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
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html
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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