ML4000 faults...

[Solstace]

'Lo!

 

Sorry - Have just come back for lunch after a busy morning at work, where we're having a few problems with our desk. The post is a long-un - but I'm trying to include as much info as I can to prove to myself (if no-one else) that I did in fact check everything!!!

 

By way of background, I've known about some problems with Aux Masters 9 and 10 on our desk. On some occasions, the signal will disappear from the meters and outputs on these channels, and I notice that moving the faders slightly will bring the signal back. Not a huge problem, but as these are used to send a signal to a Lexicon with a ping-pong delay, this can cause some interesting moments when the signal comes back and is at a higher level than desired... I have replacement cards on order for both these channels in the hope that this will solve our problem. Hey - they're cheap enough and if this method is overkill, having spares is never a bad thing.

 

Today, I've come in and patched a vocal mic into channel 38. I purposely took it off the usual VCA assignment as this was not needed. As per usual, I checked that the channel was assigned to the main mix, and panned appropriately. A quick line and sound check before doors open, all's lovely and we have sound.

 

30 mintutes later, I came to use the channel and brought the fader up - nothing. Channel was showing signal (both when solo'd and on it's own meter beside the fader), was still not assigned to a VCA, was not muted, and still routed to the main mix. To all intents and purposes, it should have worked. Tried moving the fader, the pan pots and switching the EQ in and out of the circuit.

 

Between cues (and seing as this was a relatively vital mic) I decided to patch the mic into channel 39. I copied the EQ, pan, gain and rolloff settings from the previous channel, did the usual idiot checks (background signal, mute, VCA assignment, main mix button) and everything looks fine.

 

Next cue, bring the mic up, and had exactly the same problem - signal coming into the desk that could be solo'd and metered, but nothing out of the speakers. Given that other mics were working, and the channel was panned to dead-centre all round, we should have had something!!!

 

After the service finished, I decided to see if it really was a desk issue, so I plugged a CD player into Channel 39's XLR input, taking care to bring the gain down and add the 20dB pad. Mute the main outputs, bring the fader to 0dB and bring up the channel gain until I had a signal peaking at 0dB on the solo meter and the channel meter. Un-mute, STILL no sound. Again, did the usual idiot-checks in case there was a stray VCA or Mute Group assignment somewhere in the works, and for the third time confirmed that no VCA or mute group was assigned, let alone muted or pulled down...

 

As a final idiot-check, I re-assigned the channel to VCA 7 (as per it's default), brought the VCA up and sure enough, I had sound! I then took took the channel back off the VCA and still had sound, as I should have done some 45mins previously!

 

I've yet to test channel 39 in the same way, but plan to do so later this afternoon.

 

Before I run away with my own theories as to what's happening under the hood, has anyone else had a similar problem, and what was the cause, suspected or otherwise?

[griffter]

Solstace,

 

Are the circuit boards in the desk arranged in banks of 8 channels ? , Sometimes it can be that a ribbon cable has come undone on that bank which

would explain why 38 and 39 are not getting any audio to the master.....Just a thought... better get to my gig!

 

 

Cheers

 

Griff

 

 

 

Sorry just reread this, it's probably not the problem as you were getting signal when you reassigned to VCA7!

[dbuckley]

My guess is that the ribbon connector connectors are oxidised, and as such are unreliable at small signal levels, and mechanical movement or application of signal is enough to break through the oxide allowing the signal to then pass.

 

I think this is also causing your channel dropout problem; on your desk each channel's audio always passes through the VCA, the VCA group switching just determines if the supply voltage to the channel "fader" comes from a constant supply or from a VCA master. I suspect there is more contact oxidation here. Theres a bit more guesswork in this latter determineation, as I dont have the schematics in hand, but I suspect that each channel's PCB is electrically responsible for managing its VCA assignment.

[Solstace]

Cheers guys!

 

The Aux problem isn't worrying me. Looks like I'm gonna have to flip the lid and see what's going on. The thing is that our desk is used less (and more gently) than those present in most theatres or other venues, so we are a little disappointed that we're getting problems cropping up so soon.

 

We also take care to keep the desk clean, so the only thing I can think of is that either we have a "Friday afternoon" example, or the wild temperature fluctuations within the building are leading to condensation build-up...

 

Do either of these sound likely?

 

As for schematics and service manual info, I've got one on order so should be able to trace things through for myself, provided the materials get to me in good time before I leave for my new job!

 

C.

[Pete McCrea]

Our ML4 was exhibiting some strange issues while it was off on tour. A trip back to A&H seemed to sort it out. I'm not too sure exactly what the issues were, but it may pay you to have a chat with the guys at A&H who I'm led to believe are very helpful. Give them a call and they may well be able to tell you exactly what the score is.

[jim b]

Have you tried routing the offending channels to the main mix via a subgroup-the only ML4000 problem I've run into sounds similar-intermittent failure to send audio to the mix. The desk that had these symptoms had a mechanical issue (i.e. contacts not made properly on a loom or somesuch) within the desk and not a failure of the different functioning parts of the desk.

 

I wouldn't feel too bad about this issue coming up "soon" as it sounds like the problem may have existed from manufacture/initial transport and due to the channels higher up the board being used less frequently it just hasn't been come across. The fix is possibly annoyingly simple but it may also have occurred in shipping from the factory or just be down to plain bad luck. Either way it sounds like a fairly new desk so getting A&H out to put it right seems like a fairly good shout to me....as to who picks up the tab....

[Carey D]

Have you tried contacting A&H tech support? You provide a thorough description of the symptoms. That should make a good starting point for the support team to help. Email or phone (details on www.allen-heath.com).

Regards,

Carey (A&H)

[Solstace]

In all fairness to A&H - I didn't think anyone would be around in their tech support department yesterday! My previous dealings with them have all been very positive, so once I've had a chance to test the remaining "dead" input channel I'll get a detailed report to them to see what comes back.

 

Our local A&H-approved service centre seemed relatively helpul, but the costs seemed a little high compared to the "talk to A&H to work out which parts are going wrong and fix it ourselves with new/replacement parts" approach!

 

**UPDATE** 28/3/06, 20:10ish...

 

Called A&H Tech support today, who to their credit were entirely open about the structure of the desk. Given that I've already ordered some parts and a service manual, the plan is to take the lid off (to replace the Aux modules) and get busy with a multimeter on the VCA lines, as well as to check that all the ribbon connectors are seated correctly.

 

So we don't have a fix yet, but we think it's likely to be something so simple as a badly seated connector or a dry joint. Thanks all for your replies, and I'll let you know of the outcome as soon as I find it. :)