[Heathkit] SN-634 display flickers

[Heinz und Hannelore Breuer]

Hi,

I am still working on the SB-634 and noticed that the 1 on the seconds
digit flickers. I powered the SB-634 from a variac and noticed that it
doesn't lite at all when the mains is on the low side.
I know of Mike's (The Heathkit Shop) replacement display boards for the
SB-104 which probably uses the same 7 digit gas discharge displays. Is
there a replacement board available for the SB-634 also? 
I tried the variac on the high side and the display seems to lite
better. I saw that hv is produced by a half wave rectifier. I could
install a full wave bridge and get a few volts more. I suspect that
higher voltage might help but probably not for long. I built a nixie
clock in 1969 and the nixies all went bad a couple years later. They had
a limited lifespan and this might be the fact for these displays too.

The manual shows a "keep alive cathode" pin 10 on the display which is
not connected. What is it? I searched Google but didn't find any
datasheet of the displays. Maybe some signal applied here might help to
"ignite" the display.

I would appreciate any help. Maybe we can even trigger Mike to offer a
replacement board for the SB-634 as well.

73
Heinz DH2FA, KM5VT