TEC Digital Computing scale SL-2200-50-J0-GB

[3T39]

I would appreciate help with this Calculating Weighing Scale. The machine
works perfectly well, but the centre segment of the seven segment display is
very poor contrast and easily mistaken for a zero when it should read eight.
See photo http://i10.tinypic.com/4ieqkjs.jpg
I'm not used to these displays, and can't find the pinout info in google
(don't know which question to ask as I can't identify the display) I assume
they are common cathode but how are the segments switched ? Once I can find
the pinout info I should be able to do some sensible troubleshooting.

With best regards, 3T39. E-mail: rubbi...@hotmail.com

[DaveM]

"3T39" <rubbi...@hotmail.com> wrote in message
news:451fd868$0$12441$db0f...@news.zen.co.uk...

That type of display is a Vacuum Fluorescent Display (VFD), and from its
appearance it's a custom part. If it needs to be replaced, you will likely find
it only from the OEM.
There's not much to go wrong that will cause your symptoms. These displays are
typically multiplexed one digit at a time, switching Vcc to the anode of the
digit being addressed, and simultaneously switching low voltage to the segments
(cathodes) being turned on. If the low intensity is affecting the center
segment of all digits, then the likely suspect will be the transistor and
current limiting resistor connected to the center segment. It might be a bit of
a hassle to trace the traces inside the display to find which pin addresses the
center segment of each digit.
You might be able to troubleshoot by using a scope to probe all of the pins of
the display. When you probe the pin for the center segment, it will probably
show a significantly different pulse height than the other pins. Use your eyes
to trace the PC wiring to the associated resistor/transistor or IC.
Likewise, if the offending center segment is on only one of the digits, look to
the amide side (that will be the pins with high voltage pulses). Troubleshoot
as before.

Good luck with the project.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Make it idiot-proof and someone will make a better idiot.