Mackie CFX making a popping noise. Also strange fader/channel strip behaviour.

[genekrupa]

Hi folks, I'm new around here and looking for some guidance with a weird issue and a second weird issue which I seem to have created while trying to fix the first one.

I'm attempting to repair a CFX 12 which has an issue where it will generate a regular 'pop' noise on the master output. Initially this would only happen when channel 1 gain was near max and the channel, group, and master faders were near max. I could do the same with any other channel without any issue. The popping gets faster as I turn up any combination of channel gain, channel/group/master fader, tape input, headphone output, utility output. Eventually if enough of those controls are turned up high, the popping stops but the mixer either doesn't pass audio or it is very distorted.

To try and locate the problem I tested the power supply capacitors (didn't see anything wrong) and the capacitors on the channel 1 circuit strip that I could test without desoldering them. As channel 1-4 are on a secondary PCB attached via ribbon cable, I had to take that PCB out to test it before putting it back in.

Now (after reinserting channel 1-4 PCB) channel 1-4 faders produce a high level of scratching noise whenever they are moved, their mute buttons make a weird noise when pressed, and their output is much lower than it should be so they require tons of gain to pass a signal.

I've tried reseating the ribbon cables that connect that board to the main board with no effect. And here's the kicker: Now the popping noise is present when I turn up the tape/headphone/utility controls without channel 1 maxed like before. So I disconnect the channel 1-4 PCB entirely and the popping still occurs, so the initial problem doesn't appear to be on that board anyway.

Sorry if this is a bit confusing, I've tried to explain it as well as I can.
I'd definitely appreciate any ideas!

[Enzo]

OK, you took it apart, make VERY sure the ribbon is back on the proper pins: Not off a row, and not off one pin to either side.

Mackie has had a lot of problems with ribbons. Often as not, problems with Mackie mixers are fixed by pulling every ribbon connector half way off and pushing it down again. That refreshes the connector surface.

Your pop at intervals seems to me like a DC problem with a missing termination somewhere. Like an op amp input or something that is floating, and every so often it charges up until it can discharge through something, and a new cycle starts. But that lack of termination might even be due to a ribbon.

You hear a pop in the master, but does it also appear on any monitor sends? and AUXs? Any FX sends? ANd sub outs? Any inserts?

[genekrupa]

Yeah I was always careful to line the ribbon up. It goes through a slot in the PCB so it would be difficult to misalign it anyway. I've already reseated them a few times so the connections there should be decent enough as long as the ribbons themselves are fine. I've heard things about Mackie ribbons before but surely the cables themselves should be ok?
There are two ribbons connecting the channel 1-4 board to the main PCB and the only other ribbon in it is the one connecting the effects unit to the main board.

The pop is present (to some degree or another) on all the outputs: master output (xlr and trs), tape output, master inserts, auxes and group outputs. I've just discovered that it is also quietly present on the channel 1 insert point when you apply a lot of gain.
At first I'd thought channel 1 to be the origin of the fault given that it previously only did that when channel 1 was open and gained, but the noise is still present when channel 1 is muted or even disconnected now.

Edit: When you listen to channel 1 insert it it also makes a noise when you move fader 1-4 as long as they're unmuted. But it's not the scratching noise that they make out the main bus, it's the 'squelch' noise that the mute buttons make.
All other channel inserts always behave normally though.

[Enzo]

"Surely the XXXX must be OK." Famous last words. There is no surely ANYTHING must be OK, ANYTHING can fail. Ribbons have female connectors on the ends usually, and those female pins inside can be corroded, bent, oxidized, dirty, or just lazy. Of course it can be something else, but never assume some part can't be involved.

Check the power supplies, are they both up to voltage and clean? does the pop appear on either or both supplies? And while a supply might be OK, it has to get to each IC, so are all the ICs in channel 1 getting full voltage at their power pins? Also, is any op amp showing unwanted DC on an output pin?

[genekrupa]

I'm currently awaiting the delivery of my shiny new oscilloscope so I'll report back once that arrives later in the week.

[Enzo]

Hopefully it will be a flam dunk.

[Mick Bailey]

Sometimes ribbons fail at the IDC connection. Oxidization and the connector/cable eventually relaxing can give trouble. Mackie had more problems than most at one time and issued a service bulletin to replace the cables in some mixers because the cable and connectors were mismatched. It may not be a problem with your particular mixer, but if you draw a blank it may be worthwhile coming back to those ribbons and checking each pin end-to-end.

[genekrupa]

Got my new toy and checked the power supply. All the outputs (+5, +48, +15, -15) all seem pretty good. No low/unstable voltages or dips when the mixer 'pops'.
I also tested each pin on both ribbons for continuity and it seems that the ribbons are conducting fine.
Not really sure where to go from here?

Side question: I've noticed that sometimes the 'status' on the top left corner of my oscilloscope flickers very quickly between two or three things (ie: auto/armed/ready and armed/trig'd). Is this normal behaviour?
I've not used digital oscilloscopes much so this might be a daft question. Scope is a Siglent SDS1022DL.

[Enzo]

How did you test the ribbons? Not to dwell on it, but if you poke a pin down each hole at each end and measure with a meter, that is not a measure of how well the ends mate with the other gender on the boards.

I don't know your scope, but if a sweep indicator goes off, it usually means some brief event took place, like a single pulse or a glitch of some sort, or even just the test probe momentarily lost good contact with the work.

[genekrupa]

I only tested the ribbons themselves - the pads on the board are obscured by the ribbon when it's in place so I didn't bother to look at them. I'll have another play later and see if I can test it in-situ.

When the probes are disconnected or not touching anything it flickers between auto/armed/ready fast enough that it's tricky to read it. Surely it should just be showing one thing if the input is constant?
Just want to check that I'm not being stupid before I contact the manufacturer.

[Mick Bailey]

Read the manual if you haven't done so already.

[genekrupa]

The manual just explains the functions but doesn't say why it would be toggling between them so quickly.
Maybe that's normal behaviour when it's trying to find a trigger source but I'll contact the manufacturer anyway while it's still under warranty.

[g1]

I think you are correct, if the probes are not connected to anything, the trigger light will flicker.
As long as it is stable when you are actually connected, you should be ok.

[genekrupa]

I had another look at it and managed to continuity test on the solder pads on either side of the ribbon connectors, all fine.
Tried disconnecting the FX unit on a whim but that had no effect.

Beginning to think this might end up getting pretty involved.

[Enzo]

well, if it is under warranty, we shouldn't have been doing any of this, we should have given it to the nearest warranty repair center for Loud products.