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replaced triac - output still full on


Dave C

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Hi Everyone,

 

I am tying to repair an NJD logic 8000 dimmer. The outputs are all measuring 230V even when the dimmer faders are down or the front panel is disconnected, i.e. all the time.

 

Reading the manual from NJD this is described as a triac replacement fault, which I have done with the specified triacs, but I am still getting 230V on the outputs. All channels are affected.

 

This is the same dimmer board as they put in the DP10000, as far as I can tell from reading the NJD manuals.

 

Any further advice on what to look at component wise would be most appreciated. The main dimmer board has a quad comparator and what each channel has what looks like an isolator IC, but not sure if these should be replaced just to be sure.

 

I'm more comfortable working inside audio power amps and mixers than lighting kit...but am a reasonable service tech for those things.

 

Any Ideas Anyone?

 

Thanks!

 

Dave.

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I'm not familiar with these particular dimmers, but how are you measuring the output voltage? All triacs and thyristors have a small leakage and if you are using a high impedence digital multimeter you will always see 240V on an unloaded dimmer, whether or not the triac or thyristor has failed. To make a sensible measurement of the output voltage you need to have a load on the dimmer. A 150W GLS lamp will be sufficient.

 

HTH

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I'm not familiar with these particular dimmers, but how are you measuring the output voltage? All triacs and thyristors have a small leakage and if you are using a high impedence digital multimeter you will always see 240V on an unloaded dimmer, whether or not the triac or thyristor has failed. To make a sensible measurement of the output voltage you need to have a load on the dimmer. A 150W GLS lamp will be sufficient.

 

HTH

True.

....which is why you shouldn't work on any fixture still plugged in to a live dimmer, even if the dimmer is set to zero.

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Replacing the triac wouldnt generally be the most logical step in correcting an "always on" error...

The triac is the "physical switch" in the system ...

It is more likely to be the stage before the triac in the system....

 

I cant quote on your dimmer.. but in (for example strand act series that I repair mostly) there is a transformer for each channel which sometimes fails...

 

They are generally nice simple devices and depending on the layout/design parts can be swaped from one channel to another to find the fault without needing too much electronic know-how..

 

just take similar parts and swap from a good channel to the failing channel and re-test... repeat with different components till the fail moves to another channel.... last component moved is the one that needs replaced..

 

no need to complicate a simple procedure!

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Its been a long !! time since I repaired any NJD gear but I can remember doing one that had ouputs stuck on, I swapped the triacs and the problem still existed. The fault turned out to be the zero voltage crossing circuit firing the triacs all the time.

 

Like I say this was a LONG time ago and I don't even know if the Logic 8000 has zero voltage crossing !!

 

Can NJD give you a schemtic ?

 

Kev

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No load leakage can also be through snubber networks: you will register a voltage with no load (the load absorbs the leak)

 

Other areas to look at: preheat, synchronization circuit and possibly isolating elements (opto-couplers - but unlikely since all show same symtoms) a scope or similar is the usual tool here.

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Normally a triac with no gate drive, should be off! so if the triac is on, and the triac is not faulty, then the fault lies in the drive side. have you checked the opto/isolators.how did you test with a load, or a high impedance digital volt meter? these types of meters can give false readings. try with a good load,

also check for built in functions of the rack, such as pre heat or trim.

if many are on, it must be something common too them all, such as the low voltage transformers, which provide a switching reference

a circuit would be handy!

please consider taking it to a experienced technician as they can be extremily dangerous to the un wary!

 

dont forget to let us know when you find the problem.

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please be aware than NJD use low voltage sensitive gate triacs (BT08-400S the S = sensitive gate), so if somebody has changed the triac for a typical replacement this will not work correctly.

 

If you need to get in touch with the designer Ian Benton he now works for Electrovision. as NJD became a sister company a few years ago. 01744745000.

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Hello again!

 

just reporting back - the new triacs went in and all seems well. Dimming is working, although the faders are dirty, and I have installed a 32a plug on it so I can get 8 x 500w parcans running from it all within the given power ratings.

 

Thanks for the advice everyone, hope I can contribute back in the sound section soon.

 

Dave C.

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