NJD NJ942 Dimmer pack - Circuit diagram?

[pmiller056]

Hi. Does anyone have an internal circuit diagram for this 12 channel, 4A per channel dimmer pack to share? The PCB is marked NJD NJ942 Rev 5. They were also sold as a 'DPX 12/4' dimmer pack. It's to help a (registered charity) local community theatre repair their non-working one. Thank you.

[timsabre]

I don't have a diagram, but they are not very complicated, what is the fault you are trying to repair?

[pmiller056]

Hi

This is what appears to be one of the later versions of this product with an LED display and pushbuttons for configuration. There's power onto the PCB, all fuses are intact. The Triacs and power control parts are live. The display and control electronics are dead. My first line of investigation is the on-board SMPS that drives the control electronics. The SMPS 'brain' is a Power Integrations TNY274 which has the switch transistor and controller in the one package. The microcontroller driving the triacs is mains live. The DMX side of things is on an isolated supply with an optocoupler to the microcontroller. I have an isolation transformer, so I can use a 'scope if needed. In the first instance I want to know a bit about what goes where before I start work on it, but a full drawing would be ideal.

[timsabre]

Ah OK I thought it was one of the basic analogue ones they used to do.  That sounds a lot more complicated. presumably they are making a live minus 5V supply to run the microcontroller, this makes triggering the triacs a lot cheaper but is very horrible to work on as everything is live.

You might get somewhere feeding a 5V supply from a lab supply into the microcontroller, with no mains connected, that might help you track down if it's the PSU not working, or if there is a fault in the microcontroller part which is pulling down the power.

 

You could possibly try contacting Kevin Hopcroft who was Mr NJD, he is on Linkedin, might be your only chance of finding a schematic diagram for this.

Edited September 9 by timsabre

[indyld]

When investigating the control side of dimmers, am a big fan of powering it with a DC supply to gain some information as not only is it a whole lot less death inducing, but it's easy to read current and safely connect the scope. 

Offline switchers often announce their retirement by sporting a hole in the package. 

[pmiller056]

A very long time ago I did a lot of repair work on hybrid TV sets, so I'm aware of the hazards of working with this style of circuit. There's no obvious damage to any parts on the PCB. I'll follow up Tim's suggestion later today. For my next steps, I'm inclined to trace the circuit sufficiently to work out what the various supply voltages should be. Rather than use a bench power supply, I'll use 'lump in a cord' power adapter(s) as a temporary supply(s) because the commonly found ones are double insulated and will have sufficient isolation in an all-insulated case to try using the dimmer pack as intended or off an isolation transformer reasonably safely.