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Negative rail randomly shorts out. How to diagnose?
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page 1, 2  Next [all]
Author Negative rail randomly shorts out. How to diagnose?
sleepmute
Here's my current rack:


I've been randomly losing the -12v rail (my -12v LED goes out and everything stops making sound). If I switch the case off then on again, the power comes back. I've tried removing all the DIY modules, but it still happens. I can't pinpoint a specific thing I'm doing that's causing it; sometimes it'll happen right when I plug a patch cable in, and sometimes it happens when it's just sitting idle.

It's powered by a Power One supply that as 1 amp at +/-12v. The busboard is a series of small stripboards that I soldered headers onto. I'm not sure what the draw is on the Moog clone or the DIY mixer, but Modular Grid tells me I'm at 560ma, soI'm guessing it's not being overdrawn.

Any tips on how to troubleshoot this?
dKRelease
I’d remove each module one at a time (including non DIY ones) and leave that module unplugged for as long as it takes for the issue to happen again (or not, in which case you’ve found the offender). If it happens with a module unplugged, you know it’s not that one, so plug it back in and unplug another. Rinse, repeat.
GGW
Maybe it's not shorting out, but loosing connection/power on the -12v bus?
Can you hook a voltmeter to the 12v on one of the connections and see if it is dropping the power before it gets to the boards?
Revok
GGW wrote:
Maybe it's not shorting out, but loosing connection/power on the -12v bus?
Can you hook a voltmeter to the 12v on one of the connections and see if it is dropping the power before it gets to the boards?


This would be my guess too. A short on a power rail should hopefully blow a fuse.
jimfowler
Power one supplies usually have short protection built in...so no fuse to blow.
sduck
I'm confused by this - the "busboard is a series of small stripboards that I soldered headers onto" - can you post some pictures of this, back and front if possible? You're going to want to be removing all the modules anyway.
sleepmute



Here’s what I’ve got. I know it’s not great. As you can see, I have LEDs wired to the positive and negative rails. When the problem happens, the negative LED goes out. Last time it happened I measured the voltage on the power supply, and it read -0.306. I’ve been taking out one module at a time all evening, but the problem doesn’t always happen right away, so I haven’t been able to pin it down. I’ll keep trying.

Oh, and normally there’s a lexan sheet covering the back, so no exposed wires.
sleepmute
Couple other things I should note. This problem started happening recently when I rebuilt the case. It was an Ikea Rast rack originally, and I took everything apart and cut all the wood down to size. But I didn’t change the bus board or power supply. So a connection might have come loose, but I can wiggle all the cables, and nothing happens, so I don’t think it’s that. The modules might be plugged in in a different order than they used to be; not sure if that matters.

The other thing is that I’ve been working on a spring reverb driver module, and while I was testing it I accidentally plugged it in backwards once. I blew up the op amps in the module — no big deal — but I wasn’t sure if it could have any other repercussions. A little knowledge is a dangerous thing, and it’s pretty much what I have.
khakifridge
How many wires do you have connecting each of those small boards? My poor eyes see only two. How's the 0V being propagated? hmmm.....
sleepmute
It’s hard to see in the picture, but there’s a thick copper wire in the centre between each busboard.
Quinie
When it's not the power supply


you can do a 50/50 check.

Take half of the modules out.. Problem gone? it's in the other half.
Problem still there.. take half of the half out.. (1/4).
Problem still there take half out..

until you find the module that's buggy you.

The first test 50/50 can also be use to see if it's the power supply. When error in both halfs it isn't a module wink

ps. my quese would be something in the power sulpy or bus. problem started after building a new case. modules didn't change.
sduck
I'm almost 100% certain the problem is in that "busboard". I see several problems with it - too small wires, not enough of them, and too much potential for bad connections. If I were you, I'd consider just ditching it and get a decent ready made one. There are lots of affordable ones available.
sleepmute
Haha "busboard" in quotes. Agreed, it is kind of a nightmare. It never gave me problems before, though, and I didn't remove it from its board when I rebuilt the case, but I've been meaning to get a better one anyway, so now's as good a time as any. This AI Synthesis one looks about right.

Would a bad busboard explain why the negative rail always comes back when I cycle the power, though?
jimfowler
Don’t take this the wrong way but you really ought to ditch your current distribution setup and get something more, um, robust and then re-assess.

The power one supplies (some at least) are designed to shut down a rail if it detects a problem. Now I have no idea what your problem is but that you’re able to cycle power suggests that the psu is resetting only to have the fault recur. Shit, it could be the psu itself but the first thing to do is get some proper rails or a pcb busboard. Neither are expensive and both would be more reliable.
sleepmute
No offense taken. It started out as a single board for a very small setup, and I should've bought a proper busboard as soon as I wanted to expand. Even if I have to order a new PSU, I'd still rather have a better distribution system.

Thanks, everyone, for your input! I'll report back once it's installed.
sduck
sleepmute wrote:


Would a bad busboard explain why the negative rail always comes back when I cycle the power, though?


Absolutely. Even good busboards go bad sometimes. I had one in a cabinet, it was a nice one from Synthesis Technology back when they were doing MOTM stuff. For whatever reason it started having the exact same problem yours is having. Replaced it, everything's been fine since.
sleepmute
Thanks, I appreciate that. I ordered the busboard and also a smattering of ribbon cables to replace the couple of homemade power cables I have (I know, I know). I guess it was just a matter of time before this failed.
sleepmute
New busboard is in, but the negative rail still randomly goes out. It seems more likely to happen if I’m actively patching, and I’ve noticed a few times it goes out right when a patch cable touches a jack (not always, though). I’m working my way through removing modules one at a time and testing, but I’m almost at the end and I somehow doubt it’s a module.

I might end up just buying a new power supply; not sure what else to do at this point.
NV
sleepmute wrote:
New busboard is in, but the negative rail still randomly goes out. It seems more likely to happen if I’m actively patching, and I’ve noticed a few times it goes out right when a patch cable touches a jack (not always, though). I’m working my way through removing modules one at a time and testing, but I’m almost at the end and I somehow doubt it’s a module.

I might end up just buying a new power supply; not sure what else to do at this point.


At what point of inserting a plug? Some modules implement the switching lugs in jacks, with a voltage normaled to the switched lug. If this is a PSU rail normaled straight to the jack without protection it can mean shorting +/-12V to ground when inserted. You wouldn't (or at least shouldn't) see this in a production module but it can sneak into DIY builds.

The 50/50 test mentioned above is a solid test - power up half the modules, see if it still happens. Then power up the other half and try again. On that test, I would say power up all the Intellijel, MakeNoise, and XAOC modules on the first test. Those are proven modules that shouldn't be causing problems. Then on the second test, try the rest including your DIY modules.
sleepmute
I guess it would be when the tip of the plug contacts the sleeve of the jack. But it's happened a lot of times when I'm not patching anything, and it's not one particular jack.

I've now managed to reproduce the problem with each module removed from the rack, so I'm leaning towards this being a power supply issue. I noticed that the date of manufacture on the PSU is December 1999, so almost 20 years old. I can get a new Bel Power supply with an extra half amp for $120CDN, so I may just do that unless anyone has suggestions for diagnosing the PSU.
emmaker
I haven't read the full thread but.

Once the system is up and running have you tried wiggling/moving the wiring one cable/'wire at a time to see that craps out?

I'd give each connector/connection a real close inspection looking for shorts, frayed wires either shorting or not enough copper to conduct current and just dirty or corroded contacts.
sleepmute
I had, but I’m glad you suggested it again. Before giving up I decided to take the PCB off the power supply and make sure there wasn’t anything stupid like a screw behind there, and I discovered this:



You can see the broken solder connection in the middle there, which leads to the transformer. In order to mount the PSU, you have to unscrew the transformer, so it must’ve tugged on it and loosened it. Anyway, it’s all soldered up now, so hopefully the problem is solved!
jimfowler
Good find! That broken connection certainly isn’t helping. Let us all know how it works out.
sleepmute
I spent an hour or so making drones the other day, and it didn't crap out once, so I think it might be fixed!
sleepmute
Aaaand it's doing it again, the negative rail goes out. It seemed better for a day or so. I should get a new power supply now, right?
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