JTM Modules


Active Member
Hey - I have a question regarding JTM Modules.... So may be up the alley of some of the more experienced members or those who can remember back to the darn things!!

Our venue still runs 72 JTM Configuration Dimmers, 65 2.4kW modules and the remainder are 5kW modules. Also running 4 12 Channel digital racks through a multiplex.
I have recently rebuilt a 20 Way rack, in original casing with bits and pieces I found around the place, I tested all modules with a contraption I made that gives it 240v, 9v control and a load, allowing you to see if the module still dims and set the top and bottom trims.

I have heard a few people a while back discussing JTM's and they were saying that when the module no longer works or can not be trimmed, you can convert a module to Triax and resurect it. I was hoping someone knows what this process actually involves and how to do it?

I am fairly young, worked in the industry for a number of years, and despite what the younger generation of techs say, the JTM's are still an awesome work horse, and I would love to see them keep running!!



David Ashton

Well-Known Member
if your JTM is not trimming properly it is most likely that the trimpots need replacing, especially the early skeleton type, as all the components are discreet and readily available they are easy to repair.Changing to triacs while easy, is a backward step, a pair of thyristors is much better than a triac.I bought 60 units in 1972 which are running without problems, a failure rate of 1-2% a year is as good as new stuff and you only lose 1 channel, not the 4 or 12 that you lose with modern packs with their shared circuitry.I suggest a maintenance regime to change the pots and the capacitors across the power transformer every 10 years whether it needs it or not, these are the main things to fail.I have circuits etc. if you need them.


Active Member
I have a number of the CTM units (Canadian Thyristor Module) these things were designed by Strand in the UK. The JTM stands for Junior Thyristor Module and the STM stands for Senior thyristor Module. The CTM units were assembled in Canada (Toronto). My units were actually built in 1978 and still work reliably, I have the manual for these. The CTM is the Canadian version of the STM. The nice thing about these units is they are complete, each module has its own power supply, trigger circuit and isolation transformer. The input is normally -10V to 0V but can be switched to 0V to +10V just by swopping the return and signal wires over - these are not 0V and positive.

Strand actually used the same circuit card to control 2.4kW, 3.6kW, 6.0kW, 7.2kW and 10kW dimmer modules. All that changes is the thyristors and filter choke. these things are so over designed that they will run reliably at full load without ever being turned off and they are reasonably quiet (electrically). Due to the good output filters they are slower to respond than some of the rock and roll dimmers but you have to look carefully.

I agree with the previous poster that configuring for TRIACs is a backward step, thyristors are better at handling large loads than triacs. If you really wanted to do this I would suggest that the first step is to replace the pair of thyristors with a suitable TRIAC. If you only want 2.4kW then I would find a suitable 600V device that can handle at least 25A and preferably 30A. At this point thyristors are probably cheaper. The TRIAC ia a back to back pair of thyristors in one package with the gates tied together. The JTM module has two seperate gate drivers- one for each thyristor. The gate drive is derived through an isolation transformer for safety reasons, modern dimmers use an ULC approved optical isolator for the same purpose - these are the devices in the white package. To make this work you have to modify the gate drive circuit. To be honest I wouldn't recomend you try this modification if you don't know how to work it out yourself. You also have to investigate if you need to modify the snubber circuit (output choke) this is the msot expensive item in the circuit.

All in all stick with the thyristors as mentioned by the previous poster the main things that go wrong with these units is the pots which cost pennies and the elctrolytic capacitors.

By the way the isolation transformers were custom wound for Strand and are as easy to find as rocking horse droppings.

Enjoy them while you can

Just a thought when adjusting the minimum and maximum settings it must be done with a 1kW load and really requires an osciloscope to make it easy

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