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 Or maybe their reliability was poor and they were quickly superceded by the board version with the silicon transistor as soon as that technology became available?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 <[email protected]> > Subject: [cctalk] Re: Identifying This Component from VT100 Monitor Board > To: 'Brent Hilpert' <[email protected]>, > "'General Discussion: On-Topic and Off-Topic Posts'" <[email protected]> > Cc: Rob Jarratt <[email protected]> > > > 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 <[email protected]> > Sent: 03 February 2026 11:54 > To: 'Brent Hilpert' <[email protected]>; 'General Discussion: On-Topic and > Off-Topic Posts' <[email protected]> > 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-specifications-datasheet > > > > Thanks > > > > Rob > > > > From: Brent Hilpert <[email protected] <mailto:[email protected]> > > Sent: 03 February 2026 08:03 > To: [email protected] <mailto:[email protected]> ; General Discussion: > On-Topic and Off-Topic Posts <[email protected] > <mailto:[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. >
