure on a DC range, you won't get much. The DC plus pulsating AC will make both ranges work on the meter, but the numeric results are not terribly useful. But you only care if there's power coming out.
3. If no power is coming out, measure the DC resistance of the primary prongs. If that's way over 1M, chances are that the internal thermal fuse has opened, and the power adapter has become a Darkness Emitting Diode (that's a DED...     ). This is unlikely to be fixable and a new adapter is needed.
4. Measurement from the output of the meter to your fingers is only a useful measurement if you're going to spend your time with your fingers hooked up to the pedal. If you're measuring 12Vac to your fingers, you're probably measuring the AC leakage of the capacitance from primary to secondary of the insides of the power adapter.

It is always possible to misuse a meter - I do it all the time. That's why I'm so quick with "go measure something that is already known".

If the pedal does not work on batteries, is it possible that the power supply is bad, the cord out of it is shorted, the jack coming into the pedal is shorted/open/etc. It is also possible that the pedal's internal power converter that makes logic voltages out of 9V-ish stuff coming in has died, and that in turn heated the power adapter til it opened the primary thermal fuse.

[ashcat_lt]

Thanks RG.  It was kind of an "Oh sh..!" moment when you said to check where the probes were plugged in.  Unfortunately, they are in fact in the right place.  I had almost convinced myself that somehow it made sense that the two sides of the output would read 0 to each other, but correct verses some other spot...  Phase inversion...  Something...

Anyway, I think you've confirmed that it is actually a dead supply, and I think you're right that it must be something in the pedal causing it, as this is the second he's blown.

Next step I suppose is to try battery power.  If that works, I thought to just hack a DC jack in where the batteries should be.  Do you think that whatever kills the AC adapter would also kill this?

[R.G.]

First, I'm not familiar with the innards of that Line 6. However, I've seen some digital delay pedals - well, OK, done post mortems on them - and they tend to use a switching converter to make +5 or +3.6 or whatever out of whatever DC they're fed. The AC-input ones make DC from the AC and then let the converter make the logic voltages out of it. If the switching converter dies, it often dies shorted, and after that it might possibly kill a wall adapter over time.

This is raw speculation of course, so take it for the raw guess that it is.

If you put batteries in, check for the output voltage of the batteries when they're in there and the pedal is turned on. If the battery voltage drops a lot when the pedal is powered, or possibly even with batteries just plugged in, then there is something badly wrong with either the power converter or the logic chips themselves. If new, fresh batteries are pulled down to a lot below their nominal voltage, something is dead in the pedal, and the batteries will get hot and die quickly too. If that's the case, the pedal is a goner, suitable for making paperweights, doorstops, and other art objects. There is no good reason to try to fix a digital board with a dead logic controller. It's not worth the time and effort, even if you could get the chips and deal with the (probably) hundred-pin SMD logic chips.

Again, a guess. Take it for what it cost you.

[PRR]

> DC resistance through the transformer, the wall side read as open

That's 99-44/100% conclusive. Dead.

> about 12V from either of the pedal side conductors to my finger

AND....?  What is the REST of the circuit?? Where do the silly electrons go after your finger?

If "nowhere" (you are not touching ground, not even damp socks), then it flows through the universal capacitance to ambient voltage. With a high-impedance meter it is not unlikely for a "no connection" to show some noticable fraction of the wall voltage. If the PT has 120V above ground on one side of the primary break, a few hundred pFd of stray capacitance into a 1Meg meter will show around 12V.

For a near-death experience, stand on the glass coffee-table, hold one VM lead, poke the other in the live side of a wall-outlet. If you don't die (of shock or by busting the house's best coffee-table), you'll read a non-zero AC Voltage.