How Do I Test A D.I. Box?

[Mark]

With a multimeter. I think I have a dud.

[Scodiddly]

Open it up and test directly on the transformer leads?

[Mark]

Okay, I think the transformer is okay. How do I test that it's passing signal from the input to the XLR out?

Also, where should I set the meter to test the transformer? I'm guessing it's one of the Z settings, but I have no idea which one to use

[digitaldrummer]

if you can send some audio to it (even if it's just audio from a phone or whatever) then you should be able to measure some voltage (put your DMM on AC, although it will likely be under 1VAC) and then just see if you can get that on the 1/4" input, and then again across the XLR (voltage will be even lower). pin 1 is ground and pins 2 & 3 your signal. So try across pins 1 & 2, then across 1 & 3.

[Scodiddly]

Well, what *is* your meter exactly? Generally it would be an ohms setting, lower ranges. DC resistance isn't exactly the same as AC impedance, but it'll be in the ballpark.

For that matter, what's the make and model of the DI?

[Mark]

The meter is a Mercury MTB01. The D.I. is a Subzero SZ DIP-1 passive D.I.

[Nick Sevilla]

Does it pass any signal normally?

If it does not, can you send a 1kHz sine wave through it, via your DAW?

A multimeter will not tell you much if you don't open the DI box up, and start tracing the signal path.

[Scodiddly]

A really simple method is the "circuit disturbance test". You plug the DI into a mic input, crank up the gain a bit (be careful of your speakers/ears, a limiter would be a good idea) and probe the signal path with a metal tool from the mic jack inwards. You should get a "pop" every time you touch the signal path, and so when that *stops* happening you've maybe found your fault.

[winky dinglehoffer]

So a passive DI is (or should be) stupidly simple. Here's how I would test initially:
Set your pad switch to its 0 position.
Check DC Resistance between pins 2 & 3 of the XLR connector. If everything's hooked up right with the transformer, I'd expect to see 20 to maybe 60 ohms there, depending on the transformer.
Check DC resistance between Tip and Ring on the 1/4" jack (you can plug a guitar cable in & test between Tip and Ring on the cable). You should see something like maybe a couple hundred ohms, maybe higher. This is also going to vary depending on the transformer.

If all that's good, it should work fine with the pad in the neutral position. And really, I'd say the pad switch (or the parts connected to it) would be your most likely trouble point, barring a bad solder joint or loose connection somewhere.

Also, it's worth verifying your 1/4" thru connection is working. You could also use the continuity setting on your DMM to very the ground lift is lifting.

Send signal through & see how output levels change as you change positions on the pad switch. Do levels drop as they should? Is the DI functional with the switch in each of its 3 positions?

Search the web for "passive direct box schematic" if you need an idea of how a DI is typically wired. That might help you to troubleshoot.

[digitaldrummer]

we are talking a $16 DI here though? --https://www.gear4music.com/us/en/PA-DJ- ... I-Box/1SDA
I wouldn't expect to much in ease of serviceability (even though all the advice offered above is valid).

[Mark]

Set the meter to 200 Ohms. Can't get a steady reading but it seems to settle down somewhere between 3 and 3.5

[digitaldrummer]

Set the meter to 200 Ohms. Can't get a steady reading but it seems to settle down somewhere between 3 and 3.5

without knowing more about the transformer used it's difficult say anything about that other than it's not "open" and it's probably not shorted to ground. But that's a start. You really need to trace through the circuit from end to end. A passive DI is pretty simple. The labeling on this is a bit weird ("screen" is effectively ground) and yours has a PAD on it, which will just be some resistors. If you think the PAD switch is causing a problem, you should be able to bypass it with a jumper cable (alligator clips on each end are nice, but even a piece of wire twisted on might work). Can't say exactly where to connect without a photo of the insides of that thing.

[Mark]

Sorry about potato quality

[Mark]

I'm not convinced it's passing signal from the input to the XLR output. I plugged a bass (Harley Benton PJ-4) into the D.I. and then held the probes from the multimeter to each pin of the XLR jack. Zilch. You'll probably tell me I should have been playing the bass and using alligator clips on the pins, but i don't have any so that's the best I could do.

Given that it's a PCB, should I just write the D.I. off? I don't think I can get a refund given that I've opened it several times already.

[digitaldrummer]

I would try reseating (remove and reinstall) those plugs coming off the switches jacks. Sometimes miracles happen. Or if you can remove those and jumper together the pins (that should be left on the board side, but in the socket) Otherwise you should see some kind of resistance (with your DMM) across both sides of the transformer (with nothing else connected). I think you measured the XLR side before. can you measure the 1/4" side too? If there is nothing (it's open) then maybe the transformer failed or a solder joint failed. I suppose you could try just reflowing all the solder joints of the XLR and 1/4" jacks too. Then once you've spent 50 hours troubleshooting it, send yourself a bill and buy a new one.