Control/Dimming - NSI NRD 8000 Repair Question

[swegner]

I've got an NSI NRD 8000 Dimmer unit which works great except one channel stays at full 100% all the time. All the fuses check out and the LED indicator on the front panel varies up and down with the board fader. The board works fine with another dimmer unit.

I have a fair amount of electronic repair skill, but before I pop the board out and start poking around, I thought I'd post here and see if someone could point me to the likely problem. This seems like it would be common.

I'm thinking that it is probably a dead SCR. I'm not sure if they tend to fail 'open' or 'closed'. I imagine replacing them would be pretty easy and I would be lucky if that was the issue.

Thanks.

[Les]

I'm thinking that it is probably a dead SCR. I'm not sure if they tend to fail 'open' or 'closed'.

I'm thinking that's a good assumption. They do tend to fail 'open' or 'closed', and it seems to go equally either way (but a 'closed' failure is definitely easier to troubleshoot). It should be an easy repair if you're inclined to do a little soldering, and the replacement part shouldn't be expensive.

I'd check things with a multimeter first, but since I'm not exactly sure what to look for and where to check, I'll let others chime in. I've seen a lot of these types of failures, but I've never actually repaired one.

Good luck!

[dramatech]

I have repaired many dimmers of the same type. The problem is most surely the Triac (two SCRs back to back in the same case.) If you have any experience in electronics, you can identify the triacs by the fact that they are in TO220 cases. That is a flat transistor looking case with three leads and a tab heat sink for mounting. Locate the AC connector that is the errant dimmer. follow the black wire or if the color is not standard, the one coming from the brass screw and the smallest slot in the connector front. Where that wire goes into the circuit board, the nearest of the Triacs to that wire is probably your problem. In many cases it might require removing to whole circuit board assembly to desolder the triac. In the case of LIghtonics, the triac is under the circuit board and requires removing quite a bit of circuitry to get to it. If the triacs are quite visible and can have the heat sink screw removed, just cut the leads as close to the triac as possible. Put the new triac in exactly as the old one was mounted. Lay the leads of the new one on the leads of the old one. You might have to trim a little off the ends of the new leads for them to lay smoothly on the old leads. Now just solder the leads together. This is the quick and dirty way to repair these guys. The fact that it might get blown again, it may not be worth doing a job where the old leads are removed from the board. That is a choice that you have to make based on the amount of time you have and your confidence level in such activity. If you do decide to remove the old leads, don't try desoldering all three leads and pulling the triac out intact. Cut the leads and remove them one at a time. This helps keep the amount of heat used in desoldering to a minimum and will help keep the conductor laminations from being pulled up from the circuit board.
Most of the shoebox dimmers use a 12 or 16 amp triac. You can get replacements in the 20 and 24 amp models on the same TO220 case. They will work just the same without any modifications to the circuitry, and will be much more robust in avoiding a failure in the future. WARNING!! Make sure that in purchasing the replacement triac, that you get one that has the heatsink electrically isolated from all three leads, if not, you might see a lovely smoke and light show when you plug the dimmer in.
I ususally use a BTA20-600CW triac available from Mouser.com The BTA20 is the important part of the number. Considering the cost of the shipping and the low cost of the triac, I usually purchase from 10 to 20 at a time. In any case it seems silly to purchase just one, when it is one of the most common electronic parts used in the theatre. While you are ordering it, you might want to purchase a few 75176 communication ICs. They are the tranceivers used to transmit and receive DMX and there is one or more in almost everything that uses DMX.
I hope this helps.

Tom Johnson

[swegner]

Thanks both for your replies and great advice!

I think I'll move forward with removal of the circuit board and see what I can find. It will be several days to a week, but I'll post what happens.

Thanks again!

Scott

[headcrab]

In this case, the 75176 (or variant) won't do jack for you as the NRD8000 is a microplex dimmer. The 75176 is a RS485 transceiver. But the boards inside said dimmer are very easy to remove, and it's not hard to figure out how.

[swegner]

Ya cool - I figured that. But I appreciate all the advice anyhow. Actually one thing that was cool about Tom's post (there were several cool things) was the advice about cutting the leads before removal. I've removed LOTS of components by rocking them back and forth - which has often ended in heartache - and when I read his suggestion about cutting the leads and removing them one at a time... it was kind of one of those slapping my forehead - "OH GOSH!" moments where I thought: "DUH! Why didn't I think of that before!!". I just recently repaired a Peavy power amp where I ruined one PCB trace with too much heat - exactly what he described! I ended up hacking it back to life, but that little tidbit of advice would have served me well in that situation.

I'm probably going to pick up a DMX-lator and convert all of the Microplex stuff I've picked up over to DMX anyhow - so the bit about the 75176 might be handy also. products

I need like 6 SCRs - but just ordered 10 from Digikey. You can never have too many components laying around! (2n6404)

I'm pretty impressed with NSI's customer support. They sent the NRD 8000 schematic the next day. So that's cool. (attached if you are interested)

Scott