EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: gooseEL34 on September 03, 2013, 06:33:46 pm
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Hey all.
I am working on a vintage Guitar pedal. Schematic is here:
http://analogguru.an.ohost.de/193/schematics/Chandler_TubeDriver.gif (http://analogguru.an.ohost.de/193/schematics/Chandler_TubeDriver.gif)
Essentially, the unit had a "wall wart" that was two prong in and three prong out (+12v-0--12v AC).
The wall wart is broken and now I need a replacement. Any thoughts?
Here is a pic with the original wart.
http://www.strat-talk.com/forum/effect-effect/116411-bk-butler-tube-driver-3-knob-vs-4-knob.html (http://www.strat-talk.com/forum/effect-effect/116411-bk-butler-tube-driver-3-knob-vs-4-knob.html)
(scroll a bit down and there is a pic of the one with wart, and one that accepts a barrel connector).
Thoughts on what to do?
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Do you have current specs?
If not, do you have a couple of bench power supplies to power it temporarily, and a way to measure current consumption?
Having both plus and minus 12V rails is somewhat unusual in off-the-shelf wall warts. You might be able to get an isolated 24VDC supply and use a rail splitter IC to provide the common. Or even build your own rail splitter using a voltage divider feeding an op-amp buffer, keeping an eye on the current specs.
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Is it a bi-polar 12 volt DC supply or is it an AC supply? From what you posted, it's not clear. A lot of audio gear uses an AC supply and converts it to bipolar DC.
Do you still have the old supply? A pic of the ratings might be helpful or even cutting it open to tell if it has an AC or DC output.
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Replace it with a 12V-0-12V transformer.
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Replace it with a 12V-0-12V transformer.
Yep.
AC wall adapters have just about joined the dinosaurs. One with a center tap may be all but impossible to find, so you may have to build one yourself.
You might try eBay or some of the electronic surplus dealers.
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I'm a little confused. According to pictures on the net, there is a transformer in the chassis. I assume this is the 12V-0-12V transformer shown on the schematic. If that's the case then what's the point of the wall wart. And is it really 12V-0-12V?
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It appears to draw a tiny current from the negative rail, so it looks like you could use a +12VDC wall wart, and a simple (ICL7662 based?) voltage inverter inside the pedal.
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Yes, the post linked to is kind of confusing. In post #4 of that link, the upper picture is of the wall wart along with a "4 knob" version which has "a built in power cord". The "3 knob" version in the lower picture is the one actually made to use the wall wart. The schematic is for the "4 knob" which apparently plugged directly into the wall socket.
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Ok. Sorry I was not clear.
There were two versions of the pedal. One had the transformer inside the metal case and a AC three prong cord passed through a grommet in the case. In that version, you just saw a power cord coming out of the pedal.
In my version, (the one in the picture), there is a wall wart. The box itself broke open and there is a transformer housed in that plastic box. It has two primary leads (it has a two prong configuration) and the secondary has three leads (so out of the "box" of the wall wart is a three conductor cable).
Once inside the pedal, the center of the secondary is grounded and there is a series of diodes that result in +12 and -12vDC
I hope that is clearer. If I had room inside the pedal (which I might), I could mount a small transformer in the chassis, but if not, some kind of wall wart (or similarly, a CPU type supply (with a cord, then a box, then more cord) would work.
Thanks guys
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Okay. That makes sense.
In that case, what you might have to do is rig up your own external transformer. You can buy 12V-0-12V transformers at Radio Shack if you're in the US (they call them 25.2V CT transformers, I think.) If the old transformer is hosed then I'd buy one of those, a fuse holder, a fuse, and a project box. And then use an old AC cord (two prong) and the existing three-wire cord from the wall wart.
Put the fuse on the primary side of the transformer between AC hot/live and the transformer. You can put a power switch after that point if you need it.
The size of the transformer and fuse will depend on what the specs stated on the wall wart.
Be careful about the mains voltages, obviously. Use twist-on wiring caps to make all the connections and make sure mains isn't exposed.