Faulty Mini-2 Dimmer

[Spock]

When it came to the final blackout during Monday evening's performance, one channel on a Rank Mini-2 dimmer would not dim. All the pre-show checks last night went OK but the same thing happened on the final blackout, leading me to believe the problem may be caused by a component / module overheating. Does anyone have any ideas what the problem might be?

[pritch]

I'm not familiar with the dimmer in question, but the usual culprit when a channel refuses to dim is a faulty triac.

[sparkyweb]

the usual culprit when a channel refuses to dim is a faulty triac.

 

yes thats right however being a pedant now, the mini 2 has 2 thyristors instead of a triac. You have to take the control pcb off the metal plates and then the 2 thyristors are exposed

[Kazeja]

There are 2 back to back thyristors, however, if either one goes short, it tends to stay short and no dimming is possible. What I understand by your fault descrition is that the dimmer dims for some of the time. If this is the case, it could be suspect solder joints or the top and bottom set pots on their way out.

Quick visual inspection and dab with a soldering iron may cure the problem. Take note of pot settings and move them around/clean incase there is a build up of foreign matter on the wiper. Don't forget to set pots back to their original settings or as close as possible or you may find the dimmer way out of calibration.

 

Oh yes and most important, make sure the dimmer is off/isolated before attempting anything of this nature. If you're not sure, then get someone who is familiar with this type of work.

 

 

Other thing, does the problem go away if you unplug the control desk?

[Andrew C]

Take note of pot settings and move them around/clean in case there is a build up of foreign matter on the wiper.

A word of warning; whilst this may clear your current problem, it may give rise to another. Once these pots are moved the wiper now sits on the dust and can lead to drift. If you do try this, you should expect to change the pots in the near future.

 

Anyone want a "Been There, Seen That" T shirt? :unsure:

[Don Allen]

Looking at the Strand Archive http://www.strandarchive.co.uk/control/tec...dbookmini2.html these control boards are similar to the JTM so it is worth replacing the skeleton trim pots and the electrolytic caps if they look dried out on the end seal. In Australia I can buy the axial electrolytic capacitors from RS, so I assume the UK RS would also stock them.

[Spock]

Thanks for the suggestions. I have managed to get round the problem by reducing the load on that channel to half its previous level, it now dims when it should. I will pass on the suggestions to the venue's management at the end of the current run (tomorrow).