Showtec MultiExchanger: Fault or Feature

[howartp]

Hi,

 

We've got 24 channels of lights (simple par cans etc; no scanners) into 4x 6-channel dimmers (not sure what model) which we've run via analogue with a Sirius 24 board for years (borrowed annually from the local theatre c/o my cousin who works there). Now we're starting to use the lights more so we decided to get a demux and small DMX board for ourselves.

 

I bought a 2nd hand Showtec 72 ch Multiexchanger off ebay last week and installed it on Thursday (with 25-pin suitably rearranged). Ran a small cabaret show on Friday using my cousins own small DMX board which worked fine all night; also swapped in another Sirius 24 from work which isn't used anymore and the lights worked fine, although the board is stuffed and causes lightning effects when you touch it - that was the reason for trying it, to see whether it was any good. Swapped back to cousin's DMX board ready for Saturdays show.

 

Got there Saturday and the lights wouldn't operate past about 50% brightness via either DMX board or MIDI from my PC - using the overrides on the dimmers brought the lights on and off at 100% brightness, so the lights themselves are definitely ok. I managed to get through the show as we only had about 8 channels in use which happened to be grouped on the right dimmers, so I ran the show from the dimmers rather than the lighting desk; not ideal but it worked.

 

Took the Demux down to my cousin at the theatre on Monday who rigged it in place of his demux (same make/model) and tried it - everything full brightness. Brought it back to church and working fine! Grrr!

 

New Behringer LC2412 arrived today which I've been playing with tonight and eventually (once I'd read on here to disregard everything the manual says) worked out how to program it etc including chases. No problems. Ran a few chases at full speed before I found the speed controls, stopped the chase then reverted to presets. Now the lights won't come up past about 10% brightness???!!!??? At this point I realised this is possibly what happened after the Thursday show last week - the board I used is a horrible Pulsar with touch-sensitive buttons all over, so you can easily run your fingers across and back to create impressive 'chases' on the fly - and I did. Swapped in the dead Sirius 24 and same thing; lights only work to 10%.

 

So, my question is - Is the Demux turning on some safety feature to stop me burning out the lights with a fast chase, or is it a fault?

 

Either way, can I fix it / force it to allow full brightness again (power off and on doesn't work) or should I look at replacing it with a new model rather than 2nd hand?

 

Sorry for the long thread, but I don't know enough about it all so best to tell you everything than miss out that important detail...

 

Cheers,

 

Peter

[themadhippy]

Never had any problems with the multichanger,and ive used a few.fast chases coulkd be the dimmer cant keep up or the rise and fall time is so quick they dont get a change to get to 100%.Might be worth checking the voltage and polarity is set correctly apart from that pass

[mrcog]

I don't know the demux in hand, however have you checked the output voltage of the demux? Different analogue dimmers use different control voltages, which means the demux too be compatible with as many different dimmers as possible must be able too change the output voltage...

 

However if the output voltage is set too low then the effects youve got here can occur so I'd check the demux too see what its outputting :P

 

James

[howartp]

Sorry, I should have said.

 

The demux is set to +10v, which is what the dimmers take; my cousin checked that much.

 

I measured the voltage across some pins when it happened before; the max voltage was +6.x with fader at full.

 

Peter

[Ynot]

Sorry, I should have said.

 

The demux is set to +10v, which is what the dimmers take; my cousin checked that much.

 

I measured the voltage across some pins when it happened before; the max voltage was +6.x with fader at full.

 

Peter

Then that, to me, suggests where your problem is.

6 volts is - unsurprisingly - about 60%+ of your expected 10V so it's logical to say that the dimmers will only give out 60% of the max output.

[howartp]

Sorry, I should have said.

 

The demux is set to +10v, which is what the dimmers take; my cousin checked that much.

 

I measured the voltage across some pins when it happened before; the max voltage was +6.x with fader at full.

 

Peter

Then that, to me, suggests where your problem is.

6 volts is - unsurprisingly - about 60%+ of your expected 10V so it's logical to say that the dimmers will only give out 60% of the max output.

I understand that, my question is why?

 

Does this (or any other) demux have safety systems in causing it to reduce voltage to x% of its DMX command or is it a fault?

 

I'm happy if someone with a bit of experience tells me it's a fault and I've been had; at least I know where I stand.

 

Peter

[dbuckley]

The multichanger isn't clever, so it wont do anything it isn't told to. They also have a reputation as a solid bit of kit. Even I've got one :P

 

I know there is a polarity switch on the back, but do I recall there a voltage selector as well? If so, it may have gone dirty and would benefit from being flicked a few times.

 

Other than that, I'd suspect faulty.

[ianknight]

Check with the person you bought it from on eBay as to what racks they used it with. You may find they were using Mini 2's, in which case the LM324's that drive the outputs of the Multi Exchanger have been damaged.

 

This has been the only fault I've ever encountered with the Multi Exchanger (apart from the original PSU design flaw) - it's easily rectified as long as your unit has socketed IC's, just buy some from RS/CPC and swap them over.

 

The 'Technical' explanation of this fault is that the input stage of the Mini 2 sinks/draws a lot of current compared to a more modern dimmer and whilst the LM324 can live with this for a while, they eventually die.

[howartp]

The multichanger isn't clever, so it wont do anything it isn't told to. They also have a reputation as a solid bit of kit.

 

do I recall there a voltage selector as well? If so, it may have gone dirty and would benefit from being flicked a few times.

 

Other than that, I'd suspect faulty.

 

 

Check with the person you bought it from on eBay as to what racks they used it with.

 

Thanks David and Ian, I'll check the polarity and voltage dip switches, then go back to the eBay guy if I get nowhere.

 

Cheers.

 

Peter

[musht]

Also check signal ground , is same ground at every point, i.e. continuity from rack to rack and rack to desk, high impedance ground can cause odd problems.