HELP WITH HEATHKIT SB-634

[Alan 'A.J.' Franzman]

---- Original Message ----
> Date:    Tue, 10 Apr 2007 20:57:16 -0700
> From:    Murray Grandy <mgrandy at TELUS.NET>
> Subject: HELP WITH HEATHKIT SB-634
>
> I HAVE SUBJECT ITEM PURCHASED ON EBAY. IT WAS ADVERTIZED AS SEGMENT =
> =93F=94 NOT WORKING ON ALL SIX DISPLAY TUBES. I FOUND A 100K RESISTOR =
> OPEN IN THAT PART OF THE CIRCUIT. I REPLACED THE RESISTOR AND ALL =
> SEGMENTS ARE NOW WORKING EXCEPT ALL SIX NOW ONLY HAVE A SMALL PORTION =
> THAT IS LIGHTING. THE ENDS OF EACH REMAIN DARK AND THE CENTER PART =
> LIGHTS UP TO A LENGTH OF ABOUT 1/8=94. I HAVE REPALCED THE TRANSISTOR =
> (Q101) WITH SEVERAL OTHER GOOD ONES AND NO JOY. MY QUESTION IS, CAN THIS =
> HAPPEN TO SIX DISPLAY TUBES AT THE SAME TIME? SEEMS DOUBTFUL BUT DON=92T =
> KNOW WHAT ELSE IT COULD BE. ANYONE OUT THERE ANY KIND OF EXPERT ON THESE =
> DISPLAY TUBES?
>
> MURRAY

Murray,

If the display has seen a lot of use with that resistor open, then very likely
all of the segments "F" are partly cathode poisoned.  (Cathode poisoning in a
gas-discharge readout is somewhat different from cathode poisoning in vacuum
tubes).  This can usually be fixed fairly easily by removing the readouts one
at a time and over-driving the afflicted segment of each digit for a while.
For these readouts, try about 1.5 mA, using this formula to calculate a series
resistor value: R [k ohms] = (Vsupply - 135 V) / 1.5 mA.  The approximate
operating forward voltage drop of the display is 135 V, so Vsupply - 135 V is
about the voltage that will be across the resistor.

For example, if you use a supply voltage of 170 VDC, R = 23 k. (22 k would be
close enough). The pins for each segment cathode are directly behind the
middle of the segments, and the anode connection is the pin behind the little
copper-colored rectangular doohickey -- actually a spring which contacts a
transparent conductive layer on the inside surface of the glass.

If you know how to get the HVDC directly from the SB-634 display board you can
use that. If you run a segment at this current level for an hour and do not
see any improvement, you can go to a lower resistor value until you find one
that works.  It's also possible that the fix may only take a few minutes -- it
all depends on how thickly the (invisible) insulating layer was deposited.
You will probably see the segments first begin to light up the dark areas, but
dimly.  Wait a while longer and the dim areas should brighten until you can't
tell where the flaws were anymore.  Once the cathode poisoning is cleaned off,
the readout should work fine with no modifications to the display circuit.


--------------------

Alan "A.J." Franzman
NEONIXIE-L Moderator

Email: a.j.franzman [ A T ]  verizon [ D O T ] net

--------------------

-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.aspx?list=HEATH
To post - HEATH at LISTSERV.TEMPE.GOV
Archives - http://listserv.tempe.gov/archives/HEATH.html