On 2026Feb 2,, at 11:00 PM, Rob Jarratt <robert.jarratt(a)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/ <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 <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/ <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.
