B-634 Display question!!

[Alan 'A.J.' Franzman]

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Wed, 28 Oct 2009 14:49:42 -0500, "Alexander N. Gerli, Sr." <ac1y at NC.RR.COM> wrote:
Hi, gang,

The manual doesn't seem to cover this.quite.so here goes:

The "1" digit in the seconds and minute displays flickers, sometimes worse
than other times. None of the other digits do this. Now the Manual indicates
which transistors drive which segments. However, the numeral "1" appear in
more than just the "1".like 7, 8, 9.you get the idea. And none of those
numbers flicker at all when they display.

Seems like a driver transistor or two or three need to be changed, but which
ones?

I'd appreciate any thoughtful advice!

73,

Sandy AC1Y

Alexander N. (Sandy) Gerli, Sr.
4012 Bristol Road
Durham, NC 27707-5403
(919) 489-9638
CCA #AC96-00001

REAL radios glow in the dark!!!!
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I'll bet that the times when the "1" display flickers worst are when the ambient light level is lowest, right? In most instances "7" will also flicker once in a while (perhaps only when the unit is operated in complete darkness). Reasons for this odd behavior have to do with the variables that influence the ionization time of a gas discharge, and the fact that the readouts in these units are multiplexed.

Most thorough searches for solutions to this problem lead to recommendations to replace the display itself, which may solve the problem for a little while (or not), but it will come back long before the display has reached its end-of-life. The real root cause of the problem is a weak common anode drive transistor. Sorry I don't have the Heath part number or equivalent types immediately to hand, but it's a high voltage PNP type, driven in a level-shifter arrangement by another transistor (you may want to replace both). Also, check that the value of the anode series resistor is within spec (if any is used; some driver chips have built-in current regulation).

Alan "A.J." Franzman
Moderator, NEONIXIE-L Yahoo Group

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