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-specifications-datasheet
Thanks Rob From: Brent Hilpert <[email protected]> Sent: 03 February 2026 08:03 To: [email protected]; General Discussion: On-Topic and Off-Topic Posts <[email protected]> Subject: Re: [cctalk] Identifying This Component from VT100 Monitor Board On 2026Feb 2,, at 11:00 PM, Rob Jarratt <[email protected] <mailto:[email protected]> > 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.
