[time-nuts] HP5328A LEDS driver transistor

Chris Albertson albertson.chris at gmail.com
Sun Jan 22 13:41:23 EST 2012

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.

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"

Chris Albertson
Redondo Beach, California

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