Baffling voltage issue

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Baffling voltage issue

Postby Rusty the Scoob » Sat Jan 08, 2011 7:50 am

I'm working on the external power supply for my Starfire and it's really behaving strangely.

The goal is to take a wall-wart transformer power supply with 1/8" Tip + and sleeve - and convert it to to 1/4" Tip + Ring - Sleeve dead. Should be simple enough, right? Every time I plug in the 1/4", the voltage somehow goes away!

The bare 1/8" from the transformer reads 54V for some reason - supposed to be 18V according to the printing on it. When I plug it into the project box below the 54V flows to both jacks with no problem. When I plug in the 1/4" plug though, the voltage suddenly reads 0 at both jacks, even the 1/8" one.

There's no conductivity between the rip ring and sleeve of the 1/4" cable I made... so what's going on here? Do I have a bad transformer that just can't handle even the load of 25' of cable?

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Re: Baffling voltage issue

Postby waldo041 » Sat Jan 08, 2011 12:13 pm

the 1/8" jacks negative/sleeve probably needs to be isolated from the enclosure. try and see if the whole thing works without the enclosure.

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Re: Baffling voltage issue

Postby Rusty the Scoob » Sat Jan 08, 2011 1:17 pm

Hmmm. Nope, same result without the enclosure. It drops instantly from about 59V (why is it reading so high?) to 1.2V to 1.3V.
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Re: Baffling voltage issue

Postby zambiland » Sun Jan 09, 2011 1:21 am

Something is screwy. Are you sure it's a DC adapter?

Why not get rid of the box and just use a 1/4" female TRS connector instead? Clip off the mini plug and install the female 1/4" plug. It might be easier and less weight to drag around. I don't really trust putting half of the current into the box via the jack.

Without having it in front of me, though, it's hard to tell what's going on.

Good luck and let us know how it comes out!
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Re: Baffling voltage issue

Postby JonnyBoy » Sun Jan 09, 2011 1:50 pm

I know this may sound kinda insulting, but are you sure the wires aren't hooked wrong? That has happened to me with stereo jacks since they are hard to see which connection is which. In that matter, switching the meter probes would produce the desired result (which I am sure you tried) unless the ground has been mistaken. It sounds like its going to ground some how when the 1/4" plug is inserted or through the box system some how.

Ed's point sounds like a winner, to go a step further, ditch the enclosure and 1/4" stereo jack and hard wire the 1/8" female end to the cord itself, rather than having it go through the box system. just a thought, I am sure you will find the issue... ---J B
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Re: Baffling voltage issue

Postby RiverRat » Sun Jan 09, 2011 5:34 pm

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Last edited by RiverRat on Tue Feb 22, 2011 1:54 pm, edited 1 time in total.
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Re: Baffling voltage issue

Postby Rusty the Scoob » Mon Jan 10, 2011 12:18 pm

I've triple-checked the wiring, it was the first thing I suspected, but it's pretty straightforward and I did do it right the first time. Tip + ring - sleeve dead... just like I planned it.

I originally thought about just soldering a female stereo 1/4" on the transformer instead of building the adaptor box, but I felt that the box and factory 1/8" end would give me better strain relief and repairability for not much extra effort or money. I do like Waldo's idea of isolating the chassis from the voltage just in case.... will a couple of rubber or nylon washers do the trick?

RiverRat wrote:Check the continuity of the 1/4' cable and make sure nothing is wired incorrectly.

Is the wall wart getting hot/warm when you have the 1/4" cable plugged in? All wall warts are not created equal, some are just a transformer and diode rectifiers, others have voltage regulation and thermal protection and other circuitry.

54V volts from the secondary sound like to may have cooked the transformer.


A bad wall-wart is my theory because of the 54 volt reading, and because it does get really, really hot. Painful to touch it, even.

The transformer is just something I picked up at You-Do-It in Needham... This pic looks wrong but the specs are right and model # is close. http://www.altex.com/18VDC-300mA-AC-to- ... 46125.aspx Where would you experts pick up an 18V DC power supply if you needed one? (My local Radio Shack only had AC in 18V.)
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Re: Baffling voltage issue

Postby JonnyBoy » Mon Jan 10, 2011 12:42 pm

Yes only AC stuff at Radio Shack, I remember that from needing a wart for my qtron. Looks like the internet or a swift return and exchange of the product to the said store. But you are getting the proper readings from the power source without the wires and junction box, right?

Would amperage have something to do with the drop in voltage over a longer run? Just thinking out loud, don't know if that has anything to do with it. But, don't different devices need more amps to get proper power depending on how much resistance it sees? maybe cable doesn't pose that much resistance to matter, but if the amps are super small that may be an issue in the long run or future.
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Re: Baffling voltage issue

Postby Rusty the Scoob » Mon Jan 10, 2011 2:40 pm

JonnyBoy wrote:Yes only AC stuff at Radio Shack, I remember that from needing a wart for my qtron. Looks like the internet or a swift return and exchange of the product to the said store. But you are getting the proper readings from the power source without the wires and junction box, right?

Would amperage have something to do with the drop in voltage over a longer run? Just thinking out loud, don't know if that has anything to do with it. But, don't different devices need more amps to get proper power depending on how much resistance it sees? maybe cable doesn't pose that much resistance to matter, but if the amps are super small that may be an issue in the long run or future.


Nope, not proper readings... when the transformer was new it read 26V, now it reads 54V when it's plugged into nothing at all. Nowhere near the 18V it's supposed to read. Maybe exchanging it is the best way.
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