Well I am going to try those NOS parts, but not before testing the flyback and other transformers. I will also check how the replacement parts measure on the bench first.
But I am going to be away for a while so it will be some time before I can report back. Thanks Rob On 4 February 2026 00:22:40 GMT, Peter Coghlan via cctalk <[email protected]> wrote: >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were quickly superceded by the board version with the silicon transistor 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. >>
