DSTR-20 LED brightness issue?

[Markus Lemm]

I've got a bit of a conundrum with a BACP DSTR-20 Dolby digital reader. I think either the LED itself, trim pot, or something else on the circuit board is causing a low LED brightness.

So I have two DSTR-20 Dolby Digital readers that were aligned about 2 or 3 years ago. Both were working great.

I recently started up the DA20 and noticed that the status LED on the Projector Film-Tech Cinema Systems DSTR-20 was red and I figured the trim pot just need to be turned up a bit to get back into green. No such luck.

No matter how much I turned it up it never got bright enough. So I figure the LED needs to be replaced.

I replace the LED/heat sink and still no joy. I decide to put the LED/heat sink I pulled from projector 2 and put it on projector 1, and it works great. I put the working/bright LED from Projector 1 on Projector 2 and again the LED status light doesn't go to green.

The LED is noticeably dimmer on projector 2 no matter which LED I put into the DSTR-20, but on projector 1 it's fine. I also tried swapping the video cable between projector 1 and 2 and swapping them on the back of the DA20.

Always the same situation. The LED on the DSTR-20 on projector 2 is dim and can't achieve working brightness.

So I wondering if the trim pot or some other part of the circuit board could be to blame?

Anyone have any suggestions? My next step would be to directly power the LED with a bench power supply and see if I can manually get it bright enough.

Any help would be apprecated.

[Ryan Gallagher]

I’d have to read the manual, but like the 701/702s, does it do red-green-yellow as it’s low-good-high status?

Edit, just looked, yep.

I assume you are checking the lindicator with film running in the path? Without film it would most likely be the “too high” status?

[Ryan Gallagher]

If you have a scope and probes handy I would suggest metering the video voltage per the manual to confirm your suspicion of low output, rather than trusting eyes/indicator.

start looking deeper after that?

[Markus Lemm]

Thanks, I'll bring in the scope next week.

The led adjustment is red-green-yellow. And it is always red, with and without film running.

I'll also give the manual led power a go. If that gets me into the green and reading Dolby Digital again, I know it's an issue with the trim pot.

I'll update next week.

[Markus Lemm]

So I used my bench power supply to power the LED and at 1.62v it gave me a green light. I was able to adjust to about 1.7v and then I got a yellow light.

I tested the LED before and it would only go to 1.2v max with the trim pot adjustment. The working reader was at 1.68v.

The reader works great again. Now I just need to make a new power supply for the LED.

I'll probably use and old 5v phone charger, a resistor and a capacitor. Just need to experiment with the resistors until I get one that gives me the right voltage/current balance.