[TenTec] Model 544 (or that era) part source needed

[Mike Bryce]

Paul, i don’t know if you can get a high brightness GREEN LED or not.

If you put a green filter in front of the ten tec display, the RED LEDs won’t be as bright as the GREEN LED. The effect will be opposite as it is now


Mike, WB8VGE
SunLight Energy Systems
The Heathkit Shop
http://www.theheathkitshop.com/ <http://www.theheathkitshop.com/>
J e e p
o|||||||o 

A man with one clock knows what time it is. A man with two clocks is never sure. 



> On Dec 5, 2015, at 7:58 PM, Paul Christensen <w9ac at arrl.net> wrote:
> 
> Mike,
> 
> Great info.  Thanks for the heads-up on the lens filter.  That's probably the root cause of the brightness attenuation.
> 
> Referring back to the schematic, the LED segment current is limited by seven 270-ohm resistors on the cathode side, one per LED segment.  That value is to be expected with a +12V supply rail.  Since these 7 lines are strobed in a basic multiplexer, changing one resistor also affects the red LED segments in addition to  green. 
> 
> If I can keep current less than the 20 mA spec, I may  change the BJT switching transistor to a 2N7000 FET to minimize the on resistance.  Then current limit if necessary.  It would be nice to squeeze just a bit of extra brightness and still stay within HP's spec.  OTOH, I may find that max brightness has already been reached and that an increase in LED current doesn't yield any additional brightness.  
> 
> Paul, W9AC
>