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WIGGLING 'LITE' IN GUEST MODE

Testing for shorts before plugging in
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Testing for shorts before plugging in
StudentsOfTheFuture
I'm looking for a method to test modules before powering on while I wait to finish my more dedicated testing tools. I couldn't find this when I searched, but I might have searched wrong - if this has been discussed in depth, please link away!

I've read that you can get a decent idea by testing resistance between positive and negative rails and ground. I'm assuming the power header is a great place to do that.

I'm wondering - what's a good reading?

I tested a filter, a buffered mult (my control case), and some Division 6 sequencers.

The filter gave me multiple millions of ohms between positive and negative rails, and between them and ground, which felt good. The mult did the same thing.

The sequencer would only give me readings from positive to ground.

Looking at the power specs, that kind of makes sense to me, as it only uses positive 12v. But is that correct?
cornutt
You're saying you got infinity from negative to ground on the sequencer? Yes, it's possible that it doesn't use the -12V at all.

I don't know how valuable this really is; it seems unlikely to me that a new module would have a dead short between power and ground. I guess it's worth checking on a used module or DIY. I'd say that anything higher than 1K or so is all right. If it really does have a short, it's only going to read a few ohms at most. When you test, make sure you apply the proper polarity for the bus that you're testing.

If the module has reverse-polarity blocking capacitors on the power inputs, those will tend to hide evidence of a short. You have to hit the diodes with enough voltage to overcome the diode drop resistance, and most ohmmeters don't.
StudentsOfTheFuture
Yeah - asking only for just-finished DIY modules. I figure commercial builders/factories probably have a better handle on this than I do lol.

And yeah, pretty sure it was infinity (my starter meter gives me "1" for infinity and "-1" for approaching 0 on the resistor tester... I think).

Thank you - the negative thing did not occur to me at all, but that explains a lot of "inconsistencies" I was getting earlier.
mskala
I use the "diode test" range on the meter, which gives a reading as a voltage rather than a resistance. Then I expect to see a reading of at least 1.0V for each "forward" measurement (+12V to 0, 0 to -12V, +12V to -12V) and infinite on each "backward" measurement. However, exactly what you get in the backward direction depends on how, or if, the modules have built-in reverse-connection protection. Mine use series diodes; other methods may not read infinite in the backward direction.

The main thing is that you don't want to see zero on any of the six possible measurements, nor less than about 0.5V or 100 ohms on any of the forward ones.
basicbasic
I usually just do a continuity/beep test between 0v and +12 and -12 before I plug it in.
heapish
Never thought of this. I just inspect all my work as I do it and do an overall inspection at the end.
basicbasic
After I meticulously built a Synthrotek Verb only to find there was a short on the PCB itself between -12v and 0v (one of the power pins to be precise) I can't recommend doing this enough. Dead Banana
wackelpeter
Best way is to measure at the power header... resistance depends on IC's how many pot's are used as voltage dividers between the rails for Controlling stuff and also what kind/value of resistance is spread between the rails around the circuit...

Another good advice maybe to turn all the pots that are connected to the rails in each direction while measuring... I usually build on stripboard and once i forgto to cut a trace so in one turning direction of the pot everything seemed to be fine,. I measured the something like 30 or 50k between + and gnd while the 100k from the wiper to the other lug of the pot were there, but when i turned it the other way around it was going down to zero ohms which gave me a nice short between + and gnd...

Also worth a try is to include 10-15R resistors instead of beads, so they will handle the current of a short circuit or to use for simple first power up a small adaptor with fuse holders and some fast fuses of say 200mA or a little bit more or less.

I happily blew a lot of fuses but not that many IC's... The latter only when i wrongly inserted the wrong logic chips (those that can only handle 5V supply voltage HC4xxx??) into any of my modules.
wackelpeter
cornutt wrote:

I don't know how valuable this really is; it seems unlikely to me that a new module would have a dead short between power and ground. I guess it's worth checking on a used module or DIY. I'd say that anything higher than 1K or so is all right. If it really does have a short, it's only going to read a few ohms at most.


When you insert the wrong logic chips into your module, they will most likely blow up and from what i experienced also give a short between positive and negative rail...

Had this experience once with some 4016 IC's which weren't CD4016 as needed but HCF4016 or something like that, with 5V supply voltage... Just grabbed them out of the box only looking if it's a 4016and not what type then inserted them into their sockets and voila... they didn't survived it and gave me a solid short between gnd and positive rail. Also a good reason to use sockets always...
abelovesfun
basicbasic wrote:
I usually just do a continuity/beep test between 0v and +12 and -12 before I plug it in.


This is what I do as well.
StudentsOfTheFuture
wackelpeter wrote:

a small adaptor with fuse holders and some fast fuses of say 200mA or a little bit more or less.


As soon as I get my head around how to build one of these I'm making one. I've seen a couple of modules that expose a power header on the front of the case but none of them seem to have a fuse option, even the allegedly testing-focussed ones.

Continuity and diode testing (same setting on my meter) worked better for me than resistance was, and they tell me more about the circuit in the process - thanks for the tips!

(and mskala, off topic but I LOVED the Cruel Angels Thesis demo on your octave controller. When I eventually grab one, that's gonna be why lol)
bonzai
basicbasic wrote:
I usually just do a continuity/beep test between 0v and +12 and -12 before I plug it in.


This, and also a visual inspection using a magnifying glass and a strong light from behind the board. Happened a few times to me that a very fine splash of solder was bridging over to another rail, these buggers are really hard to find.
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