DTS soundhead LED

[Marco Giustini]

I'm trying to replace the LED of a DTS head. I've measured the current and I've taken it out to see the dimensions.

It seems to be a 2.9mm LED - the cutout on the bracket is 3mm.
There are not many 2.9mm, RED, clear lens with 1.7V forward voltage or whereabouts. I see it's being driven at 9.5mA and the "gain" trimpot doesn't affect it, I guess it affects the gain of the photocell.

I know someone has swapped these already, I was wondering if that someone would be willing to share some working LED models so I don't have to re-invent the wheel ?

So far, I think this is one of the three 2.9mm candidates on Digikey

https://www.digikey.co.uk/en/product...-3GAE4/4515685

[Gordon McLeod]

I had bought the ones I used from a Chinese electronics store and were the brightest they had they had no spec sheets on them they were just in a bulk bin. They were rather fiddley to get to line up when one siliconed them in place. The photo cell was more commonly the fault

[Steve Guttag]

The one yo linked too isn't it. It is too dim and I think the height of the LED is too high. I can't say that I've done extensive testing but I think having an LED with around 750mcd is about the right output. The height of the LED should be 4.6mm. I think the one you linked to was 5.5mm.

Not finding what I really wanted, I took a different approach, I got a brighter LED and then soldered in a current limiting resistor (330Ω) such that you can turn the adjustment pot down to minimum and be pretty close to the desired 4VDC output. That should yield a very good range/life and, if it fades too much (unlikely, given the amount of DTS film that one is likely to run, at this point), remove the resistor and start the process over again.

I used the SUN LED model: XLM2MR11W. It outputs 1495mcd...so it is a bit bright. But what would you expect with a name like "SUN?"

Digikey part number is 1497-1264-ND

Despite the sample image on the Digikey site, it really is a clear LED with a red 640nm output.

Do yourself a favor and make sure you measure the lateral guide roller spacing before taking everything apart. In case you didn't, I've found, after measuring several that the nominal spacing is .415-inches (10.54mm) from the inboard flange of the guide roller to the outside of the base plate:

image.png

This is true for either 35mm or 70mm readers.

[Marco Giustini]

Thanks Steve,
Yes, I was wondering whether one could lower the LED current and keep the gain at maximum! I wonder why DTS didn't go that way? Could it be that at higher gain there is more noise on the output?
Yes, I noticed the LED I linked wasn't the correct one after I posted - thanks for picking that up.

The one you mentioned looks perfect thanks - I think it's stored under 3mm while I searched for 2.9mm. But the datasheet says 2.8/2.9mm which is perfect.

Ah - bugger, I didn't measure that distance. Though I experimented with the oscilloscope BEFORE taking it apart, changing all the adjustments to see how the TC changes on screen so in theory I should be able to re-align the head from scratch. But thanks for the starting point!

Thanks so much for sharing, much appreciated!

[Steve Guttag]

You have to figure, when they chose their LED, which they would have bought in quantity, they would have designed their reader circuit around that. That was back in 1992 for a 1993 release...over 20-years ago. LED intensities have changed over the years/decades.

If designed today, there could be an argument for an LED current trimmer and set the gain more fixed or possibly a dual level with a link. In later reader designs, they did away with the adjustment.

I think, once you resign yourself to disassembling the whole thing (so the wires can be either reused or replaced), it goes much faster than trying to carefully not break them as one frees up the LED bracket. If you unsolder them from the PCB and unthread them, at least part way, you can go to town freeing up the bracket to get to the LED. Naturally, I tried it the hard way, the first time...broke a wire and had to string new ones (not as bad as it might seem).

[Marco Giustini]

Now I'm thinking about, the manuals say that rev. A of those boards didn't have any adjustments at all - it was added later. Still, not long after 1993.

Me being me I disassembled everything. "I'll worry about re-assembling properly later" ? It was also a good opportunity to give it a good clean. I thought about keeping the wire in place then I decided to pull it out. I should get some replacement - though I'd imagine my enamelled copper wire should do, it just won't be coloured.

I'll let you know the outcome - thanks for now!

[Mark Gulbrandsen]

Actually, the DTS jLED's very definately do have a spec set. When I worked at Claco, we had a bunch of readers that needed new LED's. We sent a brand new DTS LED to an LED manufacturer and asked them to measure it. They did, and they had an almost identical LED on their shelf. It was just a tad bit brighter. I re-LED'd quite a few readers and they worked fine, but because of digital cinema, we never did find out the actual life span... If you need LED's you might give them a call and see if they still have some. We bought 100, and didn't use mote than probably 15 give or take.

[Marco Giustini]

Of course LEDs have specifications but to be honest a brighter LED can't be that bad - dialled down to an appropriate level via current-limiting resistor of course. The wavelength is probably not so critical and I have a feeling that as long as the size is correct, illuminations is also probably going to be ok.

[Mark Gulbrandsen]

Of course LEDs have specifications but to be honest a brighter LED can't be that bad - dialled down to an appropriate level via current-limiting resistor of course. The wavelength is probably not so critical and I have a feeling that as long as the size is correct, illuminations is also probably going to be ok.

I thought so too, and bought some LED's locally back then. If you go too bright, you can overload the circuit that's in the reader. So you have to find an LED that is near to the correct brightness. The one the LED vendor recommended was about 5% brighter than a new LED taken out of a new reader. That worked out ok, but not sure it really provided any extra life to the reader. Two years later, I was pulling the entire booths out to install digital.

[Steve Guttag]

With the current limiting resistor, they aren't ANY brighter. The DTS reader will "clip" just above 4V of output. You, effectively, are raising just the black levels to the point it cannot know what it is reading. I selected the current limiting resistor for the LED I was using to bring the level down so one had the full range of the potentiometer to adjust for aging of the LED. And then, one could probably remove the resistor (jump it out) and get yet another life span.