Strange phantom power issue...

[jdurango]

Sound Workshop Series 40 console, fully recapped and gone through. Most everything works but still ironing out a few bugs. This is one.

Mic pre only works if phantom power is turned on, then off. It will then pop on and operate as normal for 8-10 seconds until it fades out. There is only one large cap in the mic pre section. I have replaced these with larger value Nichi MUSE caps (bipolar). This is the only channel experiencing any such problem. Any ideas? Thanks fellas!

Mic pre schem: https://drive.google.com/a/mobstudios.com/file/d/1oOL_WWpaKtJIT_5wVBEVFxqZQtmDPYh_/view?usp=drivesdk

Input module schem: https://drive.google.com/a/mobstudios.com/file/d/1QMJVEhI-YnayH57ng0pd_fN-rD93ADJL/view?usp=drivesdk

Video demo of issue: https://youtu.be/vyDt9moEppo

Jonny

 

[Bo Deadly]

That's pretty much normal.

Your schem doesn't show where the "large cap in the mic pre section" is but I assume it's downstream from "PH PWR 3". When you disengage, the cap is drained through a presumably smallish resistor next to it backward through the switch to ground. Because you used an unnecessarily large cap, it's just taking longer for it to drain than normal and so the mic stays on longer.

Presumably when you engage and just leave it engaged the you get signal right?

And I assume you have an actual LDC mic on the other end in front of a speaker or something?

Hopefully you're not driving it with a line out or you might be torturing your input with "The 48 Volt Phantom Menace".

 

[jdurango]

That's pretty much normal.

Your schem doesn't show where the "large cap in the mic pre section" is but I assume it's downstream from "PH PWR 3". When you disengage, the cap is drained through a presumably smallish resistor next to it backward through the switch to ground. Because you used an unnecessarily large cap, it's just taking longer for it to drain than normal and so the mic stays on longer.

Presumably when you engage and just leave it engaged the you get signal right?

And I assume you have an actual LDC mic on the other end in front of a speaker or something?

Hopefully you're not driving it with a line out or you might be torturing your input with "The 48 Volt Phantom Menace".

Sorry, memory failed me, TWO "large" caps (relatively speaking for a mic pre). C1 and C2 22uF 63v. I believe I replaced them with 100uF 100v.

I'm using a 1k @ - 50dBu test signal. Again, no issues on other channels. Never run into this problem before. And signal passes for about 8-10 seconds ONLY after cycling power on phantom. Engaging +48v and leaving it on does not pass audio. As you can see in the video, only after a power cycle (on then off), it fades in, then fades out, almost as if cycling the phantom power (not just turning it on) opens up a path for the caps to charge up fully, sustain for 10 seconds, then drain.....that's certainly what it sounds like. The ph power switch simply passes voltage from the rail and uses a Q1 to illuminate the +48v LED....not much going on there. Not sure why it would be messing with the audio signal unless there was a wiring problem. Mic ins have all been tested for correct pins, phase and ruled out improper shorts. I'll check more tomorrow, but very odd.

 

[Khron]

Stupid thought, but how about reinstalling the stock caps there, and see if it still behaves the same, or reverts to working properly?

 

[Bo Deadly]

Sorry, memory failed me, TWO "large" caps (relatively speaking for a mic pre). C1 and C2 22uF 63v. I believe I replaced them with 100uF 100v.

I'm using a 1k @ - 50dBu test signal.

Ok, but with what? A line output? Note that if you connect a live line output to a mic input with 48V on, it will cause several amps to be drawn through the diodes and 33R through the two caps. This is known as The 48 Volt Phantom Menance problem and it can damage surrounding circuitry.

Or are you using a little phantom powered device like a DI or mic in front of a speaker?

Please be explicit. You are only showing one half of the circuit. The other half is what is connected to the input.

Again, no issues on other channels. Never run into this problem before. And signal passes for about 8-10 seconds ONLY after cycling power on phantom. Engaging +48v and leaving it on does not pass audio. As you can see in the video, only after a power cycle (on then off), it fades in, then fades out, almost as if cycling the phantom power (not just turning it on)

Ok. But for the record, you do not show that in your video. You are always turning it off immediately. You should really show the actual point-of-failure in your video. And you should show how an adjacent channel works correctly.

opens up a path for the caps to charge up fully, sustain for 10 seconds, then drain.....that's certainly what it sounds like. The ph power switch simply passes voltage from the rail and uses a Q1 to illuminate the +48v LED....not much going on there.

That's not all that's going on there. When you disengage, the switch grounds phantom.

Without more explicit information it is difficult to say but clearly when the voltage across some capacitor (either the main supply capacitor, a capacitor in a device connected to the mic input or possibly the two input coupling caps) is grounded, because the voltage across a capacitor cannot change instantly, it will, for a short time, go negative. That could is apparently biasing some circuitry into it's operating range for some time until the caps discharge. Over time the caps settle and, because some surrounding circuity is damaged (such as the protection diodes and possibly mic amp circuit because of a "Phantom Meance" event) it falls out of bias.

If I were diagnosing this, I would use a scope to watch dc on the inside of the two coupling caps while toggling phantom. The should always remain near 0V. Then look on the outside and test again and keep moving upstream until you find the problem.

However, I am still suspicious of the 48V supply cap for one very simple reason: The time that it takes for the signal to die off is quite long. That means that the R and / or C in the RC is relatively large. There are no large resistors around the coupling caps. But the 48V supply RC could be several hundered ohms. You should really show the part of the schem that has those parts.

 

[JohnRoberts]

Just spit balling here but might be a dry (low current) contact that is lightly tarnished or oxidized. The application of phantom power may break through a thin insulating oxide layer, allowing it to conduct until it oxidizes closed again..

Exercise switches and jacks looking for a suspicious interconnection.

JR

 

[emrr]

I like JR’s thought. 

 

[radardoug]

Or a dry joint.

 

[Bo Deadly]

Perhaps. But it doesn't explain clearly why there would be supply when the switch is DISengaged. If a slight movement without actually switching is enough to break the oxidation and trigger a burst of current that would support your theory.

 

[jdurango]

I've been on another job but will be back in the studio today. I'll check these things out if I have time. I've cleaned the hell out of all switches and pots (soaked entire channels in isopropyl bath, working all components then finishing off with CRC protective + Stabilant 22).....that said, some are still problematic. They are late 70's era Alps switches. I'm going to try a high concentration of Stabilant and/or Switch Cream on the remaining problem switches and pots before replacing them.

Anyway, I've worked this particular switch like crazy thinking that might be the problem. Doesn't seem to help, but maybe lotsa cleaner + Stabilant will do the trick. If not, I'll break out the scope, PSU and function gen, throw it on the bench and look for stray DC. Thank fellas!