I have a guitar effects pedal (an MXR script-logo Phase 90), and a Boss DC power adapter. The pedal doesn’t have a power jack. A friend described an easy non-invasive adapter–you wire a power jack to a 9v battery clip, clip the battery clip onto the pedal’s battery clip, and voila!
I whipped up a rig as described in this thread, got the parts at my friendly neighborhood Radio Shack, after I got past the three salesman trying to get me to buy Sprint cellular service :rolleyes:
Alas, the solution doesn’t work, although everything seems wired properly. First I checked the continuity of my solder joints and got end-to-end continuity from the jack all the way into where the pedal’s leads from the batter clip solder to the input jack (neg, disconnects the battery when nothing is plugged in) and the board (pos).
Then I thought maybe I got the polarity wrong. So I put on a battery, made sure the box still worked (it does) and checked the voltage across the contact points (which was only 7.5v and the box still worked fine–that sucker is very tolerant of old batteries). Then I replaced the battery with the adapter rig, and measured 10.0v across the same contacts, same order (note: the adapter is rated as 9.6v but delivers 10v). But when I hit the switch, no sound at all.
I have a 750KB .bmp file with a diagram if my text is less than clear. I am checking the voltage at point A.
So what could be wrong here? I wonder if the box doesn’t like 10v–maybe it chokes it off if the voltage goes over some threshold? I’ve no idea. I could put a resistor inline to bring the voltage down to 9v, though I’m very rusty at this and would have to do a little thought to get the right value. But there’s no guarantee that would work.
What a weird bitmap. Black and then going to white from bottom to top as it loaded.
I wouldn’t expect it to “choke” on 10 Volts. I wouldn’t just put a resistor in there, though. It would likely work well enough (if it gets enough current to work at all) for a test, if you can make some reasonable guess at the current consumption. See if the pedal has it marked somewhere. What you will want would be 2/(current in amperes) This will give you a drop of two volts to get you down to 8.
Before I did that, though, I’d measure the voltage with the powersupply connected to the pedal and the pedal turned on. If the pedal pulls a lot of current, your powersupply might be dropping out on you. I doubt it, though, considering your remark about the pedal being tolerant of weak batteries.
The next thing I’d try is measuring the voltage at the spot where the wires from the pedal’s original battery clip go into the PC board. Maybe the new clip you’ve attached has a bad connection - either poorly soldered under the insulator or maybe the crown just doesn’t grip well.
Outside of that, I don’t know what might be wrong. Your diagram looks right, and you seem to have followed it by your description, so it ought to go.
What is the current rating in mA of the power supply. I’m betting it is small if the rated voltage at load is 9.6 V and open circuit is 10 V. This implies poor regulation and a very small load capacity.
Actualy, no that isn’t entirely correct; let me amend: It is true for most cheap off-the-shelf adaptors, which normally do have poor regulation, however, it is entirely possible for a supply to have very good regulation under heavy load, and therefore a small voltage drop. I don’t think that’s the case here, though.
I made it in MS Paint, not sure why it would load like that.
It’s not marked but I measured with a fresh battery as 5 mA under normal operating load. (Oddly my fresh 9v measured out as 8.25v.)
Did that too, got 10v, same as the power supply with no load.
The voltage across the battery clip’s contact points (one on the board, one on the input jack) are the same as across the voltage source.
I’m reading around 5 ohms end-to-end from the power jack to the contact points on the board & switch on both sides (as well as previously mentioned voltage readings), so I believe the connections are good.
Quoth Q.E.D.:
Rated at 200mA.
BTW this power supply is designed to work on multiple pedals. Boss makes a couple of pedals that have an output power jack as well as the input. Plus they make a “daisy chain” that allows you to plug into the output power jack and then run several plugs (up to 8!) wired in parallel to other input power jacks. So load on the power supply shouldn’t be an issue. Because of the presumed design goals for this application, I would think smooth, clean power is utmost in their minds.
It’s a pretty bizarre problem, and I can’t help but think I’m doing something wrong (IANAEE). But I seem to be doing all the things that people suggest. Unfortunately my knowledge craps out when I get past the battery leads’ contact points. I have no idea what I could check on the board itself or how.
I should add that I replaced the original battery clip with a heavy duty one, with a hard plastic head instead of that flimsy vinyl-covered cardboard one. The positive lead goes to the board, but I did not solder at the board; I grafted it onto the existing lead from the board, which was how the old clip was attached. The negative lead goes to a terminal on the input jack. The good news is that it still works with a battery, but that’s also why this is frustrating to figure out. If it had stopped working with a battery I would just write it off as my incompetence.
Looks fine to me too, and I can’t imagine your pedal is drawing more than 200 mA. If the power supply was designed for powering similar pedals, it should be providing good, clean, ripple-free power, so that’s not the problem. The voltage matches well within the normal tolerance for such things, and these sorts of things usually feature integrated voltage regulation, so the supply voltage can be on the high side and not overdrive the unit. Assuming your diagram correctly identifies polarity and connection orientation (double-check all of those), there’s no obvious problem there either. I’m stumped.
The DC output of the adaptor is probably rectified but unregulated. The pedal can’t cope with the weird input voltage waveform. I’ve seen this problem before with equipment that only ever expects to run on batteries.
Very few of those off-the-shelf wall transformer adaptors are regulated. Perhaps you meant filtered? I can see them leaving out the caps to save costs. If this is indeed the problem, the OP can rectify (pun intended) the situation by installing a cap, say 2200 uF, across the power wires.
Yes, because your post made me consider the possibility of a lack of filtering in the power supply, which hadn’t occured to me originally. Hey, if I’m wrong the first pint of beer is on me next time you’re Stateside.
The thing doesn’t have an input power jack, and so would have no reason for an internal regulator. That means it also has no mains ripple rejection capability. That’s why the external regulator is required, and not just a bypass cap.
Not necessarily. Suppose there’s a dual-voltage supply for 5 V TTL logic + 9 V for the analog circuitry in there. Not saying there is, but I’m just pointing out why a battery-operated device might need internal regulation. Even if the 9 V branch is unregulated, it would be the 5 V TTL stuff that would be thrown off the most by AC ripple.
I guarantee there’s no TTL inside this thing, just 6 op-amps, 4 FETs and a BJT.
But even if there was, you’d still need the regulator so that you didn’t get 60 Hz hum in the output.
The pedal in fact does have an internally derived 3 V reference, but that’s of no use in eliminating hum because the op-amps are powered directly from the 9 V.
I think I’m missing something, or we’re not on the same page here. I can build an unregulated DC power supply with next to no AC ripple with sufficient filtering using high-value electrolytic caps; on the other hand I can build a well-regulated supply with loads of ripple (though why I’d want to is another issue), if I fail to include any output filtering. When you are saying “regulated” you mean “regulated AND filtered”?
No, you can’t, unless the source impedance is negligible compared to the load impedance.
“Well regulated” according to that usage is just another example of source impedance being negligible compared to load impedance.
I mean, fitted with some kind of regulator. E.g., a 7809, a 1N4739, etc. A feedback mechanism. And of course, the regulator ain’t gonna work without a storage element like a cap.
Cooing with Gas If you missed out on what Desmo vs QED is about, most simple power supplies (plug-packs) have unregulated power. That means that the adaptor provides nearly exactly 9V whether the load draws a current as low as 30 mA or as high as 1 Amp.
It would be strange that an adaptor designed for this job wouldnt be regulated. Desmostylus, could the adapter regulator be stuffed?
I saw a great article in Silicon Chip magazine a few years ago that shows you how to convert a simple plug pack into a regulated battery replacer. Just open it up, put in the regulator and close it back up.