I asked this before a few years ago but didn't come to a firm conclusion. The two parts are shown in this picture: https://rjarratt.wordpress.com/wp-content/uploads/2023/12/img_20231221_1123…
They are from a VT100 Monitor board. There is no schematic for my version of the board, but a related schematic is on p58 of this schematic: https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.…. They are labelled Q414 and CR406, but the Feb 82 revision uses a different board and those two parts are in TO-220 packages rather than TO-3 as here..
I have removed both of them and tested them with a DMM. Neither of them measure as two diodes, they do not measure open circuit in any direction, so either they are both faulty or they are not transistors (unlikely). I have found a spec for a 2SB411 at https://alltransistors.com/transistor.php?transistor=10279. This seems to be a germanium transistor. When I last asked about this I had a conversation with Tony Duell, who was very sceptical that these could be germanium transistors, so naturally I am very doubtful that they are 2SB411 transistors. However, in the schematic Q414 is shown as NPN, the 2SB411 is PNP and this seems to match the way the part is connected on my board (Collector to HORIZ GND). I am working on a reverse engineered schematic.
Can anyone tell me what these components are so I can find a datasheet for them? Better still a suggested replacement would be really helpful, especially if they are 2SB411s because they seem to be hard to find.
Thanks
Rob
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.
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Dear administrators,
Once again Gmail users appear to have been disabled. Could you please help
and re-enable me and others who have been affected. It is only 3 or 4
months ago when this happened last. It would be nice if this could be fixed
permanently.
Thanks and best regards
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I bought a reel of paper tape at an office supply shop today.
It was in their 'bargains' room and is marked
NCR
SYSTEMEDIA
01-6388181
Telex 888634
PTI
Made in England
It cost me £0.50. I think they had two more reels. it's age is indicated by
having a London phone number that was made obsolete 30 years ago.
However, what surprises me is that it's 17.5 mm (0.69", 11/16") wide. This
seems too narrow for 7-hole tape and too wide for the tape used for
telegrams. Is it for 5-hole tape ?
Hi all,
I have a controller card (i'll elaborate later in the message) for an
ICOM 3712 8" floppy drive unit. However, i do not have an FD3712 system
to interface to it!
With Shugart style floppy emulation essentially a solved problem, i'm
wondering if similar has been done for the FD3712. As the computer side
of the floppy interface is just a parallel interface, and all the disk
geometry wizardry happens on the FD3712, i'd assume a lot of the
difficulty of Shugart floppy emulation is essentially a moot point. So
the question is, is there a project already existent to replace the
FD3712 unit with an emulated (raspberry pi based?) alternative?
I'm asking the question as i have an RCA Microboard bus CDP18S646
combined Centronics Printer interface and FD3712 floppy interface. It
also came with an MBZ80 z80 based CPU card, and i'm putting 2 and 2
together and gathering it was likely the two were used together. If i've
every got a a hope in hell of booting anything, solving the media
problem is quite important.
Cheers,
Josh Rice
As posted previously I am trying to repair my VT100 and I am having trouble
with the Monitor Board. The one I have does not match the available printset
(PDF p54 of this
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.p
df). My VT100 appears to match the older
https://bitsavers.org/pdf/dec/terminal/vt100/MP00633_VT100_Mar80.pdf
printset, but it doesn't cover the monitor board.
My monitor board looks as follows
https://rjarratt.wordpress.com/wp-content/uploads/2023/12/img_20231221_11223
3.jpg and the two big transistors on the left appear to be incorrect parts
possibly from a previous repair attempt before I got the VT100.
Does anyone have a previous revision of the VT100 with a Monitor Board that
matches mine? If so, would you be able to photograph it please, or tell me
what parts are installed for the two TO-3 transistors on the left? Better
still would be if anyone has the printset for this version of the monitor
board.
Thanks
Rob
