4 Feb
2026
4 Feb
'26
12:22 a.m.
The answers to my questions all indicate that my theory that the board could
have originally been designed for an NPN horizontal output transistor is
not correct.
On the one hand, I suppose the best lead we have at the moment is that
the original components were 2SB411s and the board already doesn't work
so there would not be much lost if you were to try another pair of 2SB411s.
On the other hand, I agree with everything Tony said. Just because a
horizontal output transistor candidate looks good on the data sheet doesn't
mean it will perform well practice and if it turns out not to be the right
one it could well die instantly. Also, some other component like the
horizontal output transformer could have failed and been responsible for
killing the horizontal output transistor and/or clamp diode. It would be
really nice to know if the transformer is good before trying more transistors.
There could also be other faulty components but if the transformer is no
good, that's a bit of a showstopper.
I don't think germanium power switching transistors enjoyed a long and
stellar reign. Maybe this acounts for the apparant rarity of this
version of the board? Or maybe their reliability was poor and they
were quickly superceded by the board version with the silicon transistor
as soon as that technology became available?
I presume the 2SB411s on offer were manufactured donkeys years ago. I
wonder does the technology involved keep well in storage? I don't know
and my record in speculating hasn't been good recently.
Regards,
Peter Coghlan.
Date: Tue, 03 Feb 2026 11:55:16 +0000
From: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Subject: [cctalk] Re: Identifying This Component from VT100 Monitor Board
To: 'Brent Hilpert' <bhilpert(a)shaw.ca>ca>,
"'General Discussion: On-Topic and Off-Topic Posts'"
<cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Actually a seller I have bought from before has some 2SB411s available. They
aren’t cheap but I don’t mind, would it make sense to get a couple of
these?
Regards
Rob
From: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Sent: 03 February 2026 11:54
To: 'Brent Hilpert' <bhilpert(a)shaw.ca>ca>; 'General Discussion: On-Topic
and Off-Topic Posts' <cctalk(a)classiccmp.org>
Subject: RE: [cctalk] Identifying This Component from VT100 Monitor Board
Thanks Brent, I have the Technical Manual and hadn’t spotted the diagrams with Q414 in
them. I can look for PNP transistors, I guess I just need an idea of the kind of spec I
need. Would you say that I have to match this spec:
https://www.uxpython.com/electronics/bjt/2sb411/pnp-transistor-specificatio…
Thanks
Rob
From: Brent Hilpert <bhilpert(a)shaw.ca <mailto:bhilpert@shaw.ca> >
Sent: 03 February 2026 08:03
To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk> ; General Discussion: On-Topic and
Off-Topic Posts <cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Identifying This Component from VT100 Monitor Board
On 2026Feb 2,, at 11:00 PM, Rob Jarratt <robert.jarratt(a)ntlworld.com
<mailto:robert.jarratt@ntlworld.com> > wrote:
I forgot to reply to this one. Thanks Brent.
Of particular interest is the description of how the monitor board is supposed to work in
the VT180 TM at page 6-102. When I have time I will check it carefully, I think there may
be clues about Q414. Interestingly the intro says that horizontal section is not
intuitively understandable from an examination of the schematic and it is a likely
candidate for failure because of high stresses in the components.
I forgot some th-of-op was also included there, and then found it again tonight in
another document:
vt100.net <http://vt100.net> has a work-in-progress html version of some VT100
Technical manual:
https://vt100.net/docs/vt100-tm/
There doesn’t seem to be a document date there but Chapter 4 has that th-of-op section on
the Elston monitor:
https://vt100.net/docs/vt100-tm/chapter4.html
Section 4.8
Neither of course has the schematic, but notably these th-of-op sections reference
component IDs that match the PCB & your RE'd schematic, and the small th-of-op
diagrams do show Q414 as PNP.
The T403 pin numbers there differ from your labeling as Digital/Elston viewed it as 6-pin
with 1 & 5 absent, rather than 4-pin.
Your pics show what appear to be some date codes from 1979.
The vt100.net <http://vt100.net/> website is aware of other field printsets from
1979:
https://vt100.net/manx/part/dec/mp-00633-00/
but they also cannot find them.
So it does appear that, in addition to the Ball monitors, there were two versions of the
Elston monitor for the VT100:
- one from 1979 with PNP HOT,
- one from 1982 modified to NPN HOT (along with other mods) (per
MP00633_VT100_Schematic_Feb82.pdf)
Double-checking with the pics, your schematic looks correct to me regarding the HOT
circuit.
Looks like the board could be modified for NPN with 1 ~ 3 trace cuts depending on how one
went about heatsinking the HOT.
Or use the search specification selectors on sites like Digikey or Mouser to find an
adequate hi-V PNP power transistor.