Mackie 1604VLZ input problem

[Billy R aka DynaFreak]

I have a 1604-VLZ mixer that has an intermittent problem on one channel. The line input works all the time but the mic input craps out on occasion. Putting a strong signal into the mic input seems to clear it up sometimes if that makes any sense. Before I open it up and look for bad solder joints and try to trace the signal, are there any other "usual suspects" for this problem? I have the schematic for the input and there are only a handful of components btwn the mic jack and the part of the circuit where the line input connects (inductor, 47uf coupling cap, small value series resistor). Thanks.

[Jazz P Bass]

Sounds like a bad solder joint at the jack.

[Enzo]

I'd agree. Other than a couple resistors and caps, the two inputs go to the same place in the circuit.

[Ronsonic]

It's been a long time since I've been in one of those, so be kind if I'm thinking of something else ...

Is this one where the XLR input runs through the contacts of the TRS jack? If so cleaning up the 1/4" jack could solve. Those are double sided boards with through holes and tight fits and not all that subject to bad solders (that much I do remember).

The overwhelming majority of "Break through" behaviors, where a unit doesn't pass signal until the input signal is increased and then it works fine for a time, as if the signal "broke through" some barrier comes from dirty connections or a failed opamp where the input circuit drifts out of bias and a strong signal "whacks" it into working right. Yeah, I know technical terms

First clean the TRS, then check the black things in the input circuitry.

[Enzo]

You are thinking of something else. On this one, the 1/4" and XLR inputs are more or less parallel.

The XLR pins 2 and 3 run through a couple 4.7 ohm,then a pair of 6k8 pullups to phantom, a couple series 47uf e-caps, and into the transistor bases. Those bases are terminated to ground by a couple 1k.

The TRS runs tip and ring through a pair of 47uf e-caps, and then a couple series 10k, and that connects to the same transistor bases. The series 10k and the 1k to ground form a 10/1 voltage divider to reduce the hot line level signal.

SO conceivably you could run both inputs at once.

I'd agree that those "breakthrough" situations usually involve dirty contacts, that is the classic description of FX loop jacks needing cleaning.

[wayward bluesman]

In my experience with the 1604, 90% of the time the problem is either dirty jacks/pots/faders or those infamous ribbon cables. Recently, I had a 1604 on the bench with a couple of intermittent channels and, sure enough, the slightest movement in those ribbon cables would cause the signal to come in and out. After a thorough cleaning, opening the board up and checking those cables would be the next suggested move.

Unfortunately, you can't get those cables individually; they only come as a set. You can still get them directly from Loud Tech (Mackie's parent company) even if you're not an authorized service center. I believe they cost close to $80 including shipping.

[Enzo]

Silly me, you're right, completely forgot about the ribbons. Total pain in the ass disassembly to get at them too. Abd usually pulling them half way off and shoving them back down refreshes the contact, and teh thing works another year or two.

[Billy R aka DynaFreak]

thanks for the responses. I might be wrong but if the line input always works doesn't that rule out a ribbon cable problem?

[Enzo]

Probably true.

But if you have to disassemble it enough to get at the jacks take the time to reseat any ribbon connectors you can access.