JTM Dimmers pairing issue

[Paul Roberts]

Just to try and clarify.

 

The desk has 12 channels.

To use all the channels, there needs to be 12 sockets on dimmer pack/s.

Lights plugged into the first socket will be controlled by the first channel etc, etc

Is this right?

 

I think I need some schooling on the basics of lighting. :D

 

This is pretty much right, except that the word "socket" can be misleading - some dimmer packs have 6 dimmers, but 12 sockets, to allow you to pair lanterns, as Frag_Me has said.

 

I don't know if this happens with every dimmer pack that has 2 sockets per channel, but certainly with the JTM's in my school if 2 lanterns are plugged into one channel the light output decreases slightly. Should this be happening or is it normal with older dimmer racks?

[Bryson]

Split this one off - it deserves a new topic.

 

B

[Paul Roberts]

Split this one off - it deserves a new topic.

 

Ta very much.

[Nicktaylor]

I don't know if this happens with every dimmer pack that has 2 sockets per channel, but certainly with the JTM's in my school if 2 lanterns are plugged into one channel the light output decreases slightly. Should this be happening or is it normal with older dimmer racks?

Sounds to me like there is built in current limiting, hence the dim when it gets to full load. The Zero 88 Alphapack has this so the sum of all three channels does not exceed the maximum rating. Does the same thing happen if you use a double adaptor on the output rather than two different sockets?

 

EDIT:- Excess quotage snipped

[Frag_Me]

This is fairly common in schools from what I have heard, as students aren't going to know (although it is fairly obvious) that you can't plug 2 2000W floodlights into the same channel on their dimmer.

 

Don't get me started on schol equipment though, I used a room the other day where one lighting bar gave less output than the other, whatever dimmer channel it was in. Made for a very interesting double spot effect which HAD to be in one channel. One side considerably brighter than the other.

[Paul Roberts]

This is fairly common in schools from what I have heard, as students aren't going to know (although it is fairly obvious) that you can't plug 2 2000W floodlights into the same channel on their dimmer.

 

I couldn't even plug one 2000w floodlight into a channel. The school system still uses 5 Amp so I can only run about 1.2kw safely and usually have 2 500W lamps plugged into each channel at any one time.

 

 

Does the same thing happen if you use a double adaptor on the output rather than two different sockets?

 

Unfortunately I dont have a double adaptor to try this, though I would presume the same thing would happen.

[Tomo]

Don't get me started on schol equipment though, I used a room the other day where one lighting bar gave less output than the other, whatever dimmer channel it was in.

I find that very disturbing - assuming the lanterns are the same and are both clean, to me that says that there is a high resistance in the line somewhere, which could cause excessive heating.

 

You should get that checked out!

[paulears]

back to the JTMs - There was no in-built in current limiting built into JTMs or STMs. The thristor dimmers were fairly crude by todays standards. The HRC fuses tended to cope well with shorts from lamp fillaments failing and accidental overloads from over enthusiastic students with a bucket of grelcos. If plugging two lanterns in to one dimmer produces a lower output, something is wrong and needs investigating. Strand did have current limiting capability, but to the best of my knowledge it was implemented on an individual basis, being integrated into products when required. It is possible these dimmers have current sensing built in - pretty easy to check - look for current transformers on the incoming mains feed.

[Paul Roberts]

back to the JTMs - There was no in-built in current limiting built into JTMs or STMs. The thristor dimmers were fairly crude by todays standards. The HRC fuses tended to cope well with shorts from lamp fillaments failing and accidental overloads from over enthusiastic students with a bucket of grelcos. If plugging two lanterns in to one dimmer produces a lower output, something is wrong and needs investigating.

 

I'll see about getting a spark in to have a look in September when I'm back in school. The difference in output with two lanterns plugged into one dimmer isn't that noticeable, though I felt it necessary to ask as I didn't think it should happen. It is a pain if you want the optimum light output.

[Ynot]

back to the JTMs - There was no in-built in current limiting built into JTMs or STMs. The thristor dimmers were fairly crude by todays standards. The HRC fuses tended to cope well with shorts from lamp fillaments failing and accidental overloads from over enthusiastic students with a bucket of grelcos. If plugging two lanterns in to one dimmer produces a lower output, something is wrong and needs investigating.

 

I'll see about getting a spark in to have a look in September when I'm back in school. The difference in output with two lanterns plugged into one dimmer isn't that noticeable, though I felt it necessary to ask as I didn't think it should happen. It is a pain if you want the optimum light output.

I agree with Paul - if it's happening as you describe it needs a proper looking at...

BUT (usual caveat) don't just get a domestic sparkie in - get someone from a recognised theatre supplier. May cost a little extra on paper, but if they charge by the hour (likely) then a qualified theatre electrician is going to identify and fix a dimmer fault MUCH quicker than Joe Bloggs Electrical from down't road...!

 

:)

[Paul Roberts]

Thanks for that Tony, I'll speak to the relevant staff at my school. So long as I don't mention cost it should be ok. :)

[paulears]

with a school, it is always worth pointing out that there could well be a health and safety risk - et voila, the budget for the repair suddenly opens up as it usually comes from central funds rather than a department budget! This doesn't work every time - sometimes, they simply withdraw it from use totally with no replacement, so you need to suss the lie of the land before jumping in.

[Paul Roberts]

I could well imagine the dimmers being withdrawn from use if the bill is too expensive. If the headteacher or drama teacher saw the difference in light output from two lanterns powered by one dimmer, I expect nothing would be done as they would see the difference is not that significant and mostly likely tell me to put up with it. I don't think they have been looked at much since they were installed circa 1974/75 - maybe if they had been regularly maintained then repair bills could be kept down, or little things like this wouldn't annoy me like they do. :)

[Don Allen]

Jtm maintenance is very cheap and basic BUT in Australia has to be done by a tech with a restricted electrical license for safety reasons. The scr heat sinks ARE LIVE so it is easy to electrocute yourself.

 

There is a bottom set and a top set trim pot for each channel and you need to use a load of at least 60 watts and a true rms meter. The original Strand setup was to set the input control volts to 10% or 1 volt in a 10 volts system and set the output volts to 5% so for a 240 volt mains, set it to 12 volts. Set the control volts to 10 volts or full and set the output volts to 96% or about 230 volts.

 

The bottom 5% was used as a preheat voltage to lengthen filament life, but some theatres do not like it as you will not get a "black" blackout due to the faintly glowing filaments. The 96% top was used to level out discepencies between dimmers and also helped to slightly lengthen lamp life.

 

Jtm trigger cards have electrolytic capacitors on them that dry out eventually and can be replaced with new axial lead ones from RS. The trim pots were skeleton type and were affected by moisture etc so should be replaced by new enclosed ones when they become scratchy. You need good trim pots to speed up set up times.

 

JTM's that have been set up properly only need trimming about every five years, so apart from faults, there is very little maintenance needed for jtm's. The only two legitimate complaints people can make about jtm's is their dimmer curve does not match modern dimmers, but most professional desks can modify any channels dimmer curve and there is no Remote Device Management which is a legitimate criticism and if you have the budget, then buy new dimmers.

[Kazeja]

Hmmm.

 

Things to check.

 

The following must be performed by a competent person who is familiar with working on (chopped) live mains. It can prove lethal to the uninitiated. Do not try this unless you are fully aware of "ALL" of the potential hazzards involved.

 

Basically you need to know what are the fluctuations in the output voltage of the dimmer when the extra load is plugged in? Does it momentarilly dip and then recover or does it stay at a lower value? Does it happen on just 1 dimmer or all dimmers? (Idealy need a true RMS meter to measure this).

 

If it dips and recovers then it could be symptomatic of a "soggy" mains supply. Check all power terminations (internal and external) and secure.

Has anyone ever seen their lights dip at home when they turn on their electric kettle?

 

If it dips and stays low, then the input voltage to the dimmer rack needs to be monitored to see if it is affected. Does the input voltage to the dimmer rack stay constant?

Check power terminations as before.

 

Remember, these are live (potentially chopped mains) voltage measurements. It must be done by a suitably qualified expert who knows about, and is familiar with this kind of thing.

 

That's my bit for now.