[time-nuts] HP5328A LEDS driver transistor

shalimr9 at gmail.com shalimr9 at gmail.com
Mon Jan 23 09:26:39 EST 2012

At the end of the day, what defines how much power is burned in the drive circuit is the current in the LED, not how its generated (resistor or current source), unless you use a switching regulator.

In most cases, you should get sufficient current stability using resistors if you start from a 5V source, but as the equipment gets older, LEDs may age differently and it is not unusual that 20 y.o. equipment would have uneven brightness while some other is just fine.

I have a 20 y o LED clock that was just fine for a long time, but over the last 5 years has started to degrade and brightness now is very uneven. It is my X-10 controller, so I don't really use it to check the time , but it really needs new 7 segment displays.

Didier KO4BB

Sent from my BlackBerry Wireless thingy while I do other things...

-----Original Message-----
From: Brooke Clarke <brooke at pacific.net>
Sender: time-nuts-bounces at febo.com
Date: Sun, 22 Jan 2012 11:30:59 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] HP5328A LEDS driver transistor

Hi Chris:

To get the LED segments to be the same brightness some form of current limiting is needed and this usually involves 
dropping resistors and an active current limiting circuit and that burns up a lot of power.

What does the drive circuit look like?
For example if you connect a scope to a working transistor and look at the base, emitter and collector voltages, what do 
you see?
If there's a resistor in series with the LED segments what's the voltage across it doing?

The mux speed for an LED display is fairly slow so any PNP transistor that can handle the voltages and currents involved 
should work.
Have you tried any jelly bean PNPs like the PN2907, 2N3906, 2N4403?

Have Fun,

Brooke Clarke

Chris Albertson wrote:
> I have an eBay HP5328A counter with two dead digits on the display.  I
> figured out the problem was two dead transistors.  I can swap
> transistors with a good digit and the problem moves.
> I'd not worked on LED displays before.  Turns out only one digit is
> lit up at a time, they strobe the digits in sequence.  The dead
> transistor is the one that controls the all the anodes in the
> 7-segment LED module.  The service manual describes the transistor
> like this:  "part number = 1853-0326", "description = TRANSISTOR PNP
> SI ... FT-50MHZ"
> The p/n 1853-0326 cross references to a Motorola MPS-U51.  The MPS-U51
> data sheet matches the part that fails so I'm sure I got a correct
> cross ref.
> I took a photo of the dead transistor.  It is on .1" perf board for
> scale.   You can read the "3-326" p/n and see the Motorola "M" logo.
> http://www.dropbox.com/gallery/28915695/1/HP5328A?h=da35c1
> I look up the spec in the mps-u51 and see it is a to-220 like case and
> can handle 1W.  I'm really surprised it burned out as I doubt an LED
> requires 1W even if showing an "8".  Reading the mps-u51 spec sheet I
> see it has a low saturation voltage.  Maybe that is why the selected
> it as it is being driven by 7400 TTL logic that goes through a
> connector and has some resistors involved.
> Question:  These seem to be hardtop find.  Can anyone suggest a good sub"
> Thanks,
> Chris Albertson
> Redondo Beach, California
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