Mackie CFX 12 mixer repair

[bruce]

Does anyone have any repair experience - or a circuit diagram - for a Mackie CFX series 1?

 

I've got the block diagram that's in the user manual, but no service manual.

 

The symptoms:

 

All seems to work OK, apart from mic channels 3 and 4 dead. All other channels OK.

 

When you put an input on to 3 or 4, the "signal" led flashes as expected. But you get no output from the channel.

 

The signal gets as least as far as the "insert" jack and the EQ section.

 

It also gets as far as the "Solo" bus - you can solo it and listen on headphones.

 

But it doesn't get to the groups, or Auxes.

 

 

Comparing these symptoms with the block diagram on http://www.mackie.com/pdf/CFXMKIISeries_OM.pdf (which admittedly is for the MkII version) it's almost as if the "mute" switch is stuck. But doesn't appear to be a mechanical fault.

 

Only affects channels 3 and 4.

 

 

Any suggestions before I get the screwdrivers out and pull off hundreds of knobs....?

[Mixermend]

There is one op amp - the two halves of which are shared between pairs of channels. This is the 10dB make up gain following the main channel fader - so bearing in mind that you have Solo or PFL outputs (and I would have thought that you might have had pre-fade AUX outputs present?) - this is the one to change!

 

PM sent

[bruce]

There is one op amp - the two halves of which are shared between pairs of channels. This is the 10dB make up gain following the main channel fader - so bearing in mind that you have Solo or PFL outputs (and I would have thought that you might have had pre-fade AUX outputs present?) - this is the one to change!

 

PM sent

 

Don't think I'vgored anything on pre-fade aux - but will check later tonight.

 

Thanks

 

 

UPDATE:

 

You were quite right! When switched to post fade aux, no signal, but when aux is switched to preface I get output.

[bruce]

Just to close this one off - the mixer is now back in full working order. John (Mixermend)'s diagnosis was spot on - an op-amp was faulty. he even identified which one!

 

Simple to replace - well, not quite!

 

 

1. Remove about a dozen screws from the casing, which allows the top panel to be removed. Unplug the multipin cable that links the main PCB to the PSU board.

 

2. The mixer is actually on 2 main boards. The "master" board has 4x mic channels, 2x stereo, and the master section. The "slave" board has 4x mic channels. On the bigger mixers in the range, there are additional slave boards. The fault affected channels 3/4, so that was on the slave board. That was good news, as fewer bits to disconnect. Disonnect the 2 ribbon cables joining the board.

 

3. Remove the slave board. Pull off about 40 caps from the pots, and pull off 4x fader caps. Take out 8 self-tapper screws from the XLR connectors. Then there's 10 hex bolts holding the board to the front panel. Removing these was a pain, as they were imperial hex bolts and I only had metric. Even buying a cheapie imperial set from the pound shop didn't help, as the bolts were tight and mangled the cheapie hex keys. Got some decent quality tools, board removed - now we can see the faulty chip. Poked around with a voltmeter and confirmed fault.

 

4. Swearing when we saw the chip was surface mount. At least it's in an accessible part of the board. Sourced a suitable equivalent, took the mixer into the workshop at work and dug out the good soldering gear.

 

5. It's been ages since I last soldered surface mount. Dismayed how much my eyesight has degraded since then.

 

6. Soldered. Tested. Works. Reassembled.

 

 

Thanks for the info, John.