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
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Mike Bryce
Sent: Saturday, December 05, 2015 6:07 PM
To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] Model 544 (or that era) part source needed
one thing to take into account is the fact there is a RED plastic filter in
front of the LEDs, that’s going to cut the brightness of the green LED by a
large degree. There’s probably nothing wrong with the display
Mike Bryce, WB8VGE
the heathkit shop
SunLight energy systems
J e e p
o|||||||o
> On Dec 5, 2015, at 3:53 PM, Paul Christensen <w9ac@arrl.net> wrote:
>
> Found this message thread a bit late. I am in the processes of
> localizing a display brightness issue with my Triton 544 --
> specifically the green 7-segment LED, manufactured by HP, part number
> 5082-7671. The original poster was looking for a replacement. The
> LED is used to display the Triton's 100 Hz digit. My first thought
> was that the green LED required replacement, but in thinking this
> though, I'm more inclined to believe it's working correctly and that
> the driving circuitry requires further investigating. I cannot
> imagine ALL 7 segments loosing equal brightness over time -- unless
> the LED driving current is at or near the absolute current rating as shown on
> the HP datasheet.
>
> The HP 7-segment LED uses a common anode. Two transistors, a PNP/NPN
> pair are responsible for functioning as a switch and for regulating LED
> current.
> I believe the 220K resistor from the LED driver chip is responsible
> for setting LED display current. In looking at the HP specs,
> achieving equal luminance across the red and green light spectrum
> requires different LED current. However, Ten Tec used one resistor
> value (220K) for all red and green LEDs.
>
> In the next week or two, I plan on rebuilding the PTO. I'll dig into
> the brightness issue at that time and measure voltage across the LED
> segments and compare with the HP data sheet. The green segments
> should show voltage about 0.3V more than red. If they're equal, owing
> to the same 220K resistor value, that will likely explain the slight
> brightness disparity. Red LED current should be about 1.8V versus 2.2V for
> the green LED. The Plexiglas
> display lens may also be contributing to some green spectrum attenuation.
>
> Paul, W9AC
> ----------------------------------------------------------------------
> ------
> ----------------------------------------
>
> Those LED displays date back to the 1970's, or maybe even a bit earlier.
>
> The 54 used HP-5082 series LEDs for all counter sections... these 7
> section LED displays were .43" (10.92mm) and common cathode, with
> right hand decimal points.
>
> The correct LED for the green display is a HP 5082-7671. You could
> substitute a red display--HP 5082-7651, or a yellow display, HP 5082-7661.
> The red displays are extremely common and show up on eBay. The green
> and yellow displays are tad more uncommon, but this should still be a
> 1 to 5 dollar part on the surplus market since zillions were made.
>
> regards
>
> Pete k1zjh
>
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>
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