HELP WITH HEATHKIT SB-634

[Mike Bryce]

Murry..

the gas discharge tubes used in this unit are multiplexed.

all segments of all tubes are connected together. all "a" segments,  
all "B" segments and so on


the transistors are used to turn on the juice to the display, while  
the segments are controlled

Those segments are controlled by a single transistor that is assigned  
to each of the segments.

Confused?

I'm not.. but look at this way.. all those "A" segments are tied  
together on all of the displays. A single transistor controls that  
"A" segments on all the displays.

ok..

six displays, you'll see six transistors.

those transistor "sink" voltage from the display. The displays work  
by high voltage and not current. Had the display been LED, then the  
transistor would sink current.

but I have wondered a bit.

all those transistors are controlled by the mulitplex output of the  
clock chip.

Now, the transistors on the top side of the display control the  
voltage to the gas discharge tubes
and they are in fact also controlled by the clock chip.

so when you put it all together... here's how it works

when the clock chip displays the time by scanning through from the  
first display to the sixth display and then starts over. As the chip  
scans, each display is turned on for a few microseconds. The top  
transistors are turned on depending on which display will be on for  
that given microsecond.

so to display a "2" on the first display, the display is turned on.  
Then segments "A" "B" "C" and "D" would be turned on.
But since only the first display is turned on, none of the other  
displays will show the "2"

the clock chip then gates the segment drivers and they light up the  
required segments.

This happens so fast we don't see the individual displays light up.  
Instead we see the complete display that is showing 09:12:34 or  
whatever the time maybe!

the fix? Check to make sure you have replaced the correct transistors.

Next, carefully remove the clock chip and reseat it a few times. This  
will break off the crud on the pins

I wonder why the 100k went up.

A leaky transistor could also cause the problem.

there's a zillion wires going to that display board. check to make  
certain none are touching each other or anything else for that matter

Hope this helps

oh... as you might as well imagine, all that switching can cause a  
lot of rfi! Multiplexed displays are noted as being "noisy"

Mike

Mike, WB8VGE
SunLight Energy Systems
The Heathkit Shop
http://www.theheathkitshop.com/
J e e p
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