[time-nuts] HP5328A LEDS driver transistor

Brooke Clarke brooke at pacific.net
Sun Jan 22 14:30:59 EST 2012

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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